
Editor's Choice
One of the most effective methods of generating of terahertz radiation is based on the effect of optical rectification of the subpicosecond and femtosecond laser pulses in the crystals with quadratic optical nonlinearity. In this case, an optical photon decays in the nonlinear medium into two photons, one of which has a terahertz frequency. The Cherenkov’s condition of synchronism, under which this generation takes place, follows from the conservation laws of the energy and momentum for this elementary process and has the following form: $\nu_g cos \theta = c/n_T $. Here $c $ is the speed of light in vacuum, $\nu_g $ is the group velocity of optical pulse at its carrier frequency, $n_T$ is the terahertz refractive index, $\theta $ is the angle between the propagation directions of optical and terahertz signals. Note that the optical and terahertz pulses propagate in different directions under this condition. As a result, the efficiency of the generation weakens. To increase this efficiency, the technique of tilted fronts of optical signals is used in experiments. In such a case, $\theta $ is the angle between the group and phase wavefronts of optical pulse. Then, the terahertz signal is fed permanently by the energy of the optical pulse, and the efficiency of the generation is increased significantly. The terahertz pulses generated by the optical method contain about one (or even half) period of electromagnetic oscillations, i.e. they have properties of extremely short (or unipolar) pulses. Therefore, the approximation of slowly varying envelopes, which is standard for the quasimonochromatic signals, is not applicable in theoretical studies of the interaction of these pulses with matter. At the same time, the optical pulse is quasimonochromatic. Therefore, this approximation is valid for it. In order to describe theoretically the process described above, we derive in this paper the new nonlinear equations for the envelope of the electric field of optical pulse and for the electric field of terahertz signal. We refer to these equations as the Yajima – Oikawa – Kadomtsev – Petviashvili (YOKP) system. This system contains opticalterahertz and purely terahertz quadratic nonlinearities, dispersion and diffraction of both components. Also, we found the solution of the YOKP system in the form of optical $E_0$ and terahertz $E_s$ solitonlike pulses propagating in a bound mode (see figure). The angle $\theta $ between the phase and group wavefronts of the optical soliton is determined in this case by the Cherenkov's condition. At the same time, purely terahertz unipolar soliton $E_T$, which is a solution of the Kadomtsev – Petviashvili equation, propagates in the direction of movement of the phase fronts of the optical pulse. The polarities of the terahertz components $E_s$ and $E_T$ are opposite. The relationship between the temporal durations and amplitudes of the terahertz components is found from the condition of equality of their "areas". It turns out that the soliton component $E_s$ should be much shorter and more intense than the component $E_T$ in a case of $LiNbO_3$ crystal.
Schematic representation of the propagation of opticalterahertz $E_0 + E_s$ and purely terahertz $E_T$ pulses under the angle $\theta $ between the phase and group velocities of the optical signal; the phase fronts and the terahertz soliton propagate along the $z$ axis, and the group fronts propagate along the $ z' = z~ cos \theta + x~ sin \theta $ axis. The soliton mode of the generation described above is possible if the dispersion parameter of the group velocity of optical pulse is positive and exceeds the critical value determined by the angle $\theta $ of inclination. In this case, the nonlinear susceptibility of the second order corresponding to the carrier frequency of the optical pulse should be negative.
S. V. Sazonov and N. V. Ustinov
Since the recent experimental discovery of anyonic statistics of quasiparticles in the 1/3 fractional quantum Hall effect regime, this system has been of exceptional interest. In this work we investigated the spectra of resonance reflection of light from a twodimensional electronic system in the conditions of formation of Laughlin liquid in fractional state 1/3. It is shown that the main lines in the spectra of resonant reflection of light do not correspond to singularities in the twoparticle density of states of the excited electronhole system, but are associated with the birth and destruction of neutral excitations. Thus, the resonant reflection of light in fractional state 1/3 is an analogue of the Raman process with the creation and destruction of neutral excitations in transitional scattering states, while twoparticle (excitonic) optical transitions are not observed experimentally. The suppression of twoparticle optical transitions is presumably due to the incompressibility of the ground state of a twodimensional electronic system.
A.S. Zhuravlev, L.V. Kulik, L.I. Musina, E.I. Belozerov, A.A. Zagitova, I.V. Kukushkin
Experimental results on the coherent properties of a recently discovered new collective state, the magnetoexcitonic condensate, are summarized in the present letter. The condensation occurs in a fermionic system, a quantum Hall insulator (filling factor $\nu = 2$), as a result of the formation of a dense ensemble of longlived (experimentally measured lifetimes achieve ~1 ms) triplet cyclotron magnetoexcitons (TCMEs), composite bosons with spin S = 1. The magnetoexcitons are formed by an electron vacancy (Fermi hole) at a completely filled zero electron Landau level and an excited electron at an empty first Landau level. At temperatures T < 1 K and TCME concentrations n_{ex} ∼ (110)% of the density of magnetic flux quanta a transition occurs to a qualitatively new phase. The condensate shows a sharp decrease in viscosity and the ability to spread over macroscopically large distances, on the order of a millimeter, at a speed of ~10^{3} cm/s. This work is devoted to the study by interferometric methods of the degree of spatial coherence in the magnetoexcitonic condensate.
The main method for detecting TCMEs is photoinduced resonant reflection of light. This method finds photoexcited Fermi holes that are part of cyclotron magnetoexcitons (TCMEs themselves are “dark” quasiparticles that do not interact in the dipole approximation with an electromagnetic field). The figure shows the profile of interference fringes (red) in Michelson interferometer with a mirror in one arm, and a right angle prism in the other, which are observed in the light resonantly reflected from magnetoexcitonic condensate. Here, the envelope of fringes profile is nothing more than a firstorder correlator g^{(1)} as a function of distance $\delta $. The blue line is the theoretical curve (instrumental function) that best describes the central peak corresponding to thermally excited noncondensed TCMEs. The black curve is the result of adding with weights of 0.8 and 0.2, respectively, of the instrumental function and its convolution with $exp (−\delta/\xi )~ at~ \xi = 10 \mu m.$
A.V. Gorbunov, A.V. Larionov, L.V. Kulik, V.B. Timofeev Identification of solidlike clusters is important problem of condensed matter physics. Here, we use the bond orientational order parameters (BOOP), introduced by P. Steinhardt to characterize the arrangement of neighboring particles with respect to central one. Set of rotational invariants (RI) being calculated via BOOP method for each atom describes the fine details of the local orientational order of the system of atoms. We propose a new method to identify distorted solidlike clusters, including difficulttodetermine bcclike clusters. Within the method we calculate the rotational invariants of second (q4, q6) and third (w4, w6) orders by using a fixed number of nearest neighbors (NN) which is typical for close packed structures: NN = 12. In that case ideal bcc lattice gives two sets of RIs only, which are well separated from another close packed structures (fcc, hcp, ico). Using 2D distributions of RIs (shown in Figure) the most important solidlike clusters (even being strongly distorted) can be easily identified.
Distribution of distorted atoms of different symmetry (fcc, hcp, bcc, ico) on the plane of rotational invariants (q4q6) and (w4w6). The distributions were calculated via fixed number of nearest neighbors (NN), which corresponds to close packed (NN = 12) structures. In that case ideal bcc lattice degenerates into two sets of rotational invariants only which are; this method provides easy way to identify any type of symmetry of distorted solidlike clusters.
B.A. Klumov
Anderson localization is observed in a highly disordered twodimensional (2D) electronhole system in a HgTebased quantum well, the behavior of which is significantly different from that observed in widely studied twodimensional onecomponent electron and hole systems. It was found that a twostage localization occurs in the system: the twodimensional holes are localized first, as particles with an effective mass almost an order of magnitude greater than that of electrons. Then the electrons are localized. It was also found that there is no metalinsulator transition in the system under study: even at values of conductivity σ > e^{2}/h, a dielectric temperature dependence is observed. At electron densities (N_{s}) exceeding those of holes (P_{s}), when the transport is determined by electrons, localization behavior is not described by oneparameter scaling despite the smallness of the interaction parameter (r_{s} < 1). Probably it is necessary to take into account the electronhole and the holehole interaction, as well as the spinorbit interaction to get the right description of the Anderson localization in the electronhole system. Obviously, further experimental and theoretical research of the discovered phenomenon will be of interest.
Figure. (a)  Resistivity gate voltage dependences at different temperatures, (b)  Resistivity temperature dependences at N_{s }> P_{s }, (c)  Resistivity temperature dependences at P_{s }> N_{s} , (d) 
Z.D.Kvon, E.B.Olshanetsky Simulation of quantum systemson a quantum computer using the ZalkaWiesner method with allowance for quantum noise is considered. The efficiency of the developed methods and algorithms is demonstrated by the example of solving the nonstationary Schrödinger equation for a particle in the Pöschl–Teller potential. The developed analytical theory of the effect of quantum noise on the simulation accuracy is compared with the results of numerical calculations by the MonteCarlo method. The forecast of the accuracy of the solution of the Schrödinger equation for a multibody electron system is carried out depending on the number of electrons and for various noise levels. To estimate the accuracy of the ZalkaWiesner algorithm we analyze the accuracy of the gates included in the QFT circuit. Based on these values, we obtain an estimate of the QFT algorithm accuracy, which can be easily extended to the case of the ZalkaWiesner algorithm. The main advantage of this approach is the ability to evaluate quantum circuits with an extremely large number of qubits. The figure shows the level of influence of quantum noise on the Schrödinger equation solution accuracy obtained on a quantum computer. The quantum state evolution of a 9 qubits register was considered over a time interval $0\leq t \leq1 $ with a time step $\Delta t= 0.05$ at a noise amplitude level $e = 0.01$.
Illustration of the density distribution evolution in the coordinate representation. Initial state – dashed line, final state at $t = 1$  solid line, noisy ZalkaWiesner solution is represented by a set of points.
Yu. I. Bogdanov, N.A. Bogdanova, D.V. Fastovets, V.F. Lukichev ^{13}C is usually recognized a good example of a "normal" nucleus well described by the shell model. Its level scheme is reliably determined up to the excitation energies 10 MeV. However, some new ideas and results renewed interest in ^{13}C. The most ambitious among them is hypothesis about possible existence of 𝛼particle BoseEinstein condensation (𝛼BEC). Some features of the condensate structure were predicted and observed in the second 0^{+}, 7.65 MeV state of ^{12}C (so called Hoyle state). It was also suggested that the structures analogous to the Hoyle state may exist in neighbor nuclei ^{13}C. Recently a hypothesis was put forward about a new type of symmetry in the ^{13}C  𝐷′_{3h }symmetry. On the basis of this symmetry, the rotational nature of a whole group of lowlying ^{13}C states was predicted. If this hypothesis is confirmed, our understanding about the ^{13}C structure will radically change. To solve these questions our group has made experiments on scattering of 𝛼particles on ^{13}C at (𝛼) = 65 MeV and 90 MeV. New experimental data was got for the 1/2^{−}_{3}, 11.08 MeV state. Obtained data was analyzed using Modified diffraction model (MDM), developed by our group. rms radius of this state within errors coincides with the radius of the 1/2^{−}_{2} 8.86 MeV state in ^{13}C and the Hoyle state in ^{12}C (see Fig.). This result is an argument for close cluster structure of these states.
^{1} Previously our MDM analysis showed that 3/2^{−}, 9.90 MeV in ^{13}C is compact and has decreased by 10% rms radius. This unusual result we tested via consideration of its isobaranalog state (IAS) in ^{13}N – 3/2^{−}, 9.48 MeV state. We found that this state has normal nonincreased radius. Also we clarified our previous result for the rms radius of the 9.90 MeV state and obtained that within the error limits, the value of the radius obtained for the 9.90 MeV in ^{13}C coincides with the radius of the 9.48 MeV in ^{13}N. Obtained normal radius for the 3/2^{−}, 9.90 MeV destroyed one of the rotational bands predicted by 𝐷′_{3h }symmetry in ^{13}C.
Demyanova A.S., Danilov A.N., Dmitriev S.V., Ogloblin A.A., Starastsin V.I., Goncharov S.A., Janseitov D. During the last several decades the study of lowdimensional electron systems became one of the main and actively developing research areas in condensed matter physics. Such interest was caused by, on the one hand, the possibility to study new fascinating physical phenomena, and the opportunity for technological applications, on the other hand. Continuous progress in fabrication of 2D structures, quantum wires and quantum dots helped to create new unique systems for investigation and had enormous impact on development of planar semiconductor devices. In that case, study of transport properties of a 2DEG became extremely important. Such investigation revealed an intriguing effect of giant oscillations of longitudinal magnetoresistance in a 2DEG with sufficiently high mobility in the presence of weak magnetic field and illuminated by microwave radiation, named MIRO [1, 2], and led to the discovery of the zeroresistance states (ZRS). Observed phenomena created a new branch of nonequilibrium physics and demonstrated how combination of weak microwave radiation and weak Landau quantization could drastically change transport properties of a 2DEG. Despite the fact that MIRO has been actively studied for more than twenty years, the physical understanding of its origin is still a subject of wide discussion. Two mechanisms which consider the bulk origin of the phenomenon, did not explain a number of experimental results. These contradictions led to the creation of alternative theories that associate the causes of MIRO with the influence of edges and nearcontact area. As a result, an experimental study of the contribution of these regions to microwaveinduced magnetoresistance oscillations is of great interest. Present work is devoted to the contactless measurements of microwaveinduced oscillations of highfrequency conductivity in the relatively new 2DES  ZnO/MgZnO heterojunction. Experimental technique was based on the analysis of a transmission signal between two Tshaped antennas, capacitively coupled to a 2DES (Fig. 1(a)). Absence of Ohmic contacts or deposited metallization on the sample surface allows to eliminate the influence of nearcontact regions on MIRO and testing how universal are the properties of MIRO obtained earlier on a completely different material system such as ZnO/MgZnO heterojunction (Fig. 1(b)). Such measurements provide additional information for understanding the nature of the MIRO origin.
Fig. 1. (a) Schematic drawing of the experimental setup. (b) Typical dependencies of the variation of the output voltage on the magnetic field B induced by an exciting microwave radiation f = 64; 74 and 84 GHz. The voltage variation $\delta V$ was normalized by the voltage value at zero magnetic field $V_0$. The positions of the first oscillations are indicated. The sample temperature was equal T = 1:5 K.
[1] M. A. Zudov, R. R. Du, J. A. Simmons, and J. L. Reno, Phys. Rev. B 64, 201311(R) (2001). It is well known that in parametric downconversion in a nonlinear crystal, the pump photon decays into two photons with lower frequencies. Such photon pairs form quantum biphoton states, which have long been used in quantum optics and information, absolute calibration of radiation brightness, and nonlinear interferometry. Usually the frequencies of both photons are in the visible or nearIR range. However, if the frequency of one of the photons is very close to the pump frequency, then the frequency of the second one is several orders of magnitude lower and may lie in the terahertz range. The possibility of generating terahertz radiation using parametric downconversion has been studied for more than ten years, but opticalterahertz biphoton states have not yet been registered. One of the difficulties in studying the opticalterahertz biphoton field is the large wavelength of the terahertz photon, comparable to the width of the pump beam. This leads to a complex structure of spatial modes of biphoton radiation. In this paper, it is shown that the nonlinear interaction operator describing the production of opticalterahertz biphotons can be diagonalized in the space of azimuthal angles. As a result, it is possible to obtain the azimuthal eigenmodes of the scattered radiation, shown in the figure. In the basis of these eigenmodes, it is easy to obtain a scattering matrix that describes any correlation properties of opticalterahertz biphoton radiation at arbitrary values of the parametric gain. The obtained scattering matrix was used to calculate the correlation function of the intensities of the optical and terahertz scattered radiation and the dispersion of the difference in the numbers of optical and terahertz photons depending on the angular apertures of the photodetectors used in the experiment. The obtained results allow us to clarify the conditions under which it is possible to register the nonclassical properties of opticalterahertz biphoton fields.
An example of the structure of azimuthal eigenmodes of opticalterahertz biphoton radiation (at a terahertz radiation frequency of 0.5 THz).
P.A.Prudkovskii
A stable solitary wave is commonly called a soliton in physics. Solitons are classified according to various criteria. Distinguish between conservative and dissipative solitons. Conservative solitons are formed in the media where the irreversible energy losses can be neglected. In these cases, the solitons save the information about conditions at the input to the medium. Therefore, they have the continuous free parameters. The specific values of these parameters are depending on the input conditions. For example, the amplitude and the velocity of propagation of a soliton continuously depend on its temporal duration, which can be chosen as a free parameter. Besides, after passing of the conservative soliton the medium returns always to its initial state. In nonequilibrium media with irreversible losses and a source of energy, dissipative solitons can form. Such solitons do not have a continuous free parameter: their amplitude, velocity and duration cannot be arbitrary. These characteristics are dependent on the parameters of a medium. This property can be explained by the fact that in media with dissipation, the information about the input conditions will not be preserved. One of the trends in the development of modern nonlinear optics and laser physics is the creation in laboratory conditions of light pulses of ever shorter durations. By now, pulses have been created that contain about half of the electromagnetic oscillations. Such objects are called as unipolar impulses. In this work, the possibility of the formation of unipolar saltlike structures of an electromagnetic nature in a nonequilibrium medium has been investigated. This medium is formed by twolevel atoms embedded in a homogeneous matrix. In this case, the twolevel atoms and the matrix are not in a state of thermodynamic equilibrium with respect to each other.
The temporal duration $\tau_p$ of unipolar pulses is longer than the decay time $T_2$ of the dipole moments of molecules, but shorter than the relaxation time $ T_1$ of the populations of stationary quantum states. It is shown that, in this case, localized unipolar objects characterized by an electric field $E $ (Fig. (a)) possess the properties of both conservative and dissipative solitons.
Like the conservative solitons, these structures have a continuous free parameter $\tau_p$ . Hence, the memory of the input conditions is remain. In particular, the pulse amplitude is inverselyproportional to the parameter $\tau_p$. At the same time, after the passage of the soliton, the
medium passes from the initial nonequilibrium state to another metastable (also nonequilibrium) state with a lifetime $ T_1$ (see Figs. (b) and (c)). Therefore, the observation time $\Delta t $ of such solitons lies in the interval $T_2 \ll \Delta t \ll T_1$ . This can be possible in solids, where $T_2 / T_1 \sim 10^{2}  10^{5}$.
At an inverse initial population of the states of twolevel atoms ($W > 0$ ), the population difference $W$ decreases as the solitonlike pulse propagates (Fig. (b)). If the initial population of quantum states is not inverse ($W < 0$ ) and the matrix temperature is higher than the temperature of twolevel atoms, then the propagation of the soliton is accompanied by an increase of the population difference $W$ (Fig. (c)). In both cases, after the passage of the soliton, the new metastable state of the medium becomes closer to the equilibrium state.
(a) The profile $E(\zeta)$ of the electric field of a solitonlike pulse, $E(\zeta) tz/ \nu , t $ is the time, $z$ is the propagation distance, $\nu$ is the velocity of the pulse; the amplitude of a signal $E_m \sim 1/\tau_p$ .
(b) The profile $W(\zeta)$ of the difference between the populations of states of twolevel atoms with an inverted initial population; the velocity of the soliton decreases with a continuous shortening of its duration $\tau_p$ .
(c) The profile $W(\zeta)$ of the difference between the populations of states of twolevel atoms at a noninverted initial population; the velocity of the soliton increases with a continuous shortening of its duration $\tau_p$ .
S.V.Sazonov
JETP Letters 114, issue 3 (2021)
We have recently shown that the use of micropillar resonators, which comprise a cylindrical semiconductor cavity sandwiched between the Bragg mirrors can substantially increase the quality factor preserving the mode volume, and thus substantially enhance the local fields [Optics Letters Vol. 45, 1, 181183 (2020)]. Here, we show that these structures can facilitate the significant enhancement of the second harmonic generation efficiency. We provide a specific design of the AlGaAs/GaAs pillar microcavity and use the numerical modelling to directly show the resonant enhancement of the SHG efficiency in socalled quasiBIC (bound states in the continuum) regime. In this regime the quality factor of the first harmonic drastically increases due to the destructive interference of two lowquality modes of cavity. QBIC regime appears at specific geometric parameters of cavity that results in approximately two orders gain in second harmonic generation efficiency.
Kolodny S.A., Kozin V.K., Iorsh I.V. Chains of ultracold ions trapped with varying electric fields are one of the most promising platforms for quantum computations, which is being actively studied at the moment. It features long coherence time, welldeveloped and highfidelity techniques for quantum state initialization and readout as well as a strong Coulomb interaction between particles, which allows to efficiently entangle them. One of the approaches to this platform further development is a search for more suitable ion species or new ways of encoding quantum information in their electronic structure. In this letter, we experimentally investigate quantum information encoding in an optical quadrupole transition in ^{171}Yb^{+} ion, which is already widely used for quantum computations but with microwave qubit encoding. Optical qubits are easier to individually address with laser beams than microwave ones as there is no need for bichromatic laser emission from different directions and only one beam is sufficient. Initialization and readout of optical qubits are also usually more accurate. These properties may help to overcome one of the major issues with ion quantum computers – scalability problems. On the other hand, optical qubits suffer from shorter coherence times. We compare proposed optical qubit with microwave qubit in ^{171}Yb^{+} ion as well as with the most widespread at the moment optical qubit in ^{40}Ca^{+}. We also experimentally demonstrate and characterize fidelity of a singlequbit PauliX operation and fidelities of our preparation and detection schemes.
Level scheme of ^{171}Yb^{+} ion, showing both microwave qubit in the ion as well as proposed optical qubit. States proposed to use for qubit encoding are shown as 0> and 1>.
The microwave photoconductance of a short (100 nm) constriction (QPC) in a twodimensional electron gas under its irradiation at a frequency of (23) GHz has been studied for the first time. The experiment and conductance calculations showed a giant QPC photoconductance in the tunnel mode and negative photoconductance in the open mode. According to the developed model, this behavior results from cophase harmonic electric field additions to the gate voltage V_{g} and to the measuring voltage applied to QPC, determined by the frequency and power P of the microwave source. The voltage dependences of conductance G(V_{g}) at 4.2 K don’t show a pronounced quantization in units of G_{0} = 2e^{2}/h due to the small constriction length, but exhibit anomalous bending at (0.7–0.5)G_{0}. The microwave replicas of these anomalies were found in the form of peakdip features at the lower step of phototransconductance. The basic behavior of G(V_{g}) remains qualitatively the same at 77 K; this result opens possibility of development of a new kind of microwave detectors.
((a, b) The measured gate characteristics of conductance G(V_{g})/G_{0} and transconductance dG(V_{g})/G_{0}dV_{g} at = 4.2 K for various microwave power P/P_{0} at 2.4 GHz frequency (G – conductance, V_{g} – gate voltage, G_{0} =2e^{2}/h) in the transition of a short QPC with split gate from the tunnel to the open mode. Line type and color in each panel with a common scale in V_{g} and on insert to (a) correspond to the indicated P/P_{0}.
V.A. Tkachenko, A.S. Yaroshevich, Z.D. Kvon, O.A. Tkachenko, E.E. Rodyakina, A.V. Latyshev
In a series of numerical experiments, within the framework of the incompressible 3D Euler equations, we have studied evolution of the high vorticity regions, which arise during the onset of developed hydrodynamic turbulence. These regions represent compressing pancakelike structures (thin vortex sheets), which can be described locally by a new exact selfsimilar solution of the Euler equations combining a shear flow with an asymmetric straining flow. The vorticity maximum on the pancake ω_{max} increases exponentially with time, while its thickness l_{1} exponentially decreases, with the Kolmogorovtype scaling relation between the two,
ω_{max} ∞ l_{1}^{2/3}.
This law is confirmed numerically for most of the pancakes, and is also supported by analytical arguments in terms of the socalled vortex line representation.
Normalized second component of the vorticity ω_{2}/ω_{max} as a function of x_{1}/l_{1} at different times, demonstrating the threelayer internal structure of the pancake.
D.S. Agafontsev, E.A. Kuznetsov, A.A. Mailybaev
According to the measurements of the electric component of the electromagnetic field in the frequency range 2 kHz  10 MHz recorded by the Japanese ERG satellite, two generation regions of radiation are defined: the kilometric “continuum” radiation type and new hectometer “continuum” radiation type. It is shown that the kilometric “continuum” radiation is observed mainly on the dayside of the magnetosphere, its source is located near to the plane of the geomagnetic equator, and the source size does not exceed ± (0.1–0.3Re) across this plane, where Re is the Earth's radius. The hectometer radiation mainly observed in the nightside of the magnetosphere has two sources. One of them is located near to the plasmasphere and could be far from the plane of the geomagnetic equator up to 3Re. The second source is located near to the Earth at distances not exceeding 2Re. It was shown earlier that "continuum" radiation was observed on all planets with a magnetic fields. The high stability of the “continuum” radiation indicates the possibility of its use as a second marker of exoplanets with a magnetic field. The first marker is the Auroral Kilometric Radiation (AKR), which is characterized by high amplitude but relatively short lifetime. The “continuum” radiation is weaker than the AKR by 3  5 orders, but high stability of the “continuum” radiation makes it possible to carry out a longterm accumulation of the signal and thus second marker could be formed. The presence of two markers will increase the reliability of detecting exoplanets with a magnetic field by 8 times.
The figure shows the change in the polarization of hectometer radio emission when the satellite crosses the radiation source. The upper panel is a dynamic spectrogram of the electric field component amplitude (in logarithmic scale) and the lower panel is a spectrogram of the polarization coefficient (in linear scale).
Mogilevsky M.M. et al.
The threeparticle multichannel Coulomb scattering problem is an important milestone of the multichannel quantum scattering theory. Being in principle numerically solvable on modern computers without any approximations it would allow one to observe and check the concepts and effects of multichannel scattering of charged particles with applications in atomic, molecular and nuclear physics. However, there is still a number of theoretical issues to overcome in order to mark the problem as “solved”. Moving along this path, we treat the threeparticle multichannel Coulomb scattering problem with rearrangement channels by the potential splitting approach incorporated into the framework of differential FaddeevMerkuriev (FM) equations. These equations have been designed to treat uniformly the elastic, excitations and rearrangement processes. We have developed a highly efficient theoretical and computational approach based on solving the FM equations which in total orbital momentum representation are reduced to a finite set of threedimensional partial differential equations. In this letter, we outline our approach and apply it to calculations of the antihydrogen formation cross section for antiproton scattering off the ground and excited states of the positronium. This reaction is of utmost importance for the AEgIS and GBAR experiments on antimatter based on the use of the Antiproton Decelerator facility that are planned and performed at CERN. Using moderate computational resources we have achieved a supreme energy resolution of both total and partial cross sections that allows us to obtain with high quality such cross section peculiarities as Feshbach resonances.
The Pwave partial cross sections for formation of the antihydrogen in the ground state (blue) and the first excited state (red) in the process of antiproton scattering off the ground state of the positronium. Vertical dashed lines mark positions of resonances obtained with good accuracy in independent calculations.
V. A. Gradusov, V. A. Roudnev, E. A. Yarevsky, S. L. Yakovlev
In quasionedimensional systems (e.g., carbon nanotubes or 2D semiconductor nanoconstrictions with gates) with low concentration of impurities the quantization of transverse electronic motion is essential, and the conductivity shows Van Hove singularities when the Fermi level $E$ approaches a bottom of some transverse subband $E_N$ (see Figure 1). In experiment the observed Van Hove singularities may have quite complex
structure, which is often attributed to Fano resonances.
In the present work we study the resistivity $\rho$ of a conducting tube with shortranged scatterers placed on its surface, in the immediate vicinity of Van Hove singularity. The nonBorn effects lead to quantum suppression of scattering. This suppression effect is, however, destroyed when two scatterers approach each other. As a result, $\rho$ is dominated by scattering at rare "twin'' pairs of close defects, while scattering at solitary impurities and multiimpurity complexes is suppressed. The predicted effect is characteristic for multichannel quasionedimensional system, it can not be observed in strictly onedimensional one.
A tube with two pointlike impurities on its surface. b) Spectrum of electron versus longitudinal momentum $k$ in the case of ideal tube. Subbands of transversal quantization (enumerated by $m$) and Fermi level position $E$ are shown.
Ioselevich A.S. and Peshcherenko N.S.
JETP Letters 114, issue 1 (2021)
After seven years of construction of the Nuclotronbased Ion Collider fAcility (NICA) at the JINR in Dubna, Russia, the first in the chain of three proton synchrotrons – the Booster  has its beam! We present in our paper the first run of the commissioning of the Booster. The singlecharged helium ions were injected into the Booster at energy 3.2MeV/nucleon and a stable ion circulation was obtained.
The measured value of the beam lifetime τ_{exp} = (1.32 ± 0.06) s is comparable with the theoretical calculation τ_{theor} = (1.74 ± 0.50) s, obtained using original computer with heavy ions. The NICA complex will allow to study
A. V. Butenko, A. R. Galimov, I. N. Meshkov, E. M. Syresin, I. Yu. Tolstikhina, A. V. Tuzikov, A. V. Philippov, H. G. Khodzhibagiyan, V. P. Shevelko
The interest to high energy processes near black holes increased significantly after the work \cite{ban}. It was shown there that if two particles move towards the Kerr extremal black hole and collide in its vicinity, the energy $E_{c.m.}$ in their center of mass frame can become unbounded, provided one of two particle (called critical) has finetuned parameters. This is called the BañadosSilkWest (BSW) effect. The close analogy of this effect exists also for extremal charged static black holes [2]. However, as far as the Killing energy $E$ of debris detected at infinity is concerned, the situation differs radically for two aforementioned cases. For rotating black holes, the energy $E$ of an escaping particle at infinity is bounded [35]. Meanwhile, there is no such a bound for the extremal ReissnerNordstrom (RN) black hole. This was obtained in [6] and later confirmed in [7]. The process with unbounded $E$ at infinity is called the superPenrose process (SPP).
As far as nonextremal black holes is concerned, two problems existed here. First, it was widespread belief that extremality is a necessary condition for the BSW effect, so deviation from extremality weakens the effect [8, 9]. However, it was shown in [10] that if instead of one particle being exactly critical, a nearcritical particle is used, and deviation from the critical state is adjusted to the proximity of the point of collision to the horizon in a special way, the effect survives. Moreover, one can add a force acting on particles and this is consistent with the BSW effect [11]. Second, it was unclear how to realize the BSW effect physically. The most relevant situation corresponds to particles falling from infinity. However, for rotating black holes, the centrifugal barrier prevents the critical particle from reaching the nonextremal horizon [10] (see also case 2i in [12], Sec.2 of [13] and [14]). This can be repaired, provided additional constraints are imposed on the scenario, because of which the turning point is situated closely to the horizon [15].
However, there is an interesting question that, to the best of our knowledge, was not posed up to now: whether or not the SPP is possible for nonextremal black holes. It is considered in the present work. We show that this is indeed possible. In this sense, there is a sharp contrast between extremal and nonextremal black holes. One can think that this observation may be useful for astrophysically relevant black holes since they are nonextremal. It possesses some universal features in what any particles moving in the background of a nonextremal black hole (even in the Schwarzschild metric) and experiencing the action of some force can exhibit this effect.
[1] M. Bañados, J. Silk and S.M. West, Kerr black holes as particle accelerators to arbitrarily high energy, Phys. Rev. Lett. 103 (2009) 111102 [arXiv:0909.0169].
[2] O. B. Zaslavskii, Acceleration of particles by nonrotating charged black holes. Pis'ma v ZhETF 92, 635 (2010) (JETP Letters 92, 571 (2010)), [arXiv:1007.4598].
[3] M. Bejger, T. Piran, M. Abramowicz, and F. Håkanson, Collisional Penrose process near the horizon of extreme Kerr black holes, Phys. Rev. Lett. 109 (2012) 121101 [arXiv:1205.4350].
[4] T. Harada, H. Nemoto and U. Miyamoto, Upper limits of particle emission from highenergy collision and reaction near a maximally rotating Kerr black hole, Phys. Rev. D 86 (2012)
024027 [Erratum ibid. D 86 (2012) 069902] [arXiv:1205.7088].
[5] O. B. Zaslavskii, On energetics of particle collisions near black holes: BSW e¤ect versus Penrose process, Phys. Rev. D 86 (2012) 084030 [arXiv:1205.4410].
[6] O. B. Zaslavskii, Energy extraction from extremal charged black holes due to the BSW effect. Phys. Rev. D 86, 124039 (2012) [arXiv:1207.5209].
[7] H. Nemoto, U. Miyamoto, T. Harada, and T. Kokubu, Escape of superheavy and highly energetic particles produced by particle collisions near maximally charged black holes, Phys. Rev. D 87, 127502 (2013) [arXiv:1212.6701].
[8] E. Berti, V. Cardoso, L. Gualtieri, F. Pretorius, U. Sperhake, Comment on "Kerr black holes as particle accelerators to arbitrarily high energy", Phys. Rev.Lett. 103, 239001 (2009), [arXiv:0911.2243].
[9] T. Jacobson, T.P. Sotiriou, Spinning black holes as particle accelerators, Phys. Rev. Lett. 104, 021101 (2010) [arXiv:0911.3363].
[10] A. A. Grib and Yu. V. Pavlov, On particles collisions in the vicinity of rotating black holes, Pis'ma v ZhETF 92, 147 (2010) [JETP Letters 92, 125 (2010)].
[11] I. V. Tanatarov, O. B. Zaslavskii, BañadosSilkWest e¤ect with nongeodesic particles: Nonextremal horizons, Phys. Rev. D 90, 067502 (2014), [arXiv:1407.7463].
[12] O. B. Zaslavskii, Acceleration of particles as universal property of rotating black holes, Phys. Rev. D 82 (2010) 083004 [arXiv:1007.3678]
[13] S. Gao and C. Zhong. Nonextremal Kerr black holes as particle accelerators, Phys.Rev. D 84, 044006 (2011) [arXiv:1106.2852].
[14] S. Krasnikov and M. V. Skvortsova, Is the Kerr black hole a super accelerator?, Phys. Rev. D 97, 044019 (2018) [arXiv:1711.11099].
[15] O. B. Zaslavskii, Can a nonextremal black hole be a particle accelerator? Phys. Rev. D 102, 104004 (2020) [ arXiv:2007.09413].
[16] O. B. Zaslavskii, Schwarzschild black hole as accelerator of accelerated particles, JETP Letters 111, 260 (2020), [arXiv:1910.04068].
O. B. Zaslavskii
JETP Letters 113, issue 12 (2021)
Light bullet is a wave packet extremely compressed both in space and in time. It occurs during the filamentation of a femtosecond radiation under condition of anomalous group velocity dispersion in transparent dielectrics. The estimation of its duration according to measurements by different methods is ambiguous and depends on the diameter of the aperture used in the experiment. In this letter one introduced absolute parameters of a light bullet, determined by the spatiotemporal distribution of electric field strength in the area of localization of a strong light field. Introduced parameters are independent of the spatiotemporal deformations of a wave packet, its spectrum transformation during nonlinear optical interaction with the medium, and are not linked with the size of an aperture. For the considered midIR radiation the increase in the carrier wavelength λ_{0} leads to the monotonous increase in the radius of a light bullet from 1.2λ_{0} to 3.3λ_{0}, the duration does not change and is equal to 1.8 periods of optical oscillation. Obtained estimations of light bullet parameters one can consider as a lower limit of experimental measurements. The developed approach to determining the parameters of optical radiation on the basis of spatiotemporal distribution of the electrics field strength generalizes the characteristics of a quasimonochromatic wave packets to light bullets, the radius and duration of which are close to the wavelength and the period of the light field, respectively.
Spatiotemporal distribution of electric field strength in the light bullet during filamentation in LiF of a femtosecond pulse at the wavelength of 3100nm. E.D. Zaloznaya, A.E. Dormidonov, V.O. Kompanets, S.V. Chekalin, V.P. Kandidov JETP Letters 113, issue 12 (2021)
The efficiency of practically used quantum electronic interferometers is limited by rather stringent requirements, for example, very low temperature for interferometers based on superconducting SQUIDs or the requirement of very strong magnetic fields for interferometers based on the edge states of Quantum Hall Effect systems. A promising opportunity for a technological breakthrough in this direction is associated with the discovery of topological insulators, which are materials insulating in the bulk, but exhibiting conducting onedimensional helical channels at the surface or at the boundaries. The electron transport via helical edge states is ideal, in the sense that electrons do not experience backscattering from conventional nonmagnetic impurities. We review recent studies of the spindependent tunneling transport via AharonovBohm interferometer (ABI) formed by helical edge states. We focus on the experimentally relevant case of relatively high temperature, T, as compared to level spacing, Δ. The tunneling conductance of helical ABI is structureless in ballistic case but shows sharp periodic antiresonances as a function of magnetic flux  with the period of one half flux quantum  in the presence of magnetic impurities. The helical ABI with magnetic impurity may serve as an effective spin polarizer. The finite polarization appears even in the fully classical regime and is therefore robust to dephasing. There is also a quantum contribution to the polarization, which shows sharp identical resonances as a function of magnetic flux with the same period as conductance. This polarization survives at relatively high temperature. The interferometer can be described in terms of ensemble of T/Δ fluxtunable qubits giving equal contributions to conductance and spin polarization. With increasing the temperature number of active qubits participating in the charge and spin transport increases. These features of tunneling helical ABI open a wide avenue for applications in the area of quantum computing.
Strong magnetic impurity blocks transmission of one component of the electron spin. For open setup this leads to 100 % polarization. Polarization reverses sign, when strong impurity is moved from upper to lower shoulder.
Niyazov R.A., Aristov D.N., Kachorovskii V.Yu.
In condensed matter the states with negative temperature have been experimentally studied in detail, and even the magnetic phase transitions occurring at negative temperature have been detected. The equilibrium thermodynamics at negative temperature is, however, not possible, because the environment has positive temperature. The heat will be transferred from the negative temperature system to the environment, and the whole system will relax to the conventional state with positive temperature. The negative temperature states are possible for the quantum vacuum in the relativistic quantum field theories. The Universe with negative temperature is obtained using the Dirac picture of the quantum vacuum. The conventional Dirac vacuum represents an infinite sea of particles with negative energy (left figure). In the vacuum on the right figure all the positive energy states are occupied and the negative energy states are empty. This vacuum with inverse population can be obtained by the PT symmetry operation, where P and T are space and time reversal transformations correspondingly. Due to the symmetry between the vacua the inverse vacuum has exactly the same physics as the vacuum on the left. If it fills the whole Universe, this vacuum becomes thermodynamically stable. The matter in this mirror Universe has negative energy, and thermodynamic states are characterized by negative temperature. However, inhabitants of the mirror Universe would think that they live in the normal Universe with positive energies for matter and positive temperature. With respect to our Universe their temperature and energies are negative. But with respect to their Universe it is our Universe, which looks strange.
G.E. Volovik Topological insulators form a class of materials for which surface electronic states with the Dirac dispersion relation (and, consequently, zero effective mass) necessarily appear due to specifics of the bulk energy band structure. Mercury cadmium telluride solid solutions Hg_{1x}Cd_{x}Te exhibit a transition from the topological phase at x < 0.16 to the trivial one at x > 0.16. Previously, we have observed unusual PTsymmetric terahertz photoconductivity in heterostructures based on thick Hg_{1x}Cd_{x}Te films being in the topological phase [1]. The films were grown on a GaAs substrate via several intermediate buffers and a graded gap Hg_{1y}Cd_{y}Te layer for which the cadmium telluride content y gradually decreases and crosses the critical y = 0.16 value (see the inset in Fig.1). The photoconductivity was excited by short ~ 100 ns terahertz laser pulses in magnetic field directed normally to the sample surface. The photoconductivity amplitude turned out to be not an even function of the magnetic field applied which is equivalent to the T (time reversal) – symmetry breaking. It is also different for two mirror symmetric pairs of potential leads of a Hall bar which corresponds to the P (parity) – symmetry breaking. At the same time, changing both factors simultaneously keeps the photoconductivity amplitude intact (PTsymmetry) (Fig.1). It should be stressed that the equilibrium characteristics of the structures, such as magnetoresistance, are both P – and T – symmetric, so breaking of these symmetries is observed only in nonequilibrium situation. Later on, it was demonstrated that appearance of the PTsymmetric photoconductivity comes up as a result of superposition of the conventional photoconductivity and the unusual chiral nonlocal photoconductivity [2]. The latter one corresponds to appearance of chiral photocurrents flowing along the sample edge around it. The photocurrent direction, i.e., its chirality, changes to the opposite one every time the magnetic field of the electric bias applied is reversed. The chiral photocurrent is absent if the electric bias or the magnetic field is zero. The nonlocality clearly demonstrates that the chiral photocurrents responsible for appearance of the PTsymmetric photoconductivity flow in the interface area between the trivial buffer layer and the topological film. In this paper we show that though the PTsymmetric photoconductivity reveals itself at the interface, the source of nonequilibrium electrons providing the effect is the bulk of a film. When the active layer thickness decreases, the PTsymmetric photoconductivity drops, and it is not observed in films thinner than 1 mm anymore (see the right panel of the Fig.1). Apparently, the photoexcited electrons diffuse from the bulk to the interface area, where they provide appearance of the effect.
Observation of the PTsymmetric photoconductivity does not require too sophisticated equipment. A question arises, why it was not observed previously. The results of this paper give an answer. Two conditions for the observation are necessary: existence of an interface between the topological and the trivial phase and an active layer of not less than 1 mm thickness. Hg_{1x}Cd_{x}Te single crystals widely studied back in 19601990s, possessed no interface with the trivial phase material. Later on, with advent of 2D heterostructures, the experimental attention
Fig.1. Right panel – magnetic field dependence of the photoconductivity amplitude for two mirrorlike pairs of potential leads 12 and 34. The inset shows the experiment electric circuit and geometry. Left panel – dependence of the photoconductivity amplitude asymmetry on the active layer thickness. The inset shows the heterostructure composition.
[1] Scientific Reports, 10, 2377 (2020). DOI: 10.1038/s41598020592800
A.S.Kazakov, A.V.Galeeva, A.V.Ikonnikov et al. Selfassembled Ge quantum dots epitaxially grown on Si are of particular interest as they are fully compatible with SiCMOS and can be applied for 1.3– 1.55 µm optical communication applications. Despite the recent progress in fabrication of nearinfrared Ge/Si quantum dot photodetectors, their quantum efficiency still remains a major challenge and different approaches to improve the quantum dot photoresponse are under investigation. It was recently demonstrated that the integration of Ge/Si heterostructures with arrays of metal nanoparticles on the semiconductor surface leads to a significant increase in the nearinfrared photocurrent. The results were explained by the excitation of surface localized plasmon modes by the light wave. A drawback of this approach is the large ohmic losses in the metal and the small penetration depth of the plasmon field into the semiconductor. In this letter, we have implemented an alternative approach based on the concept of photonic crystals. At present, the effects of the interaction of optical transitions with modes of various microcavities, including radiation modes of photonic crystals, are actively used to enhance luminescence signals in structures with a low efficiency of radiative recombination, including laser and LED structures. The idea of the approach proposed in this work is to use photonic crystals in processes opposite to emission: optical absorption in thin layers of quantum dots embedded in photonic crystals. We found that the incorporation of Ge/Si quantum dot layers into a twodimensional photonic crystal leads to multiple (up to 5 times) enhancement of the photocurrent in the near infrared range. The photonic crystal was a regular triangular lattice of air holes in a Si/Ge/Si heterostructure grown on a silicononinsulator substrate. The results are explained by the excitation of planar photonic crystal modes by the incident light wave propagating along the Ge/Si layers and effectively interacting with interband transitions in quantum dots.
(a) Image of a fragment of the profile of the band diagram of the Ge/Si heterostructure with Ge quantum dots and possible interband electronic transitions leading to the exitation of a photocurrent in the near infrared range. (b) Schematic section of a planar photodetector with Ge quantum dots in a Si matrix on a silicononinsulator substrate embedded in a photonic crystal. (c)  Schematic image of a photodetector representing a twodimensional photonic crystal in the form of a periodic lattice of subwavelength air holes in Si/Ge/Si layers. (d, e)  Images of a fragment (d) of the surface and (e) of the crosssection of a triangular lattice of circular holes in the Si/Ge /Si heterostructure, obtained in an electron microscope.
A.I. Yakimov et al. Topological materials with the Berry phase monopoles in the spectrum of Weyl fermions provide the possibility to study quantum anomalies, such as the AdlerBellJackiw chiral anomaly and the gravitational anomaly. The analogue of the gravitational anomaly is produced by the effective gravitational fields acting on Weyl fermions: tetrads, spin connection and torsion fields. We show that the electromagnetic field in chiral Weyl superconductors plays the role of spin connection in the effective tetrad gravity. As distinct from the conventional chiral anomaly, the gravitational anomaly in chiral superconductors leads to the AdlerBellJackiw equation with the extra factor 1/3. In neutral chiral superfluids with Weyl fermions, such as superfluid 3HeA, the gravitational anomaly is produced by the analogue of the gravitational instanton. The latter is the process of creation or annihilation of the 3D topological objects, hopfions. The creation of hopfions is accompanied by the anomalous creation of the chiral charge. This is the gravitational analogue of the KuzminRubakovShaposhnikov electroweak baryogenesis.
G.E. Volovik
Multi – fermion systems appear in solid state physics and in the description of fermionic superfluids. Such systems are also used as the building blocks for the construction of certain Unified theories in high energy physics. The general (though, rare) property of multi – fermion systems is the appearance of the two – component Weyl spinors at low energies. These spinors are formed in equilibrium systems close to the Fermi points, which are the band level crossing points in momentum space. Typically the Fermi points are unstable and exist only if protected by topology. Therefore, the effective description in terms of the two  component spinors survives in the case when the topological invariants protecting the Fermi points are nonzero. Previously it was generally believed that there is a single topological invariant N_{3} responsible for the stability of the Fermi points. It may be expressed as an integral of an expression composed of the two – point Green function. The integral is over a closed hypersurface surrounding the Fermi point in four – dimensional momentum space (Brillouin zone and Matsubara frequency axis). This topological invariant takes integer values. Correspondingly, these values give rise to the classification of the Fermi points. The Fermi points with nonminimal values of N_{3} may split due to perturbations into those with minimal values. Weyl fermions existing close to the Fermi points with N_{3} = +1 are called the left – handed, while those close to the Fermi points with N_{3 } =  1 are called the right – handed. In the present paper it is shown that in fact there exist two different topological invariants responsible for the stability of Fermi points. One of them is the mentioned above N_{3}. Another one, N^{(3)}_{3 }is composed of the Green function at vanishing Matsubara frequency. The difference between these two topological invariants was overlooked previously. Correspondingly, the Weyl points are classified according to the values of both N_{3 } and N^{(3)}_{3}. For their minimal values it is proposed to call the Weyl points according to the following table.
The difference between the Weyl points and the anti – Weyl points may be detected if both these types of Fermi points are present. Then, for example, the two left – handed Weyl points may merge giving rise to the marginal Fermi point with (N_{3, }N^{(3)}_{3}) = (2,0). In the lattice systems with discretized space coordinates and continuous (imaginary) time the topological theorem requires that the sum of N^{(3)}_{3 } over the Fermi points is zero (provided that there are no zeros of the Green function in momentum space). In this case there may exist the systems with left – handed Weyl points and left – handed anti – Weyl points without the right – handed Fermi points. If the imaginary time axis is discretized as well as space coordinates, then, in addition, the sum of N_{3 }over the Fermi points has to be equal to zero. In this case the numbers of left – handed and right – handed Fermi points are to be equal. We suppose that the proposed classification may be relevant both for the condensed matter physics, and for the high – energy physics. In the former case the anti – Weyl points may appear in the systems with strong interactions. In the latter case the Weyl points of different types may appear dynamically in quantum gravity as a result of the fluctuations of vierbein.
M.Zubkov
In this Letter, we studied the photoconductivity (PC) spectra in narrowgap epitaxial HgCdTe films at various temperatures by Fouriertransform infrared spectroscopy. It was shown that the subgap features observed in the PC spectra should be associated with transitions to shallow excited states of the mercury vacancy for neutral and singly ionized acceptors, rather than transitions to the valence band continuum.
Some of the excited states have large matrix elements for the transition from the ground state owing to the large fraction of the light holes subband in the structure of wave functions. The different rates of PC lines quenching and the peculiar shape of these lines are naturally explained by photothermal ionization of such states, paving the way to a better understanding of mercury vacancies in HgCdTe.
Kozlov D.V. et al.
JETP Letters 113, issue 6 (2021)
Ferroelectric properties of different chalcogenides are of great interest due to the underlying physics and potential applications. Recently, threedimensional WTe_{2} single crystals were found to demonstrate coexistence of metallic conductivity and ferroelectricity at room temperature. The latter usually belongs to the insulators, but it occurs in WTe_{2} due to the strong anisotropy of the noncentrosymmetric crystal structure. Outofplane spontaneous polarization of ferroelectric domains was found to be bistable, it can be affected by high external electric field. Scattering of the charge carriers on the domain walls is known to provide noticeable contribution to the sample resistance. Thus, coexistence of metallic and ferroelectric properties should produce new physical effects for electron transport, and, therefore, it should be important for nanoelectronic applications. Here, we investigate electron transport along the surface of WTe_{2} threedimensional single crystals. We find that nonlinear behavior of dV/dI(I) differential resistance is accompanied by slow relaxation process, which originates from the additional polarization current in ferroelectric WTe_{2} crystal. The possibility to induce polarization current by sourcedrain field variation is unique for WTe_{2} , since it is a direct consequence of ferroelectricity and metallic conductivity coexistence.
Schematic diagram of the domain wall region, arrows indicate ferroelectric polarization direction. Due to the coexistence of metallic conductivity and ferroelectricity, there are two possible directions of the external electric fields in our setup. Gate field Egate = Vg/d is directed normally to the WTe2 surface, while sourcedrain field Esd is parallel to it, being induced by the flowing current Esd = ρj. The achievable values of the fields are too small to align polarization of the whole WTe2 flake, so they mostly affect the domain wall regions. Thus, any variation of the electric fields leads to the additional polarization current. The latter we observe as slow relaxation in dV /dI, since polarization current is connected with lattice deformation in ferroelectrics.
N.N. Orlova, N.S. Ryshkov, A.V. Timonina, N.N. Kolesnikov and E.V. Deviatov
Photonstimulated transport (PST) has been studied for 60 years, and until recently, all its resonances have been associated with the specific features of the density of states of the structures under study. In quantum point contact (QPC), such resonances are missing due to the smooth saddle potential. However, recently, when studying the microwave and terahertz photoconductance of the QPC in tunneling regime, PST was found in just such potential. It turned out that the tunneling transmission of a onedimensional smooth barrier resonantly depends on the frequency and number of microwave or terahertz photons absorbed by an electron, leading to the appearance of giant microwave and terahertz photoconductance. The developed theory of the PST through such a barrier explains the discovered effect by a sharp increase in the probability of transition of a subbarrier electron to the top of the barrier. It also gives a radical decrease in it to zero when the photon energy transferred to the electron leads to its transition above the barrier, thereby confirming another experimental fact: the absence of a photoeffect when the frequency of terahertz radiation is increased several times.
(a) Micrographs of the Hall bridge on the basis of high mobility 2D electron gas in GaAs quantum well with two QPC options (split gate, bridged gate). (b)  Behavior of the measured (points) and calculated photoconductance (lines) for three different QPCs ((1,2)  bridged gate, (3)  split gate), when the samples are irradiated by terahertz radiation at two indicated frequencies (G_{ph} – photoconductance, G_{dark} – dark conductance, G_{0} =2e^{2}/h).
V.A. Tkachenko, Z.D. Kvon, O.A. Tkachenko, A.S. Yaroshevich, E.E. Rodyakina, D.G. Baksheev, A.V. Latyshev Studies of topological insulators (TI) are currently marked by a growing interest to the origin of strong impact of various defects and local charge inhomogeneities on the fundamental properties of surface current carriers. One of the key ingredients of the progress here consists in the ability to get the reliable information on the TI local properties, since the standard transport measurements provide only nonlocal one. In this letter we propose the contactless visualization of local charge and spin inhomogeneities using electron spin resonance (ESR) of the bulk charge carriers in the insulating region between conducting surfaces of the 3D topological insulators Bi_{1.08}Sn_{0.02}Sb_{0.9}Te_{2}S. The standard ESR technique makes it possible to obtain a signal from the bulk charge carriers with a given gfactor. An analysis of the properties of the observed ESR signal allows one to conclude that the current carriers participating in the resonance are arranged in a random array of electron or hole droplets of nanoscale sizes which are located at large distances from each other. It is essential that electrons and holes from these droplets do not participate in transport, since they cannot travel from one droplet to another. The importance of the above results is due to the fact that such droplets, being in the vicinity of the TI surface, can affect surface current carriers. Surface current carriers can penetrate into these droplets via tunneling and interact inelastically with the current carriers located in them. Then, after some time, they can tunnel back to the surface, which should undoubtedly affect their transport properties and, in particular, lead to nonzero backscattering.
The experimental scheme. A plate sample placed in the magnetic field of the ESR spectrometer is excited by an alternating magnetic field of a given frequency (wavy line). By changing magnetic field, the spin resonance of bulk current carriers (black resonant peak) can be achieved. The analysis of the resonance response shows that the current carriers participating in the resonance are organized into a random set of nanosized hole and electron droplets (grey circles) separated by a large distance.
Sakhin V., Kukovitsky E. , Talanov Yu/ , Teitel’baum G. Selforganized quantum dots (QDs) grown by the epitaxial method are considered as the basis for various applications in quantum photonics due to their unique properties, such as small spectral linewidth, fast radiative decay time, and high quantum efficiency. Among such applications is the generation of single photons with a high degree of indistinguishability, which is necessary for the implementation of linear optical quantum computing schemes. Most modern quantum computing protocols require a sufficiently large number of parallel channels with indistinguishable photons. One of the approaches to their formation is the use of many independent QDs emitting photons identical in all parameters. Another approach is based on the use of only one perfect QD, which emits with a high efficiency a sequence of singlephoton pulses, which are then demultiplexed over N parallel channels. In this letter, we demonstrate the possibility of combining these two approaches by creating highquality singlephoton sources, which in principle allow integration within a single semiconductor chip. For this purpose, structures were fabricated with a selfassembled InAs/GaAs QD placed in a columnar optical microcavity with distributed Bragg reflectors, possessing a relatively low Q factor. The experiment on measuring twophoton interference, performed in the HongOuMandel scheme at various delays between two photons successively emitted under resonant coherent excitation of a single QD, showed the possibility of achieving up to 93% indistinguishability at a 250 ns delay. It is assumed that the use of such microcavity structures with a low Q factor and a sufficiently wide spectral resonance will simplify the precise tuning of the singlephoton generation wavelength, which will make it possible to increase the number of parallel channels in the circuits of optical quantum computers by integrating several independent sources of indistinguishable photons with a degree of indistinguishability sufficient to effectively demultiplex the photon flux emitted by each source.
A histogram measured in the HongOuMandel scheme of twophoton interference with a delay between photons of 250 ns under conditions of resonant coherent excitation by a πpulse of a microcavity with a single InAs/GaAs QD.
Galimov A.I., Rakhlin M.V., Klimko G.V. et al.
Discovery of the Higgs boson in 2012 by ATLAS and CMS experiments finally confirm the truthiness of the Standard Model (SM), but there still remain many open questions. Among them: inability of SM to explain the neutrino oscillation and baryon asymmetry, the problem of the particle mass hierarchy etc. This gave rise to the development of the new theories which extend the SM  Beyond Standard Models (BSM): Two Higgs Doublet Model (2HDM), Minimal Supersymmetric Standard Model (MSSM), Higgs Triplet Model (HTM) etc. These models predict new resonances in the extended Higgs sector, e.g. in 2HDM the electroweak symmetry breaking leads to five Higgs particles: two neutral Higgs bosons that are CPeven (scalar) ℎ, 𝐻, one neutral and CPodd (pseudoscalar) 𝐴, and charged Higgs boson 𝐻^{±}. A search for new particles from the extended Higgs sector were performed in the ATLAS and CMS experiments and covered many decay channels and final states. As a result of these searches the upper limits on the production cross sections or on the masses of new heavy resonances and the constrains on the BSM extensions parameters were obtained. In this paper we review the recent and most significant results on heavy Higgs bosons searches obtained by the ATLAS and CMS experiments and based on the data collected in LHC Run I (20112012) and Run II (20152018) with protonproton interactions at $\surd s$ = 7, 8, 13 TeV.
Excluded regions (light shaded or dashed) of the hMSSM model parameters 𝑚_{𝐴}, 𝑡𝑎𝑛 via direct searches for heavy Higgs bosons and fits to the measured rates of observed Higgs boson production and decays obtained in ATLAS experiment.
Yu.G. Naryshkin
The enhancement of nonlinear Raman interactions paves a way towards implementing onchip Ramanbased technologies, such as Raman amplification and lasing, sensing and superresolution imaging. Specifically, this allows us to reduce the size and pumping power requirements of nonlinear Raman devices. In recent years, the enhanced nonlinearities have been demonstrated using microresonators, waveguides, plasmonic nanostructures and alldielectric antennas. The underlying materials of these structures fall into two groups: dielectrics (positive real permittivity) and metals (negative real permittivity). A disadvantage of dielectric structures is that their size cannot be enough small compared to the wavelength of light. Whereas metallic nanostructures suffer from high ohmic losses.
Incident light (input) is converted into longerwavelength emission (output) through stimulated Raman scattering. The enhancement of the Raman nonlinearities of ENZ media allows to perform a frequency conversion on the nanoscale and suppress a nonlinear threshold
A.P.Gazizov, A.V. Kharitonov, S.S.Kharintsev Gyrometric devices based on new physical principles is a topical and actively investigated area of research. Advances in experimental techniques of creation and control of cold atomic ensembles and, particularly, atomic BoseEinstein condensates (BEC) allow using them for building perspective inertial sensors. The existing proposals for quantum gyrometric devices with cold atoms rely on direct registration of matter waves, which implies destruction of spatial coherence and atom loss. In this Letter, we propose and theoretically investigate a new scheme of quantum gyrometry which does not involve imminent decoherence of the condensate.
Figure 1: A concept of atomoptical quantum gyroscope. The rotation axis is assumed to be orthogonal to the plane of a ring trap.
V.A. Tomilin and L.V. Il'ichev Superconducting spin valve based on superconductor/halfmetal system with record values of the effect has been created. In the last two decades of the 21st century there has been tremendous theoretical and experimental interest in the development of logic elements for superconducting spintronics. In addition to the basic elements for computers of the future, passive elements are also needed that will turn on/off the superconducting current. Such a device can be a superconducting spin valve (SSV). Superconducting spin valve is an alternating sequence of ferromagnetic (F) and superconducting (S) layers. By combining the number and sequence of layers of F and Smaterials, it is possible to control the properties of the spin valve. This is due to the fact that the properties do not change abruptly at the boundary of the S/F layers  there is a region of interpenetration of the properties of two materials. This phenomenon is called S/F proximity effect. In this work, we have studied the superconducting spinvalve effect in F1/F2/S heterostructures containing the Heusler alloy Co_{2}Cr_{x}Fe_{1x}Al_{y} as one of two ferromagnetic (F1 or F2) layers. We used the Heusler alloy layer in two roles: as a weak ferromagnet on the place of the F2 layer and as a halfmetal on the place of the F1 layer. In the first case, the full switching between the normal and superconducting states is realized with the dominant aid of the long range triplet component of the superconducting pair condensate which occurs at the perpendicular mutual orientation of magnetizations. In the second case, we observed separation between the superconducting transitions for perpendicular and parallel configurations of magnetizations reaching 0.5 K. We also find a good agreement between our experimental data and theoretical results. The results obtained in this work are recordbreaking for F1/F2/S structures.
The record value of the magnitude of the superconducting spin valve effect in F1/F2/S structure.
Kamashev A.A. , Garifullin I.A. We have developed a sensitive spectroscopic technique for study of a dilute ultracold plasma (UCP) using a laser induced autoionization of Rydberg atoms. In our experiment the ultracold ^{40}Ca Rydberg atoms and ions are prepared in a magnetooptical trap by several cw lasers. The laser beam diameters are order of 2×10^{3} m. The technique allows to detect the plasma with ion and electron densities below 10^{9} m^{3}. For observation of the autoionization effect we used the twophoton Rydberg transition 4s3d ^{1}D_{2} – 90 ^{1}D_{2} (with lasers 672 nm and 798 nm) and the ionization twophoton channel with lasers 423 nm and 390 nm. The autoionization resonance is observed as a variation of the resonance fluorescence of the ^{40}Ca ions at a wavelength of 397 nm. The dependence of the autoionization resonance magnitude on the ion density is recorded. The ability to create an UCP with wellcontrolled parameters allows us to calibrate of the autoionization resonances. The technique can be applied to detect small electric fields by means of ^{40}Ca Rydberg atoms. The developed technique can be useful for the measurements of the small fields in development of the ultraprecise atomic clock, as well as for experimental simulations of the ultracold lowdensity plasma in the Earth's ionosphere.
Dependence of the resonance amplitude at the 4s3d ^{1}D_{2} – 90 ^{1}D_{2} Rydberg transition on the power P_{390} of the ionizing laser (λ = 390 nm) and the ion density in the UCP. The peak density of the neutral atoms is $n_a = 10^{15}$m^{3}.
B.B. Zelener, E.V. Vilshanskaya, S.A. Saakyan, V.A. Sautenkov, B.V. Zelener, V.E. Fortov
In the past few decades, the intensive development of angleresolved photoemission spectroscopy (ARPES) made it possible to experimentally observe the electronic band structure for various classes of materials with a high instrumental resolution and in a wide binding energy range. The corresponding ARPES data
(a) LDA + DMFT spectral function for NaFeAs in the MGM direction, (b) taking into account the experimental details, (c) ARPES. Fermi level  zero energy (white dotted line).
I.A. Nekrasov, N.S. Pavlov
A review is given of unusual manyparticle effects discovered in strongly interacting twodimensional electronic systems in quantizing magnetic fields in MgZnO/ZnO heterostructures. The studied twodimensional systems have unique properties  strong Coulomb interaction, characterized by the high values
A.B. Vankov and I.V. Kukushkin Seven years ago, IceCube neutrino telescope has discovered neutrinos of Petaelectronvolt energies coming from yet unidentified astronomical sources. Active Galactic Nuclei (AGN) powered by supermassive black holes ejecting relativistic jets are considered as possible source of the IceCube astrophysical neutrino signal. Direct verification of this hypothesis is however difficult because of the low statistics of the neutrino signal and moderate angular resolution of the IceCube telescope.
Interactions of highenergy protons and atomic nuclei that result in production of astrophysical neutrinos in AGN inevitably produce also gammarays, electrons and positrons that initiate electromagnetic cascade releasing its energy into Gigaelectronvolt (GeV) to Teraelectronvolt (TeV) range. Thus, it is natural to expect that the sources of astrophysical neutrinos have GeVTeV gammaray counterparts. However, contrary to expectations, arrival directions of astrophysical neutrinos detected by IceCube do not correlate with positions of brightest gammaray emitting AGN detected by Fermi LAT gammaray telescope. At the same time, surprisingly, recent analysis of correlation between neutrino arrival directions and positions of AGN brightest in the radio band by Plavin et al. (2020) has revealed significant correlation.
The interfaces between superconductors (S) and ferromagnets (F) are known to be the origin of rich physics associated with the proximity effect. The exchange field inside the ferromagnets converts the spinsinglet Cooper pairs into the spintriplet ones. Such unusual spin structure of superconducting correlations is responsible for the spatial oscillations of the Cooper pair wave function and a great variety of resulting interference phenomena. Recently, it has become clear that the proximity effect also drastically modifies the electrodynamics of S/F structures. As an example, spintriplet pairs can damp the usual diamagnetic Meissner response down to zero, and its vanishing was shown to be the hallmark for the emergence of the peculiar FuldeFerrellLarkinOvchinnikov (FFLO) phase with the superconducting order parameter modulated in the plane of the layers [1, 2]. Another electromagnetic consequence of the proximity effect is the anomalous longrange transfer of the magnetic field from the ferromagnet to the superconductor even in the case when the F layer does not produce a stray magnetic field [3, 4]. This socalled electromagnetic proximity effect originates from the generation of the superconducting currents inside the F layer due to the direct proximity effect and the subsequent appearance of the compensating Meissner currents flowing in the S layer. In this paper we review the recent results related to the physics of the inplane FFLO states and electromagnetic proximity effect in S/F hybrids. Also we analyze the interplay between these two phenomena revealing through the boosting of the spontaneous magnetic field generated in the S layer due to the electromagnetic proximity effect in the vicinity of the phase transition from the uniform superconducting state to the inplane FFLO phase.
Leakage of the magnetic field from the ferromagnet to the superconductor due to the electromagnetic proximity effect and qualitative plot illustrating the increase in the amplitude of the spontaneous magnetic field when approaching the transition to the FFLO phase (with the decrease of temperature).
[1] S. Mironov, A. Mel’nikov, A. Buzdin, Phys. Rev. Lett. 109, 237002 (2012)
S. V. Mironov, A. V. Samokhvalov, A. Buzdin, A. S. Mel’nikov Halfmetals are rather unusual and promising materials. The Fermi surface of a halfmetal is completely spinpolarized. Namely, electronic states with only one spin projection value reach the Fermi energy. States with the other spin projection are pushed away from the Fermi level. This makes halfmetals useful for spintronics. Typically, halfmetallicity arises in strongly correlated electron systems, or when localized magnetic moments are present. We demonstrated that doping a densitywave insulator even in the weakcoupling limit may stabilize new types of halfmetallic states, such as spinvalley halfmetal and chargedensity wave (CDW) halfmetal. In a simple model Hamiltonian describing two Fermi surface pockets (or valleys) with nesting, the electronelectron repulsion generates spin or chargedensity wave state, see Fig.1(a). If charge is added or removed from such a system, the situation becomes less clearcut: several states with close energies are competing. Such possibilities as incommensurate density wave, electronic phase separation, stripes, etc. are discussed in the theoretical literature. We demonstrate that yet another type of manybody state is available. In the doped system, the twovalley Fermi surface emerges. One valley is electronlike. It is composed mostly of states of electron band, with spin σ. Another valley is holelike, composed predominantly of states from hole band, with spin σ'. These Fermi surface valleys have halfmetallic character: the states in electron band with spin σ, as well as states in hole band with spin –σ', do not reach the Fermi level and have no Fermi surface. Depending on the parameters, the spin polarizations of the electronlike valley and holelike valley may be parallel (σ = σ') or antiparallel (σ = σ'), see Fig.1(b,c). The former case is similar to the usual halfmetal: quasiparticles at the Fermi surface are completely spinpolarized. In addition, the system exhibits a finite CDW order parameter. For this reason, we refer to such a state as the CDW halfmetal. When σ = σ', the total spin polarization averages to zero. It is proven, however, that in this situation, the socalled spinvalley polarization is nonzero. Thus, the state is called the spinvalley halfmetal. The specific features of these halfmetallic states are discussed. Namely, we demonstrate that the electric current can be accompanied by the transfer of spin or of the spinvalley quantum number. Such effects could be of interest for spintronics and pave the way to spinvalleytronics. We also discussed the possibility of using the inelastic neutron scattering to detect the halfmetallic states.
Band structure of (a) undoped density wave, (b) spinvalley and (c) charge density wave halfmetals. Horizontal line shows the Fermi level, arrows indicate spin polarizations of the Fermi surface.
A.V. Rozhkov, A.O. Sboychakov, D.A Khokhlov and A.L. Rakhmanov and A.D. Kudakov The magnonic Bose condensed state was first discovered in superfluid ^{3}He under magnetic resonance conditions. The repulsive interaction between magnons stabilizes this state. The transfer of magnetization by a magnon supercurrent was also discovered [1]. Quite similar phenomena were observed in a nonplanar magnetized film of yttrium iron garnet (YIG), but at room temperature. When the deviation of the magnetization in YIG is more than 3 degrees, the density of nonequilibrium magnons exceeds the critical one [2], and a magnon Bose condensate is formed. Due to the superfluidity of magnons, the BEC state can fill the entire sample and the angles of magnetization deviation exceed 20 degrees [3]. Magnon BEC was studied in a YIG film epitaxially grown on a gadolinium gallium garnet (GGG) plate 0.5 mm thick. The Gilbert attenuation determines the field shift of the BEC observation. It has been found to be highly frequency dependent. It increases significantly when the frequency matches the standing sound waves in the GGG (peaks A in Fig. 1). The magnetoelastic interaction excites phonons, which dissipate energy. Unexpectedly, we also detected antiresonant signals (dips B in Fig. 1). We can explain this by the coherent mediation of circularly polarized phonons, which return their angular momentum after being reflected from the other side of the GGG plate. This observation shows the coherent transfer of the angular momentum of phonons through nonmagnetic material on a macroscopic distance.
[1] G. E. Volovik, J. Low Temp. Phys., 153, 266 (2008) [2] Yu.M.Bunkov, V. L. Safonov, J. Mag. Mag. Mat., 452 30–34 (2018) [3] P. M. Vetoshko, G. A. Knyazev, A. N. Kuzmichev, A. A. Cholin, V. I. Belotelov, Yu. M. Bunkov, JETP Letters 112, 313 (2020).
P. M. Vetoshko, G. A. Knyazev, A. N. Kuzmichev, V. I. Belotelov, Yu. M. Bunkov
The ability to explain when and why an isolated quantum mechanical system can be accurately described with equilibrium statistical mechanics is one of the key challenges in modern statistical physics. Such description may be possible even for timedependent Hamiltonians, and much attention has focused on the emergence of quasiequilibrium states in manyparticle periodically driven systems. Numerous approximate methods have been developed to describe dynamics of such systems, known as Floquet dynamics. Interesting results were previously obtained when the external driving frequency significantly exceeds the strength of the interaction in the system in frequency units (the averaging condition). NMR in solids was one of the first areas where experimental and theoretical investigations of dynamics and thermodynamics in periodically driven systems were performed. The powerful experimental technique of NMR and relatively simple analytic tools allowed the creation of “spin alchemy” with very interesting results. In this letter we work out a numerical method to investigate Floquet dynamics in the simplest multipulse NMR experiment in a system of 14 spins connected by dipoledipole interactions. We discover that a quasithermodynamic equilibrium is established under the averaging condition. When this condition is not met, instead of a quasiequilibrium state, we find that the polarization decays to zero.
The decay of the polarization in multipulse NMR spinlocking with π/8 RF pulses. The initial polarization equals 1. The horizontal line is the thermodynamic equilibrium polarization.The number of the spins is 14. The averaging condition is satisfied.
G.A.Bochkin, S.G.Vasil’ev, A.V.Fedorova, E.B.Fel’dman The problem of searching new highenergydensity materials (HEDM) is very actual from both applied and fundamental points of view. Choosing nitrogen as a promising element for creating HEDMs have several reasons. Under normal conditions, nitrogen exists in the form of diatomic N_{2} molecules with a triple covalent bond, which is one of the strongest covalent bonds in nature, its energy is 4.9 eV/atom. The energies of double and single bonds for nitrogen are 2.17 eV/atom and 0.83 eV/atom, respectively. Those for nitrogen the sum of three single bonds energies is much less than energy of triple bond; therefore, singlebonded nitrogen crystal structures will store energy. At the same time, the release of energy is an environmentally friendly process. In this article, the existence of a metastable, singlebonded crystalline nitrogen phase with symmetry P62c is predicted theoretically. This phase is a directgap semiconductor and can store the largest amount of energy among all nitrogen crystals predicted to date, which are stable at low pressures. This structure of nonmolecular nitrogen has all the necessary attributes of dynamic (in terms of the phonon spectrum) and mechanical (in terms of elastic moduli) stability of a bulk medium at pressures less than 40 GPa, including zero pressure. In the entire pressure stability range phase P62c is metastable. For its synthesis, it is necessary to search new methods, for example, synthesis through excited states.
K.S.Grishakov and N.N.Degtyarenko JETP Letters 112, issue 10 (2020) After the discovery of Mott insulating states and superconductivity in the socalled magic angle twisted bilayer graphene in 2018, the study of this material became a hot topic in condensed matter physics. In singleparticle approximation, the system under study has four almost flat almost degenerate bands near the Fermi level. The electronelectron interaction lifts this degeneracy stabilizing some order parameter in the system. The mottness of the ground state of the magic angle twisted bilayer graphene manifests itself in the sequence of conductivity minima observed for several doping levels. The nature of the ground state of the magic angle twisted bilayer graphene is not yet known. Here, we assume that the emerging nonsuperconducting order parameter is a spin density wave, and study the evolution of such ordered state with doping. We show that in the range of electron densities, where the order parameter is nonzero, the homogeneous state of the system can be unstable with respect to the phase separation. Phases in the inhomogeneous state are characterized by an even number (n = 0, ±2, ±4) of electrons per a superlattice cell. This allows us to explain some features in the behavior of the conductivity of the system with doping. Thus, we are able to explain the fact that the conductivity minima, that could occur at doping levels corresponding to an odd number (n = ±1, ±3) of electrons per supercell, are absent in some samples under study (phase separation occurs) and are present in other samples (phase separation is suppressed by the longrange Coulomb repulsion).
Free energy of the system as a function of doping. The solid (red) curve corresponds to the free energy of the homogeneous state. The energies of the inhomogeneous states obtained by the Maxwell construction are shown by dashed (green) lines
A.O. Sboychakov, A.V. Rozhkov, K.I. Kugel, and A.L. Rakhmanov Recently emerged new field of alldielectric resonant metaphotonics (also called “Mietronics” aims at the manipulation of strong opticallyinduced electric and magnetic Mietype resonances in dielectric nanostructures with high refractive index. Unique advantages of dielectric resonant nanostructures over their metallic counterparts are low dissipative losses combined with strong enhancement of both electric and magnetic fields, thus providing competitive alternatives for plasmonics including optical nanoantennas, nanolasers, biosensors, and metasurfaces. Importantly, highindex dielectric nanoparticles supporting multipolar Mie resonances are building blocks of advanced metamaterials. By combing both electric and magnetic multipolar modes, one can modify farfield radiation patterns and also localize the electromagnetic energy in open resonators by employing the physics of bound states in the continuum. Changing the resonator parameters or combining the resonators into a planar geometry of metasurfaces allow achieving much higher values of the Q factor. This minireview highlights some recent advances in the field of alldielectric Mieresonant metaphotonics driven by the development of highQ dielectric structures for nonlinear nanophotonics, nanoscale lasing, and efficient sensing applications.
Example of 310 nm nanolaser based on lead halide perovskite CsPbBr_{3} nanocuboid and operating at room temperature. Multipole decomposition of the lasing mode demonstrates the dominant contribution of the thirdorder magnetic dipolar Mie mode.
P.Tonkaev, Y.Kivshar The theoretical prediction of the early seventies about the existence in a solid of a new state of "quantum spin liquids" is now finding real experimental confirmation. "Spinliquid" compounds have a specific frustrated lattice consisting of triangles, at the vertices of which there are magnetic atoms that do not allow establishing longrange order. Due to quantum fluctuations and strong correlations between spins, frustrated magnets remain disordered even near absolute zero. This work presents results of an experimental study of the electronic system of a highly frustrated quasitwodimensional organic metal κ (ET) 2Hg (SCN) 2Cl by the Shubnikovde Haas quantum oscillation method. At temperatures above 30 K, this compound behaves like a metal with a halffilled band with strong electronelectron correlations. In the region of T = 30 K, a Mott metalinsulator transition is observed in the compound, and at low temperatures the system passes into the state of a quantum spin liquid (N.M. Hassan, and all, npj Quantum Materials 5, 15, 2020). Organic conductors are fairly soft materials, and application of pressure can significantly change the conduction band and affect their physical properties. The application of a hydrostatic pressure of 0.7 kbar suppresses the metalinsulator transition and restores the metallic state of κ (ET) 2Hg (SCN) 2Cl. This enables studying the behavior of the interlayer magnetoresistance at helium temperatures. The field dependence of the magnetoresistance shows an unlimited growth according to a power law, which is a rare phenomenon for organic conductors and may indicate the presence of the polaron mechanism in interlayer transport. The spectrum of the detected oscillations of the magnetoresistance facilitates better understanding of the shape and dimensions of the Fermi surface and to estimate the parameters of the electron system.
Field dependence of the interlayer longitudinal magnetoresistance in κ (ET) 2Hg (SCN) 2Cl at T = 0.47 K and p = 0.7 kbar. Inset 1: Fourier spectrum of magnetoresistance oscillations. Inset 2: schematic representation of the Fermi surface.
R.B. Lyubovskii, S.I. Pesotskii, V.N. Zverev, E.I. Zhilyaeva, S.A. Torunova, R.N. Lyubovskaya With the recent progress in observing new “locally incompressible” fractional quantum Hall states (FQHE), at the forefront of physics of twodimensional systems (2DES's), there arises a necessity to develop experimental approaches for the direct monitoring of bulk FQHE states. Since the transport characteristics of the FQHE insulators are not very informative (only the edge channels spatially separated from the bulk states contribute to conductivity), we employ optical techniques that can provide the required information. One of the confirmed experimental techniques for studying bulk electronic states in the QHE and FQHE regimes is the resonant reflection. However, the resonant reflection technique, because of its high complexity, is not suitable for routine studies of FQHE states. Application of the nonresonant reflection for the same purpose is impossible for an uncontrolled photoinduced contribution to the experimental results. Up to now, all attempts to employ the photoluminescence technique for analysis of the FQHE states have not lead to reasonable results, despite the fact that in the QHE regime, nonresonant photoluminescence is one of the most powerful tools for studying bulk states. The reason for the incorrect use of this experimental technique became obvious only recently. In the nonresonant photoluminescence, the contribution to the signal is produced not only by twoparticle excited states of 2DES, for which the conditions of “hidden symmetry” are satisfied but also by threeparticle states, for which there are no symmetry restrictions on the spectral characteristics of the photoluminescence signal. The photoluminescence signal of threeparticle complexes in the FQHE regime can have a complex structure with several spectral components due to the nontrivial dispersion of twoparticle complexes (magnetoexcitons), from which threeparticle complexes are constructed. In the presented work, we employ the resonant photoluminescence for studying FQHE state 1/3, with which we have got rid of unwanted photoluminescence of threeparticle complexes. In this case, no violation of the “hidden symmetry” is observed, however, the amplitude of the resonant photoluminescence signal from the FQHE 1/3 state modifies so dramatically, that this modification can serve as an experimental marker of the 1/3 state. On the other hand, such a change in the amplitude of the resonant photoluminescence response indicates the formation of a nonequilibrium coherent spinexcitation ensemble in 2DES, which is believed to consist of the quasiparticles with fractional charges.
L.V. Kulik et al.
Titanium dioxide (TiO_{2}) is actively used in the modern world: as an E171 additive in the food industry, in the fabrication of paints and varnishes, solar panels, gas sensors, etc.
a) Luminescence decay for socalled green luminescence band at 2.3 eV, measured for a micropowder (grey curve), and its approximation by a powerlaw t^{x} dependence with x = 0.8 (red dashed line). b) Luminescence signal decay of the same band, measured for the toxic nanopowder (grey curve), and approximation of its fragments by power dependences with x = 0.5 (yellow dashed line) and x = 1.44 (red dashed line).
V.S. Krivobok et al.
The planar phase of superfluid 3He has two Dirac points in the quasiparticle spectrum – the Berry phase monopoles. The quasiparticles with fixed spin behave as Weyl fermions. While in the chiral superfluid $^3He$A the spinup and spindown fermions
G.E. Volovik JETP Letters 112, issue 9 (2020)
The question of the influence of potential disorder on superconductivity has a rich research history dating back to the celebrated Anderson theorem about the insensitivity of the superconducting critical temperature to the disorder strength. However, a large body of empirical evidence indicates that the transition temperature is typically suppressed with disorder, which is in particular prominent for superconducting films of a mesoscopic thickness. This effect is conventionally attributed to disorderrelated enhancement of Coulomb repulsion, which provides a negative contribution to the Cooper coupling, thus suppressing superconductivity. Quantitative study of this effect in the assumption of a twodimensional diffusive nature of electron motion was done in 1980ies by a number of authors. The firstorder correction was later generalized by Finkel'stein, who derived a nonperturbative expression for the critical temperature degradation as a function of the sheet resistance of the film. The latter has become a widespread tool for fitting experimental data. In this work, based on the theoretical treatment accompanied by the analysis of experimental data, it is argued that for the substantial fraction of superconducting films the main contribution to the critical temperature suppression stems from the region of threedimensional ballistics rather than twodimensional diffusion. The ballistic effects are governed by the parameter $k_F l$ (where $k_F$ is Fermi momentum and $l$ is the mean free path), which is a measure of the proximity to the threedimensional Anderson localization.
Suppression of the critical temperature is given by the integral over the momentum $q$ carried by the electronelectron interaction. The figure is a sketch of the corresponding integrand. The integral is logarithmic in the region of twodimensional diffusion, $q < 1/d$ ($d$ is the film thickness). It linearly diverges in the threedimensional region $q>1/d$, extending from the diffusion to the ballistic region with a different numerical coefficient. Therefore the main contribution comes from the upper cutoff at $q \sim k_F$.
Antonenko D.S., Skvotsov M.A. Balancing an inverted pendulum subject to a given timedependent horizontal force is a famous mathematical challenge known as the Whitney problem. For any initial and final position of the pendulum in the upper halfplane, there exists a trajectory that remains in the upper halfplane at the entire time interval. Remarkably, a nonfalling solution to the Whitney problem is unique. Assuming that the horizontal force is a random process, a formal mathematical problem of the existence of a nonfalling trajectory gets translated into the context of stochastic dynamics, with the main goal of describing statistical properties of such a nonfalling trajectory. Quite unexpectedly, the latter formulation has many notable connections with other mathematical physics problems: control theory, Burgers turbulence, theory of minimizers, rear events in stochastic differential equations, disordered superconductivity, etc. A new analytical method for describing statistics of the neverfalling trajectory on an infinite time interval has been recently developed by the authors of this Letter, in the framework of a supersymmetric fieldtheoretical approach to stochastic dynamics. In this Letter, the technique is generalized to finite time intervals and differenttime correlation functions on the nonfalling trajectory. In particular, it allows determining the Lyapunov exponent, which governs decay of memory correlations on the nonfalling trajectory.
Examples of nonfalling trajectories for the pendulum equation of motion obtained for two time intervals and the same horizontal force (a), (b). Shown are 25 such trajectories with five initial and five final positions in the upper halfplane. The memory of the boundary is lost exponentially with the rate determined by the Lyapunov exponent. (c) An inverted pendulum under the action of a horizontal force.
Stepanov N.A., Skvotsov M.A. The Standard Model unequivocally predicts parity violation in high energy hadronic interactions of polarized hadrons. However, the experimental confirmation of this prediction is still elusive. One of the possible observables is the parity violating singlespin asymmetry in scattering of the longitudinally polarized protons and deuterons. High intensity polarized beams will be available at NICA facility under construction at JINR, Dubna. The reported estimates of asymmetries in polarized protondeuteron scattering are an extension of systematic analysis [1,2] of possibilities of experiments at NICA. Experimental observation of asymmetries in the total cross sections, expected to be well below 10^{7 }, is extremely challenging, and it is suggested to take advantage of substantial enhancement of asymmetry in elastic scattering. In the case of polarized deuterons, similar enhancement is shown to persist in the deuteron dissociation channel.
[1] I.A. Koop, A.I. Milstein, N.N. Nikolaev, A.S. Popov, S.G. Salnikov, P.Yu. Shatunov, Yu.M. Shatunov, Strategies for Probing PParity Violation in Nuclear Collisions at the NICA A.I. Milshtein, N.N. Nikolaev, S.G. Salnikov, Parity Violation in Proton–Proton Scattering at High Energies JETP Letters 112, issue 6 (2020) Bilayer graphene nanoribbons (BGNR) are quasi one dimensional materials which have a wide variety of properties depending on their width, geometry of edges, defects and external influences, such as mechanical deformations or electric and magnetic fields. Combination of nanoribbons with various properties can open wide prospects of their use as two dimensional electronic devices. This work aims to investigate electronic transport in BGNR with a pore by means of the wave packet dynamics method. Wave packet (WP) is injected from metallic electrode to the BGNR and interacts with atomic structure of nanoribbon and with nonopore. The results of these calculations are the time dependent wave functions. Two types of system were considered where the electrode is connected with: (i) both layers, and (ii) with only one layer. Time dependent currents through the BGNR crosssections were obtained, both ahead of and behind the hole. It was shown that the presence of nanopore is important for the WP propagation: it complicates the pattern of WP spreading and leads to localized states formation on the pore (Fig.1). For type (ii) connection to electrode, the nanopore plays a role of the signal separator. Currents flow after passing the nanopore are significantly different in each layer. The propagation of the wave packet is influenced by many parameters of the nanoribbon, such as its width, hole geometry, defects, type of connection to electrode etc. This study may be the first prerequisite for potential use of such objects as elements of electronic circuits
Figure 1. Wave packet probability density in the bilayer graphene nanoribbon with a hole, in the layer connected to the electrode (left), and in the layer unconnected (right) at t = 4.2 fs
V.A. Demin, D.G. Kvashnin, P. Vancso, G. Mark, L.A. Chernozatonskii
The process of spontaneous parametric downconversion (SPDC) is a significant source of biphotons. Biphoton is a pair of quantum – correlated photons. Due to the high degree of correlation, biphotons are used in many areas such as quantum processing, quantum tomography, spectroscopy, etc. Recently, the generation of optical  terahertz biphotons under strongly frequencynondegenerate parametric downconversion has attracted more attention.
The experimental setup for the generation a terahertz  optical biphotons and idler radiation detecting terahertz frequency power at the PR in two modes at the pump wavelength $\lambda_p$ = 1046.7nm and $\lambda_p$ = 523.35 nm
V.D. Sultanov, K.A. Kuznetsov, A.A. Leontyev and G.Kh. Kitaeva
In 1984, a new type of superfluidity was discovered at the Kapitza Institute for Physical Problems  spin superfluidity [1]. In this effect, magnetization is carried on a long distance by the superfluid current of magnons  elementary quasiparticles of magnetization in the Bose condensed state. This phenomenon was discovered in superfluid ^{3}He at temperatures below 2mK. Magnetic analogs of all superfluid effects such as the Josephson effect, quantum vortices, second sound, critical speed, etc. were experimentally demonstrated. However, the application of these effects was difficult because of the very low temperature of superfluid ^{3}He. The basis of spin superfluidity is the Bose condensation of magnons and its stability upon the spatial superflow. The latter is provided by the repulsion interaction between magnons. A similar repulsive interaction between magnons also exists in yttrium iron garnet (YIG) films, magnetized perpendicular to the plane. In this letter, we have demonstrated a spatial magnon supercurrent similar to that observed in ^{3}He. Remarkably, the effect was found at room temperature!
The YIG film was placed in a magnetic field gradient. Magnetic resonance was excited in the region of strip 1 when the pumping frequency corresponded to the perpendicular field (B). With a decrease in the field, a magnon BEC was formed, which filled the entire space with a field less than that corresponding to the resonance field. In field (A), the magnon BEC reached the region of strip 2 and induced in it a radiation signal. Thus, it was shown that the magnon supercurrent transports the precessing magnetization from region B to region A. [1] G. E. Volovik, J. Low Temp. Phys., 153, 266 (2008) .
P. M. Vetoshko, G. A. Knyazev, A. N. Kuzmichev, A. A. Cholin, V. I. Belotelov, Yu. M. Bunkov JETP Letters 112, issue 5 (2020).
The development of the ultrafast magnetooptics during the last 15 years results in a new fundamental knowledge on the ultrafast interaction of light and magnetic materials and also in a very important practically possibility to increase the speed of writing/reading processes in computers by a factor of 10^{5}10^{6}. Several groups in the world have found longliving magnetic oscillations after femtosecond laser pump, for example in FeBO3 [1]. Within a phenomenological approach, this effect has been explained as the inverse Faraday effect. In our paper we demonstrate such oscillations within the microscopic model of the magnetic insulator with two possible multielectron terms at each cation site. The terms have different spin values and form local polarons due to electronphonon interactions with vibrations of local anions. We assume that the femtosecond pumping by the very fast chargetransfer excitations results in a switching of the initial high spin term of cation into the exited low spin term, and the dynamics of the excited state is studied within the master equation for the reduced density matrix. In the figure we demonstrate the dynamics of 3 material parameters: the concentration of high spin terms (blue line), sublattice magnetization (red line), and variation of the cationoxygen bond length (black line). The time scale is given in ps. One can see the generation of vibrons during the relaxation. After 1 ps all parameters reach their equilibrium values typical for the high spin state, and the magnetization demonstrates longliving oscillations.
1. A.M. Kalashnikova, A.V.Kimel, R.V.Pisarev, V.N.Gridnev, A.Kirilyuk, Th. Rasing, Phys. Rev. Lett. 99, 167205 (2007). https://doi.org/10.1103/PhysRevLett.99.167205
Yu.S. Orlov, S.V. Nikolaev, S.G. Ovchinnikov and A.I. Nesterov
HgTe quantum wells proved to be the most interesting and fundamental objects of modern condense matter physics due to their unique property of realization of five kinds of twodimensional (2D) electron systems depending on the well thickness: a 2D insulator with the direct gap, a single valley 2D Weyl semimetal, a 2D topological insulator, a 2D semimetal and a 2D metal. In fact, the indicated property comes from relativistic effects that play a key role in the formation of the HgTe energy spectrum. In our work the results of the experimental study of photo and thermoelectric effects in 2D topological insulators and 2D semimetals are reported. The most deep and important effect predicted in few theoretical papers and found in our studies is the circular photogalvanic effect in the 2D topological insulator. Figure shows the geometry of the experiment. Circularly polarized terahertz radiation illuminates the surface of the HgTebased 2D topological insulator and generates a chiral spin photocurrent along the edge of the quantum well. This photocurrent is generated just due to the topological helical nature of edge states of the 2D topological insulator. Circular irradiation breaks equilibrium between chiral currents of opposite directions and transforms the equilibrium helical state into nonequilibrium chiral one.
Z.D. Kvon, M.L. Savchenko, D.A.Kozlov JETP Letters 112, issue 3 (2020) It was shown recently that such a wellknown quasionedimensional conductor as (TaSe$_4)_2$I is a Weyl semimetal. Do the properties associated with the topological nontriviality of this material survive in the Peierls state when the Weyl points disappear because of the Peierls gap opening? In this letter, we present the results of such an investigation performed on (TaSe$_4)_2$I crystals. Longitudinal magnetoresistance of studied samples in all known modes of chargedensity wave motion (pinned, creeping, sliding and the ''Fröhlich superconductivity'' ) is small, positive and thus reveals no signature of the chiral anomaly. In order to check a possible contribution of charge density wave defects (dislocations, solitons), similar measurements were undertaken in focused ion beam shaped samples. In such samples, chargedensity wave current motion is spatially nonuniform and accompanied by nucleation of numerous chargedensity wave defects. A weak localizationlike nonparabolic longitudinal magnetoresistance is found to appear in relatively small magnetic fields $B\lesssim 4$ T in the nonlinear conduction regime in the temperature range 70120 K, whereas weak antilocalizationlike behavior dominates at lower temperatures in such samples. Possible role of the chargedensity wave defects is analyzed. Our results differ significantly from ones obtained earlier and raise the question concerning conditions for observation of the chiral anomaly in Weyl semimetals in the Peierls state. Image of a focusedion beam proled sample (Wtype sample)
a) Temperature evolution of the longitudinal magnetoresistance in CDW sliding regime. (bc) Evolution of the negative magnetoresistance contribution with temperature (b) and the electric field (c). Wtype sample.
I.A. Cohn, S.G. Zybtsev, A.P. Orlov and S.V. ZaitsevZotov The potential energy of a particle in external field is uniquely expressed through the wave function of the ground state provided it has a discrete energy level. This property, based on the oscillation theorem, allows one to investigate a wide class of model potentials by setting the explicit wave functions of the ground state. Some of these model potentials have physical realizations. For many such realizations the groundstate energy is pinned at zero and does not change with variation of one or several parameters describing the potential. Using the proposed inverseproblem method we study several classes of potentials in one, two or three dimensions: the potentials with a barrier and one discrete energy level, the craterlike potentials with possible application in string theory, the instantontype potentials with two local minima. A vivid manifestation of the effectiveness of the proposed method is its application to the solution of nonlinear Schrodinger equation. We show that the energy of a stationary twosoliton solution of this equation coincides with the energy of onesoliton solution. This means that the decay of a soliton into two solitons happens without the change of energy, the latter is even independent on the distance between the solitons.
The one and twosoliton solutions of the Schrodinger equation (dashed lines) with corresponding potentials (solid lines). They have the same energy E=1, denoted by the solid green line. This plot illustrates the independence of this energy level on the number of solitons and on the distance between them, as obtained using the proposed method.
A.M.Dyugaev and P. D. Grigoriev
The GoosHanchen (GH) effect is the lateral shift of the totally internally reflected light beam with respect to its specular point. The potential applications of the GH effect include chemical and biological sensors, as well as alloptical switching, which motivates numerous studies aiming at achieving higher GH shift values and providing control over them.
Left panel: Planar dielectric structure considered in this work. The permittivities of the background and guiding layer are higher than the permittivity of the cladding layers, which play the role of a tunnel barrier between the guiding core and the background.
In our letter, we show that the GH shift of the reflected and transmitted radiation can be easily controlled by spatial modulation of the phase front of the incident light beam. Figure 1 (right panel) shows the dependence of the GH shift for the transmitted light on the incidentbeam parameters (namely, the beam width $a$ and the curvature of the phase front $\alpha$, supposing that the impinging beam has a Gaussian form $E_{inc} = \exp ( 0.5x^2/a^2  0.5i\alpha x^2)$). As it seen, squarelaw modulation of the phase front, which can be achieved by focusing/defocusing lensing, considerably changes the GH shift and can even reverse its sign, which is impossible for the beams with the flat phase front.
A. A. Zharov, N. A. Zharova, and A. A. Zharov
The detection of longlived magnetoexciton levels in the QHE regime attracts interest for studying the formation of the socalled nonstationary condensate, that is, a system driven from equilibrium by an external force. The appearance of a highly coherent state is caused by accumulation of a large number of magnetoexcitons with integer spin in a small region of the phase space. This work is devoted to the study of the extraordinary behavior of the Raman's antiStokes scattering signal in ZnO based 2DES with strong correlation. At low temperatures (T ~ 0.35 K), this spectral line has an anomalously high intensity. It is shown that its origin may be associated with the appearance of longlived magnetoexciton levels. Potentially, such levels can cause the formation of nonstationary condensate.
Figure 1. Raman spectrum showing Stokes and antiStokes scattering signals. Note that the antiStokes scattering signal has a gigantic intensity, ten orders of magnitude greater than that expected at such a low temperature (T ~ 0.35 K).
B.D.Kaisin, A.B.Van'kov, I.V.Kukushkin JETP Letters 112, issue 1 (2020)
A wealth of fundamental physical phenomena as well as related applications suffer from inherently weak lightmatter interactions during involved physical processes. Prime examples include – hardly related at the first glance  Raman scattering of light and detection of farinfrared and THz electromagnetic waves. While the former process has a deeply fundamental limitation of the scattering crosssection, the drawbacks of the latter application arise from the low sensitivity of even stateofart detectors operating at room temperature conditions. A widely recognized approach for amplification of Raman signal is a surfaceenhancement Raman scattering (SERS) which nevertheless is mostly limited to optical frequencies ranging from UV to the red part of the visible region. The upcoming Letter continues the research of SERSlike effects with metaldielectric metasurfaces possesing 3D submicron features. An exceptionally strong local enhancement of a laser light field (wavelength 1064 nm) is demonstrated along with optimal structure design. The findings not only pave the way for future applications in biosensing but also serve as a bridge for extending the field enhancement approach into the region of farinfrared and THz frequencies.
V.I. Kukushkin et al. JETP Letters 112, issue 1 (2020).
In threedimensional systems magnetic susceptibility of itinerant electrons is determined by competition of two eects: Landau diamagnetism and Pauli paramagnetism, both being bandstructure dependent and modied by electronelectron interactions. In
JETP Letters111, issue 11 (2020) For the edge states in twodimensional electronic systems to be realized, a spinorbit interaction, as well as an inverted band structure must occur. In this case, the effects of covalent mixing lead to a strong entanglement of the valence band states and the conduction band states. The inversion condition noted above also plays an important role in the formation of an excitonic insulator (EI), when the spontaneous occurrence of an excitonic order parameter (EOP) is accompanied by the generation of a hybridization interaction between the states of the valence band and the conduction band. Therefore, there is also a significant confusion of the states of these bands in the EI. Correspondingly, one can expect that the edge states can also occur in the EI in case the spinorbit interaction will be taken into account. Using the model of the energy structure of the HgTe quantum well, the effect of intersite Coulomb interaction on the energy spectrum was studied. In the case when only densitydensity Coulomb interaction has been taken into account, there were three phases with s , d  and p  type of the EOP symmetry. Metastable pphase was topologically nontrivial. The ground state had stype of symmetry, for which there were no edge states.
When the exchange part of Coulomb interaction is considered, a mixed s+dphase becomes the ground state of the EI. In this case, EOP is described by a superposition of s  and d– basis functions. An important feature of the mixed s+d  phase of EI implies that the
Dispersion relation of the excitonic insulator with the spinorbit interaction. The excitonic order parameter has s+d symmetry. It is significant that there are two middle branches of the dispersion relation, plotted in green (red). In the vicinity of the crossing, the two middle eigenstates have energies deep inside the bulk gap, and so their wave functions are concentrated at the edges. These wave functions describe edge states of s+d EI with spinorbit interaction. V.V. Val’kov JETP Letters 111, issue11 (2020)
In quantum cryptography, in addition to attacks on transmitted quantum states, it is possible to detect states in the side channels of information leakage. Without taking into account information leakage via side channels, it is impossible to seriously talk about the secrecy of keys in real quantum cryptography systems. A quantummechanical method is proposed for taking into account the leakage of key information through side channels — detection of electromagnetic side radiation, active sensing of a phase modulator at a transmitting station, and back reemission of avalanche detectors on the receiving side. The method takes into account joint collective measurements of quantum states in all channels of information leakage and works at any intensity and structure of states in side channels. The choice of special basis functions of an prolate spheroid allows one to ''sew'' a quantum and classical description of signals in side channels. A connection has been established between the leak of information and the Holevo fundamental value, and a transparent and intuitively clear interpretation on the physical level of the results has been given. Figure 1a) shows the dependences of the length of the secret key for various ratios of the average number of photons in the state $k$ noise dispersion $\frac{\overline{M}}{\sigma_M}$, $\overline{M}=\frac{M_{1}M_{0}}{2}$. In the classical case, instead of the number of photons, the signal energy in the frequency band ($ E_s $) is used, similarly signal dispersion is expressed in terms of the noise intensity in the frequency band. In this case, there is a correspondence $\hbar\Omega \overline{M} \rightarrow E_s $ and $ \hbar \Omega \sigma \rightarrow \frac {N_ {noise}}{2} $. In this notation, the key length becomes the function $ \ell \left (\frac{E_s}{N_{noise}} \right) $. It can be seen from Fig. 1a) that even without an attack on informational quantum states with a large signaltonoise ratio, there is a good distinguishability of states (for example, curve 1 ($ \frac{\overline{M}}{\sigma_M} = 0.5 $, $ \left (\frac{E_s}{N_{noise}} \right) $)) even with a small number of photons in the side state $ \overline{M}\approx 5 $, the key length tends to zero. The eavesdropper, detecting states only in the side channel, and without making errors on the receiving side, will know the whole key. With a small signaltonoise ratio (curve 4 of Fig. 1a))  poor distinguishability of states allows one to obtain a key even with a large average number of photons ($ \overline{M}> 20 $) in a side state.
a) The length of the secret key when detecting only the side radiation of the transmitting station as a function of the average number of photons $\overline{M}$ for different signaltonoise ratios $\frac{\overline{M}}{\sigma_M}$ $\left(\frac{E_s}{N_{noise}}\right)$  the average number of photons to the dispersion. Parameter $\frac{\overline{M}}{\sigma_M}$ $\left(\frac{E_s}{N_{noise}}\right)$ for curves 14 next: 1  0.5; 2  0.2; 3  0.1; 4  0.05.
S.N.Molotkov
In 1959 Aharonov and Bohm [1] proposed series of experiments that demonstrate the physical significance of electromagnetic potentials. In classical electrodynamics, these quantities play the role of mathematical auxiliary quantities whereas the electric and magnetic fields have a physical sense solely. In quantum mechanics, potentials possess a primary role. To observe the Aharonov  Bohm effect (AB), it is necessary to have regions free of electromagnetic field but with nonzero potential. Unipolar pulses, in contrast to conventional bipolar multicycle pulses, have non vanishing electric area S_E≡∫ E(t)dt ( E(t) is the electric field strength, t is the time) [2]. This means that unipolar pulse in vacuum changes the value of the vector potential A. Thus, a unipolar light pulse allows observation of the optical AB effect. The experimental setup of the “electronic interferometer” proposed in [1] is shown in Fig.1. The plane electron wave 1 is divided by splitter 2 into the two packets, which after the refraction in prisms 3 and 4, pass through two spatially separated regions (shoulders of the electron interferometer). Then packets are directed by prisms 5 and 6 to the screen 7. In our optical variant, a unipolar pulse 8 passes in one of the arms of interferometer before the appearance of the electronic packet. The radiation pulse is ahead of the packet and does not intersect it. Thus the packet will have to interact with a constant vector potential, which was created by unipolar pulse and the wave function of the electrons should change the phase. In this case, on the screen 7 one will observe the shift of the interference fringes relative to their position in the absence of the pulse. Besides the fundamental interest to AB effect, its optical analogue, in our opinion, can be used for the measurement of electric area of unipolar pulses.
Fig.1. Scheme of the proposed experiment to observe the AharonovBohm effect with unipolar optical pulse, which is the source of vector potential.
M.V. Arkhipov, R.M. Arkhipov, N.N. Rosanov JETP Letters 111, issue 12 (2020)
Remarkable effect of microwave irradiation of twodimensional electron systems is the appearance of giant magnetoresistance oscillations with the resistance tending to zero in the main minima. There are different approaches proposed to explain this effect, which predict similar resistance oscillations both in the shape and position. Their applicability is still debated. One of them is based on the nonequilibrium electron energy distribution under the radiation. We have employed a different from magnetotransport experimental technique which is sensitive namely to such a distribution. The measurements are carried out with samples of fieldeffect transistors (FETs) with a channel comprising two 2D electron layers (subbands) located at different distances from the gate. Nonequilibrium occupation of electronic states leads to microwave induced electron redistribution between the layers which causes ac current between the gate and channel when the microwave power is modulated. Theoretical analysis shows that the redistribution oscillates as a function of magnetic field and is described by a product of two harmonic functions with frequencies determined by commensurability of either the subband energy separation or photon energy with the cyclotron splitting. Such oscillation pattern with the beating node has been observed in our experiment (see Fig.) giving convincing evidence of the nonequilibrium electron distribution in energy. The figure shows our main experimental result and the measurement layout (inset). GaAs/AlGaAs FET is irradiated by microwaves which power is modulated at a frequency f^{mod}. The ac photocurrent I_{photo} of frequency f^{mod} between the gate and the channel, comprising two layers L1 and L2, is converted into the ac voltage and detected by the Lockin amplifier.
S.I.Dorozhkin JETP Letters 111, issue 10 (2020)
Over the past decade, the unique properties of HgTe/CdHgTe quantum well heterostructures and their potential for practical applications in terahertz electronics and optoelectronics have been discovered and intensively studied. One of the problems impeding the advancement into the terahertz range is the carrier lifetime decrease due to recombination via impuritydefect centers, i.e. by the Shockley – Reed – Hall mechanism. It is generally accepted that the most common point defect in CdHgTe ternary alloys is a double acceptor formed by a mercury vacancy. It is natural to expect the presence of such a vacancy in HgTe/CdHgTe quantum well heterostructures. To date, only a few works investigated “below bandgap” features in the photoconductivity and photoluminescence spectra. Moreover, the relationship between the observed features and the mercury vacancy states was based on calculations only. In this Letter, we experimentally demonstrated that the interplay “below bandgap” features in the photoconductivity spectra of HgTe/CdHgTe QW heterostructures results from the ionization of a double acceptor rather than the ionization of the states of two different singlecharged acceptors. To do so, the Fermi level was driven though the bandgap by dosed blue light illumination exploiting the effect of persistent photoconductivity.
Photoconductivity spectra in the HgTe/CdHgTe heterostructure obtained under dark conditions (lower curve) and after short illuminations with blue light. Bands a and b are the observed “below bandgap” features. Transition schemes with different Fermi levels positions are shown near the spectra. The absence of the band b in the lower spectrum (when the Fermi level is in the valence band) is just the evidence that the observed features are associated with the ionization of a double acceptor. Nikolaev I.D. et al. JETP Letters 111, issue 10 (2020)
As distinct from the quantized vortices in mass superfluids, which have quantized circulation of superfluid velocity, the spin vortices in antiferromagnets have quantized circulation of spin current. That is why in the rotating container with superfluid liquid the lattice of quantized vortices represents the ground or equilibrium state of the liquid. On the other hand, in the rotating antiferromagnets the spin vortices are not formed, because the orbital rotation does not act on the spin currents. Here we discuss the spin vortices in the spin triplet pwave superfluids. We show that under certain conditions the lattice of spin vortices can be formed in the rotating vessel. The first condition is the applied sufficiently large magnetic field. The second condition is that the formation of the mass vortices must be suppressed. This condition is not the problem for some phases of superfluid 3He (the superfluid 3HeB and the polar phase). In both phases there is a large barrier for the creation of mass vortices. In experiments, the mass vortices are created under rotation if either the large critical velocity is exceeded, or if the liquid is cooled down from the normal state to the superfluid state under rotation. If the mass vortices are not formed, the superfluid in the rotating state vessel is in the Landau vortexfree state. In this state one has the counterflow of the normal and superfluid components of the liquid: the normal component experiences the solid body rotation, while the superfluid component is at rest. In the presence of magnetic field, the spin vortices feel the effect of rotation from the rotating counterflow and form the spin vortex lattice with low density.
G.E. Volovik JETP Letters 111, issue 10 (2020)
Creating a fully functional multiqubit quantum computer using superconducting, qubits is a difficult problem due to the relatively short lifetime of superconducting qubits several hundred microseconds. Such a computer requires efficient multiqubit quantum memory with significantly longer lifetime. The quantum memory can be created with high Q microwave resonators which enable increasing the lifetime of qubits to tens of milliseconds. Previous studies [S.A. Moiseev, K.I. Gerasimov, R.R. Latypov, N.S. Perminov, K.V. Petrovnin, and O.N. Sherstyukov. Scientific Reports, 8, 3982 (2018)] have shown that the system of coupled resonators with a periodic spectral structure of resonance lines is capable of playing the role of a highly efficient quantum memory and interface. Due to the spectral optimization, such multiresonator systems can efficiency store short pulses of arbitrary temporal modes in a wide spectral range. In this letter, we study the multiresonator quantum memory connected to an external resonator via a controlled switchable coupling. The memory block includes 3 resonators connected with a common resonator, whose coupling k(t) with an external resonator can be controlled in time (see Fig. 1). Using algebraic methods, we found a single set of optimal spectroscopic parameters of the resonators and its coupling constants which show the possibility of highly efficient controlled reversible transfer of information into the memory block and its multicycle storage after decoupling with external resonator (k(t)=0). The presence of switchable coupling allows use of the multiresonator system both for the longer quantum storage and quantum processing of the stored quantum state on the memory resonators.
Fig.1. A quantum memory with three resonators x_{1, 2, 3} (t) of initial frequencies (D, 0, D) connected to resonators y_{1} and y_{2} having frequencies (0,0), coupling through the switch k (t). For switched off coupling k(t)≠0, the loading/unloading the field from the mode y2(t) to the memory block is implemented. When the coupling k(t)=0 is turned off, the regime of cyclic energy exchange between the resonator modes y_{1}(t) and x_{1,2,3}(t) performs reversible processing of the stored state.
S.A. Moiseev and N.S. Perminov JETP Letters 111, issue 9 (2020). methods for processes occurring at the cellular level. For this purpose, specially synthesized particles with intense luminescent properties are introduced into cells and biotissues. At the same time, one of the main problems that inhibit the development of entire areas of molecular biology and biomedicine is the separation of the useful signal against the background of socalled autofluorescence (the fluorescence of natural fluorophores of biological tissue), whose spectrum overlaps with the photoluminescence spectrum of the nanoparticles themselves. One of the most promising ways to solve this problem is timeresolved spectroscopy. Since the fluorescence lifetime of natural fluorophores is usually units of nanoseconds, for successful visualization it is necessary to use nanoparticles with the lifetime of radiative states lying in the micro and millisecond range. In this context, ions of rare earth element are very promising, since they have relatively longlived excited states In modern biology and medicine, increasing attention is paid to the development of optical visualization
The authors of this paper study new solid solutions based on the lowtemperature modification of the NaGdF_{4} matrix doped with europium ions. The combination of small sizes of the crystalline particles and the presence of alloying impurities provides high efficiency and stability of luminescence of such materials. The paper presents the results of the Judd  Ofelt theory application for determining the lifetime of excited states of NaGdF_{4}:Eu solid solutions from their integral luminescence spectra. A good agreement between the experimentally measured and theoretically calculated lifetime values of the excited states of these complexes in suspensions showed the adequacy of the Judd  Ofelt model for describing photophysical processes in samples. The application of the Judd  Ofelt theory enables to estimate the lifetime of radiative states in solid solutions of rare earth elements without resorting to direct measurement of the luminescence lifetime; therefore, such approach can be used by researchers in the absence of appropriate experimental equipment. The obtained results indicate the prospects of using solid solutions of NaGdF_{4}:Eu for optical imaging in biotissues by timeresolved spectroscopy.
S.. Burikov et.al. JETP Letters 111, issue 9 (2020).
In the twodimensional turbulence there is a tendency to form larger and larger eddies, that leads to formation of the socalled inverse cascade. In the homogeneous case, a chaotic flow with stationary statistics is realized. However, inhomogeneity (say, presence of walls) could lead to appearing coherent structures with welldefined mean flow. Here we demonstrated possibility of arising the coherent vortex around a rotating disc immersed into a twodimensional turbulent flow. We found the radial profile of the mean azimuthal velocity in such vortex. As it was shown in our previous works, a coherent vortex without a solid disc inside is characterized by the profile with constant velocity. The mean velocity amplitude is determined by the energy production rate per unit mass and the bottom friction coefficient. The immersed uniformly rotating disc, whose axis coincides with the center of the vortex, imposes boundary conditions for the flow. We showed that the presence of the disc does not destroy the vortex, and the mean velocity amplitude tends to its unperturbed value far from the disc. We found how the disc changes the flow in its vicinity in case the velocity of the disc rotation is not equal to the asymptotic value of the mean velocity.
A.B.Buzovkin, I.V.Kolokolov, V.V. Lebedev, S. S.Vergeles JETP Letters 111, issue 8 (2020) Due to the unique properties, carbon nanotubes (CNT) are currently of great interest for different applications. One of the promising areas is their use in electronics. CNTs have a wide variety of electronic properties from metallic to semiconducting, with a band gap up to 2 eV. Electronic properties of the nanotubes are entirely determined by their geometric structure. One of the most important problems of practical implementation of CNT is the complexity of the synthesis of CNT of a predefined geometry and, therefore, with desired properties. Nanotubes with predefined width were experimentally obtained in 2013 from bilayer graphene in AApackage [1]. Bilayer graphene nanoribbons were produced using transmission electron microscope. Interaction of electrons with the nanoribbon induces closure of the edges and thinning, leading to the formation of a singlewall nanotube with a diameter less than one nanometer. This work is devoted to modelling of the new flattened carbon nanotubes (FCNT), which can be prepared from twisted bilayer graphene with the Moiré angle Θ=27.8°. Bilayer nanoribbons are cut out and their chemical active edges are folded, forming structures similar to the known compressed carbon nanotubes. We describe in detail the FCNT geometry. Abinitio calculations show energy stability and Young modulus ~0.7 TPa of the new nanotubes. Electronic band structure analysis shows metallic type of conductivity for all new nanotubes except one semiconducting with the width of 14Å and energy gap E_{g}=0.2 eV (Fig.1a). Band gaps of other FCNT open under compression/stretching deformations. We believe that such structures can be useful for nanoelectronics as transition bridges between two disordered graphene monolayers (Fig.1a) or as conductive channels between two bilayer graphene arrays (Fig.1b).
Figure 1. a) – FCNT connecting two disordered graphene monolayers, b) – conductive FCNT channel connecting two arrays of bilayer graphene with Θ=27.8° [1] G. AlgaraSiller, A. Santana, R. Onions, M. Suyetin, J. Biskupek, E. Bichoutskaia, and U. Kaiser, Carbon 65, 80 (2013)
Demin V.A., Artyukh A.A., Soroko V., Chernozatonskii L.A. JETP Letters 111, issue 7 (2020)
Turbulence in classical liquids is often encountered in real life and is important for numerous technical and engineering applications. A characteristic feature of turbulence is a complicated motion of liquid in the form of eddies carrying rotational motion at different length scales. Quantum fluids, like superfluid helium, ultracold atomic gases forming BoseEinstein condensates and interior of neutron stars, can be involved in nontrivial rotating motion only when topological linear objects, quantized vortices, are formed. How is turbulence then structured in such quantum systems? It is believed currently, based on experimental, theoretical and numerical work that at length scales significantly exceeding the average vortex separation, turbulence in quantum systems can mimic that in classical fluids. But at scales smaller than the intervortex distance, turbulence looks completely differently: instead of turbulence of eddies one expects turbulence of oscillations of individual vortices, Kelvin waves. There is no experimental data available on such turbulence so far and various approaches to its theoretical description resulted in a rather heated debate. With recent progress in experimental techniques, in particular in fabricating nanomechanical oscillators which can be used to agitate waves on individual vortices, one can hope for direct observation of the Kelvinwave turbulence. In this letter, we provide expressions relating the amplitude of the waves with the energy flux needed to support such turbulence – one of the first relations required to interpret future experiments.
Example configurations of vortex lines, agitated to generate Kelvin waves. (a) A single vortex, attached to an oscillating device. (b) An array of vortices, stretched between parallel plates and agitated by shear or torsional oscillations of the plates.
Eltsov V.B., L'vov V.S. JETP Letters 111, issue 7 (2020) The effective detection of electromagnetic radiation in THz  near IR frequency band is the problem of interest due wide modern possibilities of application of such radiation: broadband communication systems, detectors for tracing concentrations of drugs and explosives, noninvasive diagnostics, nearfield spectroscopy, surface studies using electronic paramagnetic resonance, etc. In this letter, we propose a general physical approach to increase the sensitivity and selectivity of a number of THz and nearIR detectors. The essence of this approach is in the addition of a dielectric structure (resonator) under the conducting layer (“dielectric” plasma, metal, superconductor) that is opaque for an incident wave. This allows one to increase significantly both the field strength behind the mentioned layer and absorption in the conductor. The set of dielectric resonators separated by conductive layers forms the photonic crystal with the absorption band. The properties of such detector are governed by parameters of the dielectric and conducting layers.
The photonic crystal as a set of dielectric and conducting layers forming the band of THz radiation absorption in a given spectral range. Plots of absorption versus radiation frequency are given for different number of the layers in the structure.
A.E. Schegolev, A.M. Popov, A.V. Bogatskaya, P.M. Nikiforova, M.V. Tereshonok, N.V. Klenov JETP Letters 111, issue 7 (2020)
There are several scenarios of emergent gravity. Gravity may emerge in the vicinity of the topologically stable Weyl point; the analogue of curved spacetime emerges in hydrodynamics with the socalled acoustic metric for the propagating sound waves; etc. There are two very different scenarios, which however have unusual common property: the tetrad fields in these theories have dimension of inverse length. As a result, all the physical quantities which obey diffeomorphism invariance are dimensionless. This was first noticed by Diakonov and Vladimirov in the scenario, where tetrad fields emerge as bilinear combinations of the fermionic fields. In another scenario, tetrads with dimension of inverse length emerge in the model of the superplastic vacuum. The reason is that the superplastic vacuum can be arbitrarily deformed, and the equilibrium size of the elementary cell is absent. There is no preferred microscopic length scale (such as Planck scale), and the distances (say, between the points A and C in Figure) are measured in terms of the integer positions of the nodes in the crystal.
In both systems, such physical quantities as Newton constant, scalar curvature, cosmological constant, particle masses, etc., are dimensionless. Some of these physical quantities become integervalued quantum numbers, which characterize the topology of quantum vacuum. Examples are the parameters describing the 3+1 dimensional quantum Hall effect in topological insulators and in the NiehYan anomaly.
G.E. Volovik JETP Letters 111, issue 7 (2020) Remarkable features of iron pnictide LiFeAs with critical temperature 17 K, such as superconductivity in stoichiometric state, absence of magnetism, and nontrivial band structure [1], still challenge experimenters. Unfortunately, the available experimental studies are not so numerous due to LiFeAs instability in the presence of water vapors or oxygen. In order to reveal the structure of the superconducting order parameter, here we used multiple Andreev reflection (MAR) spectroscopy. The superconductornormal metalsuperconductor (SnS) junctions with ballistic hightransparent barrier were produced by a breakjunction technique [2,3]. This technique provides local probing bulk order parameter (unaffected by surface), and enables to resolve its fine structure (anisotropy). In Li_{1}_{d }FeAs single crystals with T_{c} = 15.617 K [4], we observe three superconducting gaps: an isotropic small gap, and anisotropic middle and large gaps. Following the ARPES data interpretation [1], the observed gaps are attributed to the outer Fermi surface sheet near the G point, at the electron barrels near the M point, and at the inner Gbarrel, respectively. The directly measured temperature dependence of the gaps and their anisotropy, zerobias conductance and the Andreev excess current may be interpreted in the framework of the threegap model and demonstrate selfconsistency of our data. We also discuss the relation between superconducting and normalstate features, in particular, a flat band presence in the vicinity of the Fermi level.
Fig. 1. Directly measured temperature dependence of the three superconducting gaps in LiFeAs. Following the ARPES data [1], D_{S} develops at the outer Gbarrel, D_{L} – at electron Mpoint barrels, and D_{G} at the inner Gbarrel of the Fermi surface below T_{c}. “In” and “out” indexes mark the minimum and maximum values of Cooper pair binding energy in the k_{x}k_{y}plane of the momentum space (gap anisotropy). Dashdot lines show singlegap BCSlike curves. The inset shows the temperature dependence of the anisotropy of the middle D_{L} and the large D_{G} gaps taken as 100%(1D_{i}^{in}/D_{i}^{out}).
1.S.V. Borisenko, et al., Symmetry 4, 251 (2012). 2. S.A. Kuzmichev, T.E. Kuzmicheva, Low Temp. Phys. 42, 1008 (2016). 3. T.E. Kuzmicheva, et al., Phys. Rev. B 97, 235106 (2018). 4. I. Morozov, et al., Cryst. Growth&Design 10, 4429 (2010).
T. E. Kuzmicheva, S.A. Kuzmichev, I.V. Morozov, S. Wurmehl, B. BüchnerJETP Letters 111, issue 6 (2020)
Light bullets are bunches of the light energy localized in all directions that can propagate in both homogeneous and inhomogeneous media [1]. The formation of such objects requires at least the presence of nonlinearity, dispersion, and diffraction. In connection with possible applications of light bullets in optical communication systems, the question of their stability is very important. To date, light bullets have been studied theoretically and experimentally in media with nonlinearities of various physical nature [2  4]. In addition to quasimonochromatic light bullets [1, 3], the fewcycle light bunches were considered [2, 4]. The theory of “breathing” parametrical light bullets propagating in media with quadratic nonlinearity was developed recently for both anomalous [5] and normal [6] groupvelocity dispersions (GVD). One should emphasize that the latter case can be realized only in an inhomogeneous medium, for instance, in a waveguide. Interplay between nonlinearity, dispersion, diffraction and waveguide geometry defines stability areas of such spatiotemporal solitons. Various nonlineardispersive effects accompany the process of secondharmonic generation. Their character depends, in particular, on the magnitude and sign of GVD coefficients. Actually, the most interesting case is when one of pulse carrier frequencies locates near zero GVD.
In this paper we investigate the pulsebeam propagation in a medium with quadratic nonlinearity provided zero GVD coefficient of the second harmonic. Our study shows that such conditions don’t prevent the
[1] Ya.V. Kartashov, G.E. Astrakharchik, B.A. Malomed, and L. Torner, Nature Review / Physics 1, 185 (2019). S.V. Sazonov and M.V. Komissarova JETP Letters 111, issue 6 (2020). The phenomenon of topological superconductivity and Majorana bound states (MBSs) have attracted much attention of researchers mainly due to the prospects of implementing quantum computing that is stable against decoherence processes. From the standpoint of available technologies one of the most promising systems for detecting the MBSs is semiconductoring wire with strong spinorbit coupling in contact with a superconductor (hereinafter, superconducting wires). Then, under the influence of an external magnetic field the Majorana modes with zero energy should emerge at the opposite ends of such a hybrid nanostructure [1, 2]. As a result, in the experiment, the MBS should manifest itself as a conductance peak at zero bias voltage. In the 2010s considerable efforts were directed toward the MBS detection in these systems utilizing local tunneling spectroscopy [3,4]. However, it was shown that the MBS may not be the only reason for observing the mentioned resonances [5]. Thus, today the urgent task is to search for alternative ways to probe the MBS, in particular, using the nonlocal nature of this excitation. In this study we propose a method for detecting the MBS nonlocality in a situation where the superconducting wire connects two arms of an interference device. With an asymmetric connection of such a device to the contacts, the asymmetric Fano resonances occur in the conductance. It was found that the width of these resonances is proportional to the overlap of the Majorana wave functions localized at opposite ends of the superconducting wire. Thus, if a true MBS is realized, then these Fano resonances collapse. In the framework of the spinless model it is shown that the discovered effect is associated with an increase in the degree of degeneracy of the structure zeroenergy state leading to the appearance of a bound state in continuum.
[1] R. M. Lutchyn, J. D. Sau, and S. Das Sarma, Phys. Rev. Lett. 105, 077001 (2010). [2] Y. Oreg, G. Refael, and F. von Oppen, Phys. Rev. Lett. 105, 177002 (2010). [3] V. Mourik, K. Zuo, S. M. Frolov, et al., Science 336, 1003 (2012). [4] H. Zhang, C.X. Liu, S. Gazibegovic, et al., Nature 556, 74 (2018). [5] C.X. Liu, J. D. Sau, T. D. Stanescu, and S. Das Sarma, Phys. Rev. B 96, 075161 (2017).
S.V.Aksenov and M.Yu.Kagan
During the last decade, much attention was focused on high energy collisions of particles near black holes. This is due to the effect discovered by Banados, Silk ad West (the BSW effect, after names of its authors) who observed that under certain conditions the energy $E_{c.m.}$ in the center of mass of colliding particles can grow unbounded [1]. This happens if one of two particles is finetuned (it is called critical). There are two main scenarios in which such collisions occur. In the first one described in the original paper [1], particles collide near a black hole horizon of a rotating black hole, the critical particle has a special relation between the energy and angular momentum. There is also direct analogue of the BSW effect if a black hole is nonrotating but electrically charged. Then, a special relation should exist between the energy and electric charge of the critical particle [2]. At first, the BSW effect was found for extremal black holes, later it was shown for nonextremal ones [3]. It was believed that if a black hole is neither rotating nor charged, the BSW effect is impossible. In particular, it was shown that for the Schwarzschild black hole $E_{c.m.}$ $\leq 2\sqrt{5}m$ [4]. Meanwhile, this is true if a particle is free or moves under the action of the electrostatic force caused by interaction with a black hole. However, it is demonstrated in the present work, that a situation changes radically if some force is exerted on a particle in the background of a static neutral black hole. The analogue of the BSW effect exists eve for a simplest case of radial motion. What is especially interesting, this is valid even for the Schwarzschild black hole. I doing so, we do not specify the nature of a force. In a particular case of the ReissnerNordstrom metric and Columb interaction, the previous results are recovered. But, the results are much more general. There is also one more interesting point. Traditionally, it was believed that additional interaction of a particle with surroundings act against the BSW effect [5, 6], and one was led to prove that the BSW effect retains its validity even in spite of the presence of the force [7, 8]. However, in the case under discussion, such a presence is not an obstacle but the main ingredient of the effect. If a black hole is surrounded by external electromagnetic fields, we can suppose that the described mechanism promotes high energy collisions near black holes. The Schwarzschild metric and radial motion give us the simplest exactly solvable example but it is quite probable that qualitatively the similar results hold in a more realistic situation as well.
[1] M. Bañados, J. Silk and S.M. West, Kerr black holes as particle accelerators to arbitrarily high energy, Phys. Rev. Lett. 103 (2009) 111102 [arXiv:0909.0169].
O.B.Zaslavskii
Ion fluxes with group velocities up to 2000 km/s were detected in the plasma sheet boundary layer on highapogee spacecrafts [1]. These fluxes are formed in the current sheet of the smallscale ion beams – beamlets [2], which are accelerated by the electric field at various distances along the magnetotail of separated resonant N zones and, then, move along the magnetic field lines towards the auroral region. Experimental test [3] of the theoretically predicted scaling W_{N} ~ N^{A} (where W_{N }is the energy of the Nth resonance and A ~ 1.33) [4] shows that the real scaling of resonance energies varies in a wide range A ∈ [0.61, 1.75]. Model calculations [3] with the addition of This paper reports an experimental study of the energy scaling of beamlets (seven resonance zones N=17 with resonances R=17 were identified) using the data from SC1 and SC4 CLUSTER satellites for the event of 05.02.2003. Analysis of the ion beam signatures in the auroral magnetosphere in the range 120 kev showed that the energy of beamlets scales differently (0.04 and 0.40 for zones with resonances R=14, and 0.83 and 1.14 for zones with R=57, according to satellites SC1 and SC4, respectively). For zones with R=57, the energy scaling of the beamlets can be explained in accord with the results of the work [3]. The observed parameters A in zones N=14 may be related to the fact that the normal component of the magnetic field B_{z}, which controls the increment of the ion beams energy in the current sheet, has spatial decay lower in the region of these resonant zones than in the region containing zones N=57. Therefore, the current sheet is inhomogeneous and is characterized by various conditions of the formation of its parts. [1] K. Takahashi, and E.W. Hones, J. Geophys. Res. 93, 8558 (1988). [2] L.M. Zelenyi, E.E. Grigorenko, and A.O. Fedorov, JETP Lett. 80, 663 (2004). [3] R.A. Kovrazhkin, M.S. Dolgonosov, and J.A. Sauvaud, JETP Lett. 95, 234 (2012). [4] L.M. Zelenyi, M.S. Dolgonosov, E.E. Grigorenko, and J.A. Sauvaud, JETP Lett. 85, 187 (2007).
R. A. Kovrazhkin, A.L. Glazunov, and G.A. Vladimirova
Since the discovery of unconventional dwave superconductivity in hightemperature superconductors, physical consequences of dwave electron pairing have been intensively investigated. One of such physical properties is a fourfold symmetry of the parallel upper critical magnetic field in this quasitwodimensional (Q2D) superconductors. From the beginning, it was recognized that the fourfold anisotropy of the parallel upper critical magnetic field disappears in the GinzburgLandau (GL) region and has to be calculated as a nonlocal correction to the GL results. Another approach was calculation of the parallel upper critical magnetic field at low temperatures and even at T=0 using approximate method, which was elaborated for unconventional superconductors with closed electron orbits in an external magnetic field. Note that Q2D conductors in a parallel magnetic field are characterized by open electron orbits, which makes the calculations to be inappropriate. The goal of our article is to suggest an appropriate method to calculate the parallel upper critical magnetic field in a Q2D dwave superconductor. For this purpose, we explicitly take into account almost cylindrical shape of its Fermi surface (FS) and the existence of open electron orbits in a parallel magnetic field. We use the Green's functions formalism to obtain the Gorkov's gap equation in the field. As an important example, we numerically solve this integral equation to obtain the fourfold anisotropy of the parallel upper critical magnetic field in a d(x^2y^2}wave Q2D superconductor with isotropic inplane FS. In particular, we demonstrate that the socalled supercondcting nuclei at T=0 oscillate in space in contrast to the previous results. We also suggest the gap equation which take both the orbital and paramagnetic spinsplitting mechanisms against superconductivity.
A.G.Lebed and Sepper O. In recent years, a number of interesting papers appeared [1,2], where from the analysis of experiments on rather wide range of compounds, it was shown that in the $T$  linear region of resistivity growth, the scattering rate of electrons (inverse relaxation time) with rather high accuracy is described as $\Gamma=\frac{1}{\tau}=\alpha \frac{k_BT}{\hbar}$, where $\alpha\sim 1$ and is weakly dependent on the choice of the material. In connection with these results the notion of the universal (independent of interaction strength) "Planckian'' upper limit of inelastic scattering rate in metals was introduced as $\frac{1}{\tau_P}=\Gamma_P=\frac{k_BT}{\hbar}$ [3]. To explain this "universality'' a number of relatively complicated theoretical models were proposed [4, 5], including some rather exotic, based on the analogies taken from the black hole physics, cosmology and superstring theory (e.g. see Refs. [69]). It is shown here that the "Planckian'' limit for the temperature dependent relaxation rate actually follows from a certain procedure used in Refs. [1, 2] to derive $\frac{1}{\tau}$ from experimental data on resistivity, using the effective electron mass, determined from low  temperature experiments. Thus, the "experimentally'' observed universal "Planckian'' relaxation rate in metals, independent of interaction strength, is nothing more than a kind of delusion.
[1] J.A.N. Bruin, H. Sakai, R.S. Perry, A.P. MacKenzie. Science 339, 804 (2013) M.V. Sadovskii JETP Letters 111, issue 3 (2020)
Discovery of high critical temperatures of superconductivity in sulfur [1, 2], lanthanum, and yttrium hydrides [35] led to the active search for stable structures of hydrides of other elements, including iron. Iron hydrides are characterized by a critical temperature of ~50 K and can conditionally be classified as hightemperature superconductors. On the other hand, hydrogen is considered as one of the possible light elements of the Earth’s and planets core, which causes interest in phase relationships for the FeH system over a wide range of pressures and temperatures. In this work, within the density functional theory, the thermodynamic stability of iron hydrides Fe_{4}H, Fe_{2}H, FeH, Fe_{3}H_{5}, FeH_{2}, FeH_{3}, FeH_{4}, Fe_{3}H_{13}, FeH_{5} and FeH_{6} at temperatures up to 5000 K in the pressure range of 100400 GPa was estimated and the corresponding phase PTdiagrams were calculated. We performed a topological analysis of all stable iron hydrides. The regularity of the formation of dumbbellshaped hydrogen molecules with increasing hydrogen concentration in iron hydrides was established. [1] A. Drozdov, M. Eremets, I. Troyan, V. Ksenofontov, S. Shylin, Nature 525, 73 (2015) [2] D. Duan, Y. Liu, F. Tian, D. Li, X. Huang, Z. Zhao, H. Yu, B. Liu, W. Tian, T. Cui, Scientific Reports 4, 6968 (2014). [3] A. Drozdov, P. Kong, V. Minkov, S. Besedin, M. Kuzovnikov, S. Mozaffari, L. Balicas, F. Balakirev, D. Graf, V. Prakapenka, Nature 569, 528 (2019). [4] H. Liu, I. I. Naumov, R. Hoffmann, N. Ashcroft, R. J. Hemley, Proceedings of The National Academy of Sciences 114, 6990 (2017). [5] M. Somayazulu, M. Ahart, A. K. Mishra, Z. M. Geballe, M. Baldini, Y. Meng, V. V. Struzhkin, R. J. Hemley, Physical Review Letters 122, 027001 (2019). D.N. Sagatova et al. JETP Letters 111, issue 3 (2020)
The search of a quarkgluon plasma (QGP), where hadrons dissolve and quarks are supposed to be free and deconfined, is difficult due to the short QGP lifetime. Various signals were proposed for detection of the QGP phase, and the ''horn'', which appears in the ratio of positive charged kaon to pion, was supposed be one of them [1]. Nowdays the picture of this peak becomes more clear on the experimental side: the peak appears in the ratio of positive charged kaons and pions at the collision energy $\sqrt{s_{NN}}\sim$ 710 GeV for the largesize systems in Au+Au and Pb+Pb collisions. With decreasing system size, the sharp peak becomes lower and for Be+Be, p+p collisions the ratio demonstrates smooth behaviour [2].
On the theoretical side, the quick increase in the $K^+/\pi^+$ ratio and its decreasing and flattering with further energy increasing is interpreted as a sequence of the chiral symmetry breaking and subsequent deconfinement effect.
In our works [3, 4], including the present one, we discussed the chiral phase transition, deconfinement transition and inmedium behaviour of the pseudoscalar mesons in the framework of the SU(3) Polyakov loop extended NJL model. Using the model it was shown how $K/\pi$ ratio changes as function of $T/\mu_B$, when T and $\mu_B$ are chosen on the phase diagram along the chiral phase transition curve and discussed in this way how the chiral phase transition can affect to the $K/\pi$ behaviour. Several modifications of the model was considered, including the model with vector interaction, where the situation with the absence of the first order transition region can appear when the vector coupling constant is high enough. We can conclude that the peak appears in the range of low temperatures and high baryon chemical potential (which corresponds to low collision energy). The appearance of the peak is weakly sensitive to the type of phase transition in the high density region, as the replacement of the the first order transition to the soft crossover only leads to a changing in the peak hight. The peak structure is more sensitive to the slope of the phase transition curve at low T and the properties of the matter. For example, the hight of the peak is sensitive to the chemical potential of the strange quark. For the case with the zero strange chemical potential ($\mu_S(\mu_K) = 0$), the $K^+/\pi^+$ ratio shows smooth behaviour, and when the strangeness neutrality is introduced, the $K^+/\pi^+$ ratio does not show a clear peak structure.
1. S. V. Afanasiev et al. (NA49 Collabration), Phys. Rev. C 66, 054902 (2002); C. Alt,et al (NA49 Collaboration) Phys.Rev. C 77, 024903 (2008).
2. A. Aduszkiewicz (NA61/SHINE Collaboration) Nucl. Phys. A 967, 35 (2017). 3. A. V. Friesen, Yu. L. Kalinovsky, V. D. Toneev Phys. Rev. C 99, 045201 (2019). 4. A. V. Friesen, Yu. L. Kalinovsky, V. D. Toneev, PEPAN Letters, 16, 681 (2019).
A. V. Friesen, Yu. L. Kalinovsky, V. D. Toneev
JETP Letters 111, issue 3 (2020)
Monolayer films of transition metal dichalcogenides (TMD) (in particular, MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$) can be considered an ideal system for studying a hightemperature electronhole liquid (EHL). The quasitwodimensional nature of electrons and holes defines a stronger interaction compared to bulk semiconductors. Screening of the Coulomb interaction in monolayer heterostructures is significantly weakened, because it is determined by permittivity of the environment (e.g., vacuum and substrate), which are much smaller than that of TMD films. The multivalley structure of the charge carriers energy spectrum in TMD many times reduces the kinetic energy. This leads to increase in the equilibrium density and binding energy of EHL. The optical properties of the monomolecular TMD layers are generally determined by excitons and trions. The binding energy of the exciton $E_x$ in the TMD is hundreds of meV. For example, in the monolayers MoS$_2$ $E_x=420$ meV [1]. The binding energy of EHL on one electronhole pair is $\leftE_\text{EHL}\right\sim E_x$, and the critical temperature for the gasliquid phase transition is $T_c\sim0.1\leftE_\text {EHL}\right$ [24]. So, we can expect that EHL will be observed in TMD monolayers even at room temperature. A hightemperature strongly bound EHL with $T_c\simeq500$ K was already observed in the MoS$_2$ monolayers [5]. In this paper, we are theoretically investigating the possibility of the formation of EHL in monolayers of multivalley semiconductors. We consider a thin film of a model multivalley semiconductor on an insulator substrate in vacuum. The semiconductor has a large identical number of equivalent electron $\nu_e$ and hole $\nu_h$ valleys $\nu_e=\nu_h=\nu\gg1$. A large number of valleys can be achieved due to the presence of several monomolecular layers in the film. We found analytically the binding energy of EHL and its equilibrium density and compared the results of calculations with experimental values. [1] Y. Yu, Y. Yu, Y. Cai, W. Li, A. Gurarslan, H. Peelaers, D.E. Aspnes, C.G. Van de Walle, N.\,V. Nguyen, Y.W. Zhang, and L. Cao, Sci. Rep.5, 16996 (2015). [2] E.A. Andryushin, V.S. Babichenko, L.V. Keldysh, T.A. Onishchenko, and A.P. Silin, JETP Lett. 24, 185 (1976). [3] E.A. Andryushin, L.V. Keldysh, and A.P. Silin, JETP 46, 616 (1977). [4] ElectronHole Droplets in Semiconductors ed. C.D. Jeffries and L.V. Keldysh (Amsterdam: NorthHollalnd, 1983). [5] Y. Yu, A.W. Bataller, R. Younts, Y. Yu, G. Li, A.A. Puretzky, D.B. Geohegan, K. Gundogdu, and L.Cao, ACS Nano 13, 10351 (2019). P.L. Pekh, P.V. Ratnikov, and A.P. Silin JETP Letters111, issue 2 (2020)
Ten years after recognition of the Nobel Prize, the chirped pulse amplification technique, was first implemented [1] and the unique regime of longrange femtosecond pulse propagation was discovered [2]. This propagation regime without beam divergence, or filamentation, was studied with Ti:Sapphire laser systems centered at ~800 nm with pulse peak power of 10^{10}–10^{13} W [3]. Ultrashort pulse filamentation is accompanied by supercontinuum conical emission [4]. The atmospheric transparency window [5] in the visible range ensures lossless propagation of supercontinuum blue wing in the course of backward propagation after reflection from the cloud [6]. However, the fingerprints of atmospheric molecular pollutants are in the mid and farinfrared range [5]. Besides, the critical power for selffocusing is proportional to the squared wavelength and achieves several hundreds of gigawatts for midinfrared pulse propagating in air. This requires the pulse energy of at least several tens of milliJoules (pulse duration of about 100 fs) to form a filament on an atmospheric path. In order to target the application of femtosecond lidar in the midinfrared part of the spectrum, we suggested the generalized approach for identification of the optimum laser wavelength for supercontinuum remote sensing applications [7,8]. We also developed the gas cell [9] for pressures 10^{–3}–120 bar and temperatures up to 150°C to reach the filamentation with submilliJoule pulses. Our long cell of 75cm length provides the filamentation in highpressure gas in the quasicollimated geometry close to atmospheric path experiments. The gas dispersion in the cell can be continuously tuned from normal to anomalous in the vicinity of water absorption band at 1.35 mm. The reservoir with water is installed into the gas cell and is additionally heated. In our experiments the cell was filled with nitrogen (30 bar) and water vapor (200 Pa). The laser pulses of ~100mJ energy and 1.3mm central wavelength propagate in the cell. The nonlinearly enhanced linear absorption was revealed in the longwavelength part of the supercontinuum spectrum; this observation confirmed the theoretical prediction [7] of launching the pulse on the red (longwavelength) side of the absorption line to ensure the maximum transmission through gases.
[1] D. Strickland and G. Mourou, Opt. Commun. 55, 447 (1985). [2] A. Braun, G. Korn, X. Liu, D. Du, J. Squier, and G. Mourou, Opt. Lett. 20, 73 (1995). [3] S. L. Chin, S. A. Hosseini, W. Liu, Q. Luo, F. Théberge, N. Aközbek, A. Becker, V. P. Kandidov, O. G. Kosareva, and H. Schroeder, Can. J. Phys. 83, 863 (2005). [4] O. G. Kosareva, V. P. Kandidov, A. Brodeur, C. Y.Chien, and S. L. Chin, Optics letters 22, 1332 (1997). [5] L. Rothman et al., J. Quantum Spectrosc. Radiat. Transfer 130, 4 (2013), HITRAN2012 special issue. [6] J. Kasparian et al., Science 301, 61 (2003). [7] N. A. Panov, D. E. Shipilo, V. A. Andreeva, O. G. Kosareva, A. M. Saletsky, H. Xu, and P. Polynkin Phys. Rev. A 94, 041801 (2016). [8] N. A. Panov, D. E. Shipilo, A. M. Saletsky, W. Liu, P. G. Polynkin, and O. G. Kosareva Phys. Rev. A 100, 023832 (2019). [9] V. O. Kompanets, D. E. Shipilo, I. A. Nikolaeva, N. A. Panov, O. G. Kosareva, S. V. Chekalin “Nonlinear enhancement of resonant absorption under filamentation of midinfrared laser pulse in highpressure gas” JETP Lett. accepted for publication, December 2019. V. O. Kompanets, D. E. Shipilo, I. A. Nikolaeva, N. A. Panov, O. G. Kosareva, S. V. Chekalin JETP Letters 111, issue 1 (2020)
Quasiparticles with the Dirac spectrum arise in a number of materials. Wellknown examples are graphene, topological insulators, Dirac semimetals. More recently, it has been found that there are also materials in which the vertices of the Dirac cone are not at one or more points of the Brillouin zone, but form a line [1]. A feature of nodalline Dirac semimetals is the much higher density of Dirac states than in materials with Dirac points, which allows us to hope for a more vivid manifestation of the properties due to Dirac fermions.
R(B) = R_{0}+R_{1}(1+η^{2}B^{2})^{1/2}+bB^{2},
[1] A. A. Burkov, M. D. Hook, and L. Balents, Phys. Rev. B 84, 235126 (2011).
S.V. ZaitsevZotov and I.A. Cohn Superfluid 3He is a wellknown condensed matter whose properties are described by quantum field theory. Upon transition to superfluid states, gauge and spin and orbital rotational symmetries are violated simultaneously, demonstrating the properties of antiferromagnetic superfluid liquid crystals. In these systems, spin superfluidity was discovered  quantum transfer of spins controlled by the gradient of the magnetization precession phase. Spin supercurrents provide coherence during the magnetization precession: the precession becomes coherent even in a strongly inhomogeneous magnetic field. This leads to a longlived signal of free induction, which was observed experimentally, see Review [1]. An even more complex interaction between the spin and orbital degrees of freedom leads to the formation of an extremely long live signal, which was explained in terms of the Coleman Qball model [2]. For a long time, magnetic resonance in solidstate magnets was considered in the limit of small perturbations, which corresponds to a low concentration of no equilibrium magnons. However, at high concentrations, magnons can experience Bose condensation, as in superfluid 3He. Moreover, in the case of a repulsive interaction, magnons can form a superfluid state and exhibit spin superfluidity properties in a solid magnets [3]. In particular, manifestations of a superfluid spin state in yttrium iron garnet (YIG) at room temperature have recently been discovered [4]. This article presents the results of observations of a very longlived induction decay signal obtained in a YIG at room temperature. Its properties are partially similar to the Qball observed in superfluid 3He. Nevertheless, there are some fundamental differences with the Qball, which require the correct theoretical explanation. The formation of this longlived signal can be a manifestation of quantum field theory at room temperature. [1]. Yu. M. Bunkov, G. E. Volovik “Spin superfluidity and magnon BEC” Chapter IV of the book "Novel Superfluids", eds. K. H. Bennemann and J. B. Ketterson, Oxford University press, (2013) . [2]. S. Autti, Yu. M. Bunkov, V. B. Eltsov, et al. “Selftrapping of magnon BoseEinstein condensates in the ground and excited levels: from harmonic to a box confinement” Phys. Rev. Lett. 108, 145303 (2012). [3]. Yu. M. Bunkov, E. M. Alakshin,2 R. R. Gazizulin, et al., “HighTc Spin Superfluidity in Antiferromagnets” Phys. Rev. Lett. 108, 177002 (2012). [4]. Yu. M. Bunkov, A.Farhutdinov A. N. Kuzmichev, et al., “The magnonic superfluid droplet at room temperature” https://arxiv.org/pdf/1911.03708.pdf
Yu.M.Bunkov, P.M.Vetoshko, A.N.Kuzmichev, G.V.Mamin. S.B.Orlinsky, T.R.Safin, V.I.Belotelov, M.S.Tagirov. JETP Letters 111, issue 1 (2020)
In recent years, a rapidly developing field of science and technology  spintronics  has attracted much attention. New principles for the operation of devices have been proposed, in which the electronic spin is used along with its charge to transmit and process information. The main tasks of semiconductor spintronics are the investigations of the carrier spins injection, orientation, accumulation and detection processes and the study of the possibilities of controlling them by optical and electrical methods. Diluted magnetic semiconductors and nanostructures based on II – VI materials with manganese ions are considered as model objects for possible applications in spintronics. In such structures, magnetic Mn^{2+} ions isoelectronically replace metal ions in cationic sublattices. The lowtemperature spectra of magnetooptical photoluminescence provide quantitative information on the temperature and magnetization of the Mn ions subsystem. Indeed, the exciton luminescence line shift in external magnetic fields is directly proportional to the magnetization, which makes it possible to experimentally implement the internal thermometer of the magnetic ions spin temperature, since temperature increase leads to a decrease in the Zeeman shift of the emission band. Measurements of the lowtemperature exciton luminescence spectra with time resolution in external magnetic fields also allow one to study the dynamics of changes in the spin subsystem magnetization and temperature of diluted magnetic semiconductor structures when nonequilibrium magnetization is created in them, for example, using highpower pulsed optical pumping [1]. To determine the real interaction time of carriers with magnetic ions, it is very important to study diluted magnetic semiconductor superlattices with type II band alignment. In such structures based on (Zn,Mn)Se/(Be,Mn)Te the type II band alignment makes it possible to experimentally change the interaction time of photoexcited carriers with magnetic ions. At high levels of optical excitation inside ZnSe/BeTe superlattices, due to the high concentration of spatially separated charges of electrons and holes, strong electric fields arise, which in turn lead to strong band bending [2]. Strong band bending leads to the formation of metastable abovebarrier hole states [3], which increases the hole lifetimes in the ZnSe layer. In the present paper the magnetization kinetics in diluted magnetic semiconductor type II superlattices Zn_{0.99}Mn_{0.01}Se/Be_{0.93}Mn_{0.07}Te in external magnetic fields was studied using an optical technique with a high temporal resolution ~ 2 ps. For the first time, direct measurements of the picosecond kinetics of the process of energy and spin transfer from photoexcited carriers due to the exchange interaction with the localized spins of Mn^{2+} ions were performed and the energy and spin transfer time τ ≈ 17 ± 2 ps was determined. [1] M.K. Kneip, D.R. Yakovlev, M. Bayer, A.A. Maksimov, I.I. Tartakovskii, D. Keller, W. Ossau, L.W. Molenkamp, and A. Waag, Phys. Rev. B 73, 035306 (2006). [2] S.V. Zaitsev, V.D. Kulakovskii, A.A. Maksimov, D.A. Pronin, I.I. Tartakovskii, N.A. Gippius, M.Th. Litz, F. Fisher, A. Waag, D. R. Yakovlev, W. Ossau, and G. Landwehr, JETP Lett. 66, No. 5, 376381 (1997). [3] A.A. Maksimov, S.V. Zaitsev, E.V. Filatov, A.V. Larionov, I.I. Tartakovskii, D.R. Yakovlev, and A. Waag, JETP Lett. 88, No. 8, 511–514 (2008). A.A. Maksimov, E.V. Filatov, I.I. Tartakovskii, D.R. Yakovlev, A. Waag JETP Letters 110, issue 12 (2019)
Observation of the polar Kerr effect in $\mathrm{Sr_2RuO_4}$ [1], a layered material considered to realize the chiral $p_x+ip_y$ superconducting state, has lead to extensive theoretical investigations of the anomalous Hall response $\sigma_{xy}(\omega)$ in $p_x+ip_y$ superconductors. These studies consider either multiband superconductor models or effects of potential disorder caused by weak impurities.
[1] J. Xia, Y. Maeno, P. T. Beyersdorf, M. M. Fejer, and A. Kapitulnik, Phys. Rev. Lett. 97, 167002 (2006). Ioselevich P.A., Ostrovsky P.M. JETP Letters 110, issue 12 (2019)
Most materials found in nature exhibit negligible nonlinear optical behaviors. To observe them, it is necessary to increase the interaction length (for example, using optical fibers) and/or to amplify the pump intensity with highpowered pulse lasers. It means that the thirdorder nonlinear optical processes, for example, stimulated Raman scattering (SRS), optical Kerr effect, to name a few, do not appear within highly confined media or from single molecules exposed to continuouswave lowpowered laser light. Nonlinear enhancement of light becomes possible due to giant local electric fields and/or changes in higherorder nonlinear susceptibility. The nonlinear optical effects were found to occur in plasmonic and/or epsilonnearzero (ENZ) materials [14]. In paper [5], the authors, for the first time, have succeeded to synthesize a metaldielectric nanocomposite exhibiting the 2ENZ behavior in the visible and nearinfrared region. In such a medium, multiple plasmon resonances at different wavelengths are available. In this paper, we study SRS effects using a percolated 50 nm titanium oxynitride (TiON) thin film that exhibits the 2ENZ behavior in the visible and nearinfrared region. This film was fabricated using dc reactive magnetron sputtering in an argonnitrogen environment at elevated temperature and postoxidation in air. In order to enhance the SRS effect we have patterned the TiON thin film by making squareshaped planar nanoantennas with focused ion beam milling. Using tipenhanced Raman scattering, we have proved that this nanocomposite film can be represented as the mixture of metallic TiN and dielectric TiO_{2 }nanoparticles. The underlying mechanism to observe the SRS is linked to the enhanced effective thirdorder susceptibility due to plasmon resonances at the ENZ wavelengths. Earlier, we have experimentally demonstrated a farfield Raman color superlensing effect by showing a subwavelength resolution of l/6NA (l is the excitation wavelength, NA  numerical aperture) at different SRS overtones using multiwalled carbon nanotubes of 40 nm in diameter directly dispersed on the TiON thin film [6]. This allows one to use this material for developing a multiresonant metalens pushing a spatial resolution beyond the diffraction limit without postrecovery. The metalens serves as a SERS substrate that not only enhances a scattered light but provides the subwavelength resolution. The metaldielectric 2ENZ nanocomposite film can be used as a broadband perfect absorber for thermophotovoltaic cells. [1] Reshef O., De Leon I., Alam M. Z., Boyd R. W. Nat. Rev. Mater. 4, 535 (2019). [2] Caspani, Kaipurath R. P. M., Clerici M.,et al., PRL 116, 233901 (2016) [3] Kharintsev S.S., Kharitonov A.V., Saikin S.K., Alekseev A.M., Kazarian S. G. Nano Lett. 17, 5533 (2017). [4] Kharintsev S.S., Kharitonov A.V., Alekseev A.M., Kazarian S. G. Nanoscale 11, 7710 (2019). [5] Braic L., Vasilantonakis N., Mihai A.,et al., ACS Appl. Mater. Interfaces 9, 29857 (2017). [6] Kharintsev S.S. Opt. Lett. 44 (24), 59095912 (2019).
Tyugaev M.D., Kharitinov A.V., Gazizov A.R., Fishman A.I., Salakhov M.Kh., Dedkova A.A., Alekseev A.M., Shelaev A.V., Kharintsev S.S. JETP Letters 110, issue 12 (2019)
In 1982 Nieh and Yan introduced the quantum gravitational anomaly caused by the gravitational torsion field [1, 2]. Since that time the torsional anomaly has been debated, because the coefficient in the NiehYan anomaly term contains the ultraviolet energy cutoff, which is not well defined. In this paper we discuss the temperature correction to the NiehYan anomaly. As distinct from the zero temperature term, the $T^2$ temperature correction does not depend on the ultraviolet cutoff and thus can be universal. Such $T^2$ NiehYan term may exist not only in the relativistic quantum field theories, but also in condensed matter with Weyl fermions. In the topological Weyl semimetals and in the chiral $p+ip$ superfluids and superconductors, this term is fully determined by the quasirelativistic physics in the vicinity of the Weyl nodes. [1] H. T. Nieh and M. L. Yan, J. Math. Phys. 23, 373 (1982). [2] H. T. Nieh and M. L. Yan, Ann. Phys.138, 237 (1982). Nissinen J., Volovik G.E. JETP Letters 110, issue 12 (2019) Nematic aerogels consist of nearly parallel strands. In liquid ^{3}He in such aerogels, the strands lead to anisotropy of ^{3}He quasiparticles scattering that makes favorable new superfluid phases: polar, polardistorted A (PdA) and polardistorted B [1]. A distinctive feature of this work is that experiments were performed with ^{3}He in two samples of nematic aerogel one of which was squeezed by 30% in the direction transverse to the strands. The squeezing leads to anisotropy in a plane perpendicular to the strands that can affect superfluid phases. It was found that the superfluid transition of ^{3}He in both samples occurred into the nonchiral polar phase, where no qualitative difference between properties of nuclear magnetic resonance in ^{3}He in these samples was found. The difference, however, has appeared on further cooling, after a transition to the chiral PdA phase. The results agree with theoretical expectations and provide an additional proof of existence of the polar phase of ^{3}He in nematic aerogels. The obtained quantitative characteristics of the observed phases also agree with recent theoretical paper [2] where it was stated that Anderson theorem for swave superconductors is applicable to superfluid ^{3}He in ideal nematic aerogel.
[1] V.V. Dmitriev, A.A. Senin, A.A. Soldatov, and A.N. Yudin, Phys. Rev. Lett. 115, 165304 (2015). [2] I.A. Fomin, JETP 127, 933 (2018). V.V. Dmitriev, M.S. Kutuzov, A.A. Soldatov, A.N. Yudin JETP Letters 110, issue 11 (2019)
After the discovery of graphene with its unique mechanical and electronic characteristics, a number of other quasitwodimensional carbon structures were theoretically predicted, including octagraphene [1], pentagraphene [2], ψgraphene [3], StoneWales (SW) graphene [4], as well as their various hydrogenated versions (graphane [5], pentagraphane [6], ψgraphane [7] etc.). In this paper, SW graphane  a new allotropic modification of graphane is proposed. This quasitwodimensional structure is formed upon complete twoside hydrogenation of SW graphene. SW graphene is more thermodynamically stable than most other allotropic modification of carbon. This justifies possibility of the SW graphane formation. Unlike graphane, SW graphane is an anisotropic and soft material. Depending on the direction, its Young's modulus is 194  221 N/m, whereas in isotropic graphane it is 249 N/m. The density of phonon states in SW graphane differs from that in graphane. There are no sharp peaks in the density of phonon states of SW graphane, which are typical for graphane. The densities of electronic states in SW graphane and pristine graphane slightly differ from each other. As well, as for graphane, the main channel of thermal decomposition of SW graphane is the separation of atomic hydrogen. The desorption energies of hydrogen atoms for graphane and SW graphane are also very close. 1. X.L. Sheng, H.J. Cui, et al., J. Appl. Phys. 112, 074315 (2012). 2. S. Zhang, J. Zhou, et al., Proc. Natl. Acad. Sci. U.S.A. 112, 2372 (2015). 3. X. Li, Q. Wang, P. Jena, The J. of Phys. Chem. Lett. 8, 3234 (2017). 4. H. Yin, X. Shi, et al., Phys. Rev. B 99, 041405 (2019). 5. J. O. Sofo, A. S. Chaudhari, and G. D. Barber, Phys.Rev. B 75, 153401 (2007). 6. H. Einollahzadeh, et al., Sci. Technol. Adv. Mater. 17, 610 (2017). 7. X. Huang, M. Ma, L. Cheng, and L. Liu Physica E 115, 113701(2020).
Podlivaev A.I. JETP Letters 110, issue 10 (2019)
At present the interest to Coulomb impurity centers in semiconductors, particularly in silicon and germanium, is revived due to their natural zerodimensional origin . The specific properties of such centers and advancement in modern technology allow one to create, a qubit with optically controlled coherent states [1], or a source of the THz coherent radiation which utilizes the conventional laser scheme or stimulated Raman scattering [2]. Such applications require accurate knowledge of optical excitation and relaxation processes within an impurity center.
In weakly and moderately doped semiconductors, the lifetime of excited states for a shallow impurity center is controlled by phononassisted relaxation. Recently [3], the relaxation times for arsenic donor states in bulk germanium have been calculated; these values are encouraging and suggest that the population inversion and THz lasing can be realized under The present work is devoted to studying the lowtemperature relaxation of the excited states of As donors in Ge crystal using a pumpprobe technique. We show that the lifetime of lower odd parity 2p states are close to one ns. At the same time, experimental study of the inverse relaxation rate for the first excited state 1s(T_{2}) yields value not longer than 160 ps. The data obtained are compared with the results of theoretical calculations [3] and confirm the possibility to reach THz amplification on the 2p – 1s(T_{2}) transitions of optically excited As donors in Ge.
Zhukavin R. Kh., Kovalevskii K.A., Choporova Yu. Yu. et al. (Collaboration) JETP Letters 110, issue 10 (2019)
Electron spin resonance (ESR) is one of the most fruitful approaches for the exploration of spin physics in a great deal of different materials including twodimensional electron systems (2DES) confined in semiconductor heterostructures [1]. The conventional technique for the observation of spin resonance in a 2DES relies on the high sensitivity of a 2D electron channel resistance to the absorption of microwave radiation in the regime of integer quantum Hall effect. In the presented manuscript we propose the complementary experimental approach for the ESR detection as a sharp peak in the microwave induced photovoltage measured between the ohmic contacts to the 2DES. In the presented manuscript we have demonstrated that the suggested experimental approach works well in different semiconductor heterostructures and in various contact geometries. Detection of ESR in such a way requires no current flow through the sample, thereby protecting 2DES from potential overheating, and from resulting negative impact on subtle physical phenomena like highorder fractional quantum Hall effect [3]. Furthermore, the flow of nonequilibrium charge carriers that is responsible for the generated voltage is at least partly spin polarized, as spin dephasing time in the quantum Hall regime [4] exceeds the transport scattering time. [1] M. Dobers, K. v. Klitzing, and G. Weimann, Phys. Rev. B 38, 5453 (1988). [2] D. Stein, K.v. Klitzing and G. Weimann, Phys. Rev. Lett. 51, 130 (1983). [3] R. Willett, J. P. Eisenstein, H. L. Stoermer, D. C. Tsui, A. C. Gossard, and J. H. English Phys. Rev. Lett. 59, 1776 (1987) [4] A. V. Shchepetilnikov, Y. A. Nefyodov, and I. V. Kukushkin, JETP Lett. 97, 574 (2013).
Periodic driving transforms the stationary energy spectrum into the Floquet modes spectrum (quasienergies). This can be associated with the socalled synthetic dimension introduced by the Floquet modes [1, 2]. Perturbation frequency in this case becomes an additional degree of freedom, which opens new ways of manipulating the quantum systems spectrum. In this context, periodic driving can introduce phenomena, which are typical for higher dimensional systems, in lower dimensional samples. In a finite system, periodic driving can effectively change its topology (connectivity of tunneling paths). In present letter, we study interference features in the highfrequency conductance of a twostate model system within the Keldysh formalism for nonequilibrium Green functions in tightbinding basis. We provide a clear and illustrative correspondence between highfrequency response and stationary transmittance of spatially symmetric configurations of the model system considered. In particular, we show that the synthetic frequency dimension provides the possibility for effective degeneracy of eigenstates in a simply connected linear quantum conductor, which is impossible in statics. It turns to be the dynamical counterpart of the situation considered in [3] for stationary tunneling. In dynamical transport, this phenomenon manifests itself by the destructive quantum interference and resonance coalescence, described by an exceptional point of a generalized transmission coefficient. As a result, for instance, one can observe a dip in the real part of the conductance at resonant frequency. [1] E. Lustig, S.Weimann, Y. Plotnik et al. Nature 567, 356 (2019). [2] L.Yuan, Q. Lin, M. Xiao et al. Optica 5 (11), 1396 (2018). [3] A. A. Gorbatsevich, G.Ya. Krasnikov, and N. M. Shubin. Scientific Reports 8, 15780 (2018). Gorbatsevich A.A., Shubin N.M. JETP Letters 110, issue 9 (2019)
Specific features of the band structure of transition metal dichalcogenides (TMDCs) monolayers — the presence of two valleys, strong spin – orbit interaction — have recently become the subject of a large number of theoretical and experimental studies. Relatively few papers are available on the spatially inhomogeneous problems with TMDCs – quantum dots and quantum wires (QW). In the present work we consider a QW made of TMDCs monolayer in the form of straight strip. Our analysis is based on the Diractype Hamiltonian with the finite gap and with accounting for the spin splitting both conduction and valence bands [1, 2, 3]. We use the boundary condition for the electron wave function proposed in [4] which is a special case of the more general consideration given in [5]. Our main findings are: 1. There exists a certain critical value of the strip width L= L_{cr} that separates two types of the electron spectrum: for L> L_{cr} there are energy levels (subbands in which energy depends on the momentum along the wire) lying within the band gap of an infinite sample, while at L < L_{cr} such states are absent. Note, that in conventional QW for particles with parabolic dispersion law there are no states in the forbidden gap for any value of width. 2. The optical absorption of the QWs in question differs essentially from the one in conventional QWs. First of all, for the interband transitions there is no strict selection rule Dn=0 where n is the number of the transversal subbands in the valence band and the conduction band (cf. with conventional QWs where only interband transitions at Dn=0 are allowed). However in our case the transitions with Dn=0 are still much more intensive than others. Second, depending on the mutual parity of the numbers of size quantization subbands in the valence and conduction bands, optical transitions are characterized by significantly different threshold behavior of the absorption intensity. Namely, for the transitions even – even or odd – odd types the threshold dependence of the absorption is I µ (ww_{0})^{1/2} while for even – odd and odd – even cases we obtain I µ (ww_{0})^{1/2}.
[1] D.Xiao et al., Phys.Rev.Lett., 108, 196802 (2012). [2] A.Kormanyos et al., 2D Materials, 2, 022001 (2015). [3] V.V.Enaldiev, Phys.Rev.B, 96, 235429 (2017). [4] M.V.Berry and R.J.Mondragon, Proc.R.Soc.Lond., A412, 53 (1987). [5] V. A. Volkov and T. N. Pinsker, Sov. Phys. Solid State 23, 1022 (1981).
R.Z.Vitlina, L.I.Magarill, A.V.Chaplik JETP Letters 110, issue 8 (2019)
The discovery of superconductivity in ironbased pnictides and chalcogenides with a relatively high transition temperature has attracted considerable interest due to the unusual correlations between magnetism and superconductivity in these compounds [13]. Several theoretical models of superconductivity based on pair interactions associated with magnetic fluctuations have been proposed [37]. Much attention is paid to studying the interaction of superconductivity, nematicity of the electronic structure, and quantum paramagnetism in FeSe and FeSe_{1x}S_{x} compounds [8,9]. The coexistence of ferromagnetism and superconductivity in FeSe crystals doped with Bi_{2}Se_{3} was reported recently [10]. In the present work, the method of Mössbauer spectroscopy on ^{57}Fe nuclei was used to study magnetic correlations and possible structural and electronic transformations that are expected in the temperature range of nematic and superconducting transitions in single crystals of iron selenide doped with sulfur Fe (Se_{0.91 ± 0.01}S_{0.09 ± 0.01})_{1}_{δ}. It was found that at room temperature, FeSe_{0.91}S_{0.09 }samples have a tetragonal βFeSe structure of the PbO type (space group P4/mmm), which transforms into the orthorhombic phase when the crystal is cooled down to Ts ≈ 80 K. The temperature of the superconducting transition is = 10.1 . The temperature dependence of the hyperfine interaction parameters obtained from the Mössbauer spectra revealed a number of anomalies in the temperature range of the superconducting Tc, structural Ts, and nematic T* phase transitions. It was established that iron atoms are in a nonmagnetic state even in the region of helium temperatures, which is explained by the lowspin state of Fe^{2+} ions (3d^{6}, S = 0). It is shown that this state practically does not change at temperatures of transition to the superconducting state. This means that the lowspin state of iron ions is more likely a structural factor, and is not directly related to superconductivity. Thus, there is no effect of the suppression of magnetism by superconductivity. The electrical resistance and Mössbauer spectroscopy data show that in the Fe(Se_{0.91}S_{0.09})_{1}_{δ} crystal, the temperature of the nematic transition T* is about 200 K and is much higher than the temperature of the structural transition (Ts ≈ 80 K). The Debye temperature, obtained from Mössbauer data for the iron sublattice, is Θ_{M} = 478 K, which turned out to be much higher than in the undoped FeSe_{1}_{δ} compound (Θ_{M} = 285 K).
[1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am. Chem. Soc. 130, (2008) 3296. [2] X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, and D. F. Fang, Nature 453, (2008) 761. [3] M.V Sadovskii. PhysicsUspekhi 59(10), (2016) 947. [4] Y. Mizuguchi, Y. Hara, K. Deguchi, et al., Supercond. Sci. Technol. 23, (2010) 054013. [5] J. Paglione and R. L. Greene, Nat. Phys. 6 (2010) 645. [6] V.A. Gasparov, JETP 111(2), (2010) 313. [7] A.A. Kordyuk, Low Temp. Phys. 38, (2012) 888. [8] K.K. Huynh, Y. Tanabe, T. Urata, et al., Phys. Rev. B 90, (2014) 144516. [9] Q. Wang, Y. Shen, B. Pan, et al., Nature Materials 15 (2016) 159. [10] Y. Liu, X.Y. Pu, K. Zhao, X.S. Yang, Y. Zhao, Solid State Comm. 281, (2018) 27.
K.V. Frolov, I.S. Lyubutin, D.A. Chareev and M. AbdelHafiez JETP Letters 110, issue 8 (2019)
The original TKNN invariant [1] responsible for the Hall conductivity has been derived for the uniform magnetic field (constant both as a function of time and space coordinates). The expression for the Hall conductivity discussed in the present paper is an extension of the TKNN invariant to the case of varying (in space) magnetic fields. Therefore, its consideration is important and should be interesting for the wide audience. The non  renormalization of the Hall conductivity (given by the original TKNN invariant) by interactions has been discussed earlier [26]. But this consideration was limited by the case of constant magnetic fields. Now we present the proof that the QHE conductivity (given by our extension of the TKNN invariant) is robust to the introduction of interactions in the case of varying magnetic field. This result has never been obtained in the past, to the best of our knowledge.
In addition, the mathematical form of the topological invariant in phase space discussed here is somehow similar to the one of the topological invariant in momentum space composed of the two  point Green function. The latter topological invariant and its variations are used widely (see G.E. Volovik "Universe in a Helium droplet"). Now the Green function is substituted by its Wigner transformation depending on both space coordinates and momentum. The ordinary products are therefore changed to the Moyal (star) product, thus leading to the beautiful mathematical structure. The resulting expression may be used in the presence of interaction for the calculation of Hall conductivity. One simply has to insert to it the interacting (Wigner transformed) two  point Green function. The Green functions with larger number of legs do not contribute to the Hall conductivity (this also has been proved in the presented paper). We would also like to emphasize, that our proof is valid to all orders in the perturbation theory.
[1] D. J. Thouless, M. Kohmoto, M. P. Nightingale, and M. den Nijs, Phys. Rev. Lett. 49, 405(1982).
[2] Ryogo Kubo, Hiroshi Hasegawa, Natsuki Hashitsume, Journal of the Physical Society of Japan 14(1) (1959) 5674 DOI: 10.1143/JPSJ.14.56
[3] Q. Niu, D. J. Thouless, and Y. Wu, Phys. Rev. B 31, 3372 (1985).
[4] B. L. Altshuler, D. Khmel'nitzkii, A. I. Larkin and P. A. Lee, Phys.Rev.B 22, 5142 (1980).
[5] B.L. Altshuler and A.G. Aronov, Electronelectron interaction in disordered systems (A.L.Efros, M. Pollak, Amsterdam, 1985).
[6] S. Coleman and B. Hill, Phys. Lett. B159 (1985) 184 T. Lee, Phys. Lett. B171 (1986) 247
C.X.Zhang, M.A.Zubkov Landau quantization in a twosubband Fermi electronic system placed in an external perpendicular magnetic field B leads not only to the wellknown Shubnikov – de Haas (SdH) oscillations, but also to another type of quantum resistance oscillations — the magnetointersubband oscillations (MISO) [1, 2]. MISO are not suppressed by the temperature broadening of the Fermi distribution function and therefore allow one to study quantum transport under conditions when SdH oscillations cannot be used for these purposes [3, 4]. The present work is devoted to the study of MISO in a onedimensional (1D) lateral superlattice (LSL), where 1D periodic potential is applied to a twosubband electronic system. The 1D LSL was created on the basis of a selectively doped GaAs/AlAs heterostructure [5, 6]. The measurements were carried out using Hall bars fabricated by means of optical lithography and wet etching. The 1D LSL of period a = 300 nm was created as an array of metal strips on a planar surface of Hall bars using electron beam lithography and the method of exploding an Au/Ti bilayer metallic film. The potential modulation in the studied LSL arises without applying voltage to the metal strips. One of the reasons for this modulation is elastic mechanical stresses between metal strips and a heterostructure [7]. The measurements were carried out at the temperature T = 4.2 K in magnetic fields B < 2 T. It has been shown that commensurability oscillations (CO) of resistance coexist with MISO in the studied LSL. It has been found that 1D periodic potential in a twosubband electron system leads not only to COs but also to MISO amplitude modulation, which is caused by periodic modulation of Landau level width in a 1D LSL in external inverse magnetic field. It has been shown that increased intersubband scattering time in a twosubband system under 1D periodic potential modulation is one of the reasons of MISO amplitude damping in a 1D LSL. [1] V. M. Polyanovskii, Sov. Phys. Semicond. 22, 1408 (1988). [2] D. R. Leadley, R. Fletcher, R. J. Nicholas, F. Tao, C. T. Foxon, and J. J. Harris, Phys. Rev. B 46, 12439 (1992). [3] A. A. Bykov, A. V. Goran, and S. A. Vitkalov, Phys. Rev. B 81, 155322 (2010). [4] O. E. Raichev, Phys. Rev. B 81, 195301 (2010). [5] K.J. Friedland, R. Hey, H. Kostial, R. Klann, and K. Ploog, Phys. Rev. Lett. 77, 4616 (1996). [6] D. V. Dmitriev, I. S. Strygin, A. A. Bykov, S. Dietrich, and S. A. Vitkalov, JETP Lett. 95, 420 (2012). [7] Ivan A. Larkin, John H. Davies, Andrew R. Long, and Ramon Cuscó, Phys. Rev. B 56, 15242 (1997).
A.A. Bykov, I.S. Strygin, A.V. Goran, D.V. Nomokonov, I.V. Marchishin, A.K. Bakarov, E.E. Rodyakina, A.V. Latyshev JETP Letters 110, issue 5 (2019).
It has been shown in [1] that any photoluminescent body in thermal equilibrium obeys the following relation: $ P(\lambda_1, T) F(\lambda_1, \lambda_2, t) = P(\lambda_2, T) F(\lambda_2, \lambda_1, t) $ (1) where P(λ, T) is the Planck function, which describes the spectral density of thermal radiation at wavelength λ and temperature T, and F(λ_{1}, λ_{2}, t) is the timeresolved excitationemission matrix, which describes the probability density of emitting a photon with wavelength λ_{2} at time t as a result of absorption of a photon with wavelength λ_{1} at time t = 0. For fixed λ_{1}, the function F(λ_{1}, λ_{2}, t) is the photoluminescence spectrum PL(λ; λ_{0}, t) at time t after a shortpulse excitation at wavelength λ_{0}: PL(λ; λ_{0}, t) = F(λ_{0}, λ, t). For fixed λ_{2}, the function F(λ_{1}, λ_{2}, t) is the photoluminescence excitation spectrum PLE(λ; λ_{0}, t) detected at wavelength λ_{0} at time t after a shortpulse excitation at wavelength λ: PLE(λ; λ_{0}, t) = F(λ, λ_{0}, t). Equation (1) rearranged to $ \frac{ PL(\lambda;\lambda_0, t) }{ PLE(\lambda;\lambda_0, t) } = \frac{ P(\lambda, T) }{ P(\lambda_0, T) } $ (2) is a new universal photoluminescence law stating that for any luminophore in thermal equilibrium, the ratio of the corresponding timeresolved photoluminescence and photoluminescenceexcitation spectra, PL(λ; λ_{0}, t) and PLE(λ; λ_{0}, t), is equal to the ratio of blackbody radiation spectra at wavelengths λ and λ_{0}. For fixed λ_{1} and λ_{2}, the function F(λ_{1}, λ_{2}, t) is the kinetics of decay of photoluminescence excited instantaneously at λ_{1} and detected at λ_{2}. Since the righthand side of equation (2) does not depend on time, the lefthand side is also timeindependent. This means that the photoluminescence decay kinetics is invariant under interchange of the excitation and detection wavelengths up to a timeindependent factor. The aim of the present study is to test the relation (2) experimentally by measuring the photoluminescence decay kinetics with interchanging the excitation and detection wavelengths. This implies that when the forward process is a Stokes photoluminescence, then the reverse process is an antiStokes photoluminescence. Colloidal solutions of InP/ZnS quantumdot nanoclusters, which do not obey the Vavilov law about the independence of the photoluminescent properties of a luminophore of the excitation wavelength, have been used in the study to test the invariance of the decay kinetics under interchange of the excitation and emission wavelengths. [1]. S. A. Tovstun, V. F. Razumov, et all. // J. Lumin. 190, 436 (2017).
Razumov V.F., Tovstun S.A., Kuzmin V.A. JETP letters 110, Issue 5 (2019)
Studies of oscillatory magnetotransport effects are one of the most reliable methods for investigating energy spectrum of 2D carrier systems. A magnetic field normal to the plane of a 2D gas leads to orbital quantization of the spectrum and, as a consequence, to the appearance of oscillations of the magnetoresistance (ρ_{xx}) at low temperatures (Shubnikov de Haas oscillations). These oscillations are periodic in the inverse magnetic field and their frequency f is determined by the carrier concentration. In systems in which two (or several) branches of the spectrum E_{1,2} (k) are filled, the oscillations are observed with frequencies f_{1} and f_{2}, determined by the carrier concentration in its branch. The sum of these oscillations manifests itself as a beating of oscillations of ρ_{xx},, causing nodes and antinodes at certain magnetic fields. In the presence of transitions between the branches, new oscillations arise with a difference frequency, f_{1}  f_{2}. They are called magnetointersubband oscillations (MISO) [1,2]. The simplest qualitative examination shows that the positions of the antinodes in magnetic field must coincide with the ρ_{xx} maxima of MISO. Such mutual positions of the antinodes and the MISOs were investigated for the structures based on widegap semiconductors with double quantum wells, for wide quantum wells where two branches of the spectrum are formed due to the Coulomb repulsion of electrons, and for structures with two filled subbands of the size quantization. Along with the cases described above, the two branches of the spectrum for a single quantum well can arise due to the strong spinorbit (SO) interaction. The large SO splitting can occur for the quantum wells of narrowgap (InAs, InSb) and gapless (HgTe, HgSe) semiconductors, as well as for many new topological insulators. Such MISO oscillations were considered only theoretically [3, 4], but were never observed experimentally. This work reports an experimental study of r_{xx} in the gated structures with HgTe quantum wells of 820 nm widths with an inverted spectrum. It was found that, unlike all other cases and theoretical predictions, the mutual position of the antinodes and MISO is quite opposite. Namely, the positions of the antinodes in a magnetic field coincide with the ρ_{xx} minima of MISO. A possible reason for such unusual behavior is discussed. 1. .. , , 22, 2230. (1988) 2. D.R Leadly, R. Fletcher and R. J. Nicholas, Phys. Rev.B, 46, 12439 (1992) 3. M. Langenbuch, M. Suhrke and U. Ro^¨ssler, Phys. Rev. B 69, 125303 (2004) 4. S. G. Novokshonov, Low Temperature Physics 39, 378 (2013) G.M. Minkov, O.E. Rut, A.A. Shestobitov, S.A.Dvoretski, N.N. Mikhailov JETP Letters 110, issue 4 (2019)
Precisely mapping the phase diagram of stronglyinteracting matter is a challenging problem. Lattice simulations of QCD, the field theory of strong interactions, are reliable at zero density, but become less precise when the density is finite and at the moment are not capable to map the whole phase diagram of strong interactions.
The most dramatic phenomenon that happens when strongly interacting matter is heated to extreme temperatures is restoration of chiral symmetry, a key symmetry of QCD that largely determines properties of hadrons and interactions among them. Chiral symmetry restoration is a sharp crossover at zero density that happens at temperature $T_{0}\simeq 157\,{\rm Mev}$ accurately known from lattice simulations \cite{Bazavov:2018mes}. Various model estimates predict that at larger baryon densities the crossover becomes sharper and eventually merges into a line of firstorder phase transitions at a critical endpoint whose precise location on the $T\mu $ plane is not entirely known. Model estimates vary by a factor of a few \cite{Stephanov:2004wx} depending on the model assumptions.
The order parameter of the chiral symmetry breaking is the quark condensate $ \bar{\psi }\psi$, which has a nonzero expectation value in the vacuum. The pseudoGoldstone modes that arise from the chiral phase of the condensate: $\bar{\psi }\psi \sim \Sigma\,{\rm e}\,^{\gamma ^5 T^{a}\pi _{a}} $ are identified with pions, kaons and the $\eta $meson, which are substantially lighter than other hadrons. Chiral symmetry restoration is typically associated with melting of the quark condensate, but can also proceed via disordering of the condensate's phase. Strong pion fluctuations, such that $\left\langle \mathop{\mathrm{tr}}\,{\rm e}\,^{iT^{a}\pi _{a}}\right\rangle=0$, will restore chiral symmetry even if condensate's modulus is nonzero.
This paper studied this slightly unconventional scenario of chiral symmetry restoration. Following \cite{Zarembo:2001wr} the shape of the pseudocritical line on the $T\mu $ plane can then be predicted from the lowenergy effective field theory. An interesting consequence of this scenario is an absence of the critical endpoint. The symmetry restoration always proceeds through a crossover which moreover becomes weaker with growing baryon density.
1. A. Bazavov et al., 1812.08235.
2. M. A. Stephanov, Prog. Theor. Phys. Suppl. 153, 139 (2004). 3. K. Zarembo, JETP Lett. 75, 59 (2002). K. Zarembo JETP Letters 110, issue 3 (2019).
Recently, a number of quasitwodimensional (Q2D) hightemperature and intermediatetemperature superconductors have been discovered. The anisotropic upper magnetic critical fields in some of them can be described by the LawrenceDoniach model, which is relevant to Q2D superconductors with high anisotropic properties. On the other hand, there are many Q2D superconductors with intermediate anisotropy of the upper critical magnetic fields, which are usually described by the socalled effective masses (EM) model, partially based on the anisotropic GinzburgLandau equations. The most popular such Q2D compounds are MgB_{2 }and Febased superconductors [1]. It is possible to define anisotropic ratio in Q2D superconductors, g, as the ratio of the parallel and perpendicular upper critical magnetic fields, which is always bigger than 1, g > 1. In accordance with EM model, the ratio g doesn’t have to depend on temperature. Meanwhile recent experiments show strong temperature dependence of anisotropy g, which in the case of superconductor MgB_{2} increases with decreasing temperature. The previous explanations of this phenomenon were based on some approximate manyband calculations of the upper critical magnetic fields and were prescribed to manyband effects. In this Letter, we investigate anisotropy ratio, g, more carefully by using derivation and investigation of an integral equation for the socalled superconducting nucleus, using the Gor’kov equations for nonuniform superconductivity (see, for example, the corresponding derivations for a 3D isotropic case in Ref.[2]) . For the first time, we show that the superconducting nucleus is not of the Gaussian shape for the parallel upper critical magnetic field and even changes its sign with space coordinate. This circumstance breaks down the EM model and predicts a factor of 1.3 increase of the upper critical magnetic fields ratio, g, with decreasing temperature. We prescribe the experimentally observed increase of the parameter g in the superconductor MgB_{2 }[1] to the breakdown of the EM model suggested in the Letter. This issue is an important one since Q2D hightemperature and intermediatetemperature superconductors are good candidates for some scientific and industrial applications in high magnetic fields. [1] See, for example, review V.G. Kogan and R. Prozorov, Rep. Prog. Phys. 75, 114502 (2012). [2] L.P. Gor’kov, Sov. Phys. JETP, 37(10), 42 (1960). Lebed A.G. JETP Letters 110, issue 3 (2019).
The formation of metallic hydrogen, predicted in [1], was observed experimentally in [2]. It is also assumed that this state of solid hydrogen is a superconductor at room temperature. However, the possibility of practical application of the metallic hydrogen is significantly limited by the pressure of formation of this state. The properties of stability and metastability of metallic hydrogen depend on the structure, which determines the relevance of the theoretical study of this issue. As it was shown in [3  6], atomic metallic hydrogen at zero temperature exists in a metastable state up to normal pressure.
[1] E. Wigner, H. B. Huntington, J. Chem. Phys. 3, 764 (1935).
I.M. Saitov
There are a number of physical systems in which, under certain conditions, spatially ordered electronic superstructures are formed. Charge and spin density waves (CDW and SDW), Wigner crystals and vortex lattices in typeII superconductors in a magnetic field are examples of such systems. The interaction of the superstructure with local lattice imperfections (various point defects, impurities, etc.) leads to its pinning. In the simplest case, such a pinning (let's call it local) is divided into collective (weak) and individual (strong).
In the present work, it is experimentally shown that, in the Peierls conductor {\it o}TaS$_3$, a new type of CDW pinning appears as a result of samples quenching. It is characterized by a number of fundamental differences from pinning by local pinning centres, namely:
The presence of these features allows us to conclude that quenching defects are macroscopic (nonlocal) objects, for example, dislocations, which can glide along the crystal. They lead to a previously unknown type of CDW pinning with properties different from local pinning ones. The feature of Peierls conductors is strong CDW interaction with defects. As a result of this interaction, forced diffusion of quenching defects and their exit from the crystal takes place during lowtemperature thermocycling.
V.E.Minakova, A.M.Nikitina, S.V.ZaitsevZotov JETP letters, v. 110, issue1 (2019)
In connection with recent report on the first detection of terahertz (THz) emission due to intraexciton radiative transitions in semiconductors [1] and a number of theoretical works predicting the possibility to achieve intraexciton population inversion at intense bandtoband optical excitation of a crystal (see [2] and also [3] and other references therein) it is very important to verify experimentally the possibility of implementing an exciton THz laser. In this work, we studied the THz photoluminescence (PL) from highpurity silicon due to radiative transitions between the energy levels of free excitons under conditions of continuouswave interband photoexcitation with a maximum density of up to 120 W/cm^{2}. The appearance of the superlinear dependence of the intensity of the intraexciton THz emission on the pump intensity at temperatures above 2025 K was found. The transition from the linear to superlinear dependence of the THz PL intensity on the pump intensity occurs at a photoexcitation density of about 7 W/cm^{2}. The observed regular patterns are explained by the appearance of the THz stimulated emission and, accordingly, population inversion in the system of excitons at their high density. The THz gain spectrum was obtained, which shows the lines at 13.7 and 15.5 meV, the gain values of which are 0.5 and 1 cm^{1}, respectively, at 25 K and the photoexcitation density of order of 35 W/cm^{2}. The line at 13.7 meV is due to the population inversion between highly excited exciton states and the ground state of free excitons. The gain line at 15.5 meV possibly corresponds to the population inversion between the twoexciton and biexciton states. The values of the terahertz gain indicate that a new type of THz laser can be created on transitions between energy levels of free excitons in silicon under conditions of interband photoexcitation.
[1] A.V. Andrainov, and A.O. Zakhar’in – Intrinsic Terahertz Photoluminescence from Semiconductors – Appl. Phys. Lett., 112, 041101 (2018). [2] M. Kira, and S.W. Koch  ExcitonPopulation Inversion and Terahertz Gain in Semiconductors Excited to Resonance  Phys. Rev. Lett., 93, 076402 (2004). [3] G.K. Vlasov, and S.G. Kalenkov – Sources of Coherent FarInfrared Radiation on Hot Excitons in Crystals  Int. J. Infrared Millimeter Waves, 4, 955 (1983). Zakhar’in A.O., Andrianov A.V. Petrov A.G. JETP letters, v. 109, issue12 (2019)
Nonlinear Hall effect has been predicted in a wide class of timereversal invariant materials [1], like topological crystalline insulators, twodimensional transition metal dichalcogenides, and threedimensional Weyl and Dirac semimetals. Recently, the timereversalinvariant nonlinear Hall (NLH) effect has been reported for twodimensional layered dichalcogenides [2, 3]. It stimulates a search for the Berry curvature dipole induced NLH effect in threedimensional crystals, where Dirac and Weyl semimetals are excellent candidates.
In the experiments [2, 3] on twodimensional WTe$_2$, the the secondharmonic Hall voltage depends quadratically on the longitudinal current. On the other hand, topological materials are characterized by strong thermoelectric effects, which also appear as a secondharmonic quadratic signal. For this reason, it is important to experimentally distinguish between the Berry curvature dipole induced NLH effect and a thermoelectric response while searching for the NLH effect in nonmagnetic materials.
We experimentally investigate a nonlinear Hall effect for threedimensional WTe$_2$ and Cd$_3$As$_2$ single crystals, representing Weyl and Dirac semimetals, respectively. We observe finite secondharmonic Hall voltage, which depends quadratically on the longitudinal current in zero magnetic field, as it has been predicted theoretically. We demonstrate that secondharmonic Hall voltage shows oddtype dependence on the direction of the magnetic field, which is a strong argument in favor of currentmagnetization effects. In contrast, one order of magnitude higher thermopower signal is independent of the magnetic field direction. Thus, the magnetic field dependence allows to distinguish the nonlinear Hall effect from a thermoelectric response.
[1] Sodemann and Liang Fu, Phys. Rev. Lett., 115, 216806 (2015).
[2] Kaifei Kang, Tingxin Li, Egon Sohn, Jie Shan, Kin Fai Mak, arXiv:1809.08744 (2018).
[3] Qiong Ma, et al., arXiv:1809.09279 (2018).
Shvetsov O.O., Esin V.D., Timonina A.V., Kolesnikov N.N., Deviatov E.V. JETP Letters 109, issue 11 (2019)
Nonclassical squeezed light is one of the most attractive quantum objects. Squeezed light is in the center of scientific interest nowadays due to its unique features, such as entanglement of large number of photons, twinbeam correlations and suppressed variance of one of the field quadratures. Such light is very important for many applications in quantum information, quantum tomography and measurements with noise reduction beyond the standard quantum limit. It was shown that squeezed light can be presented as superposition of Schmidt modes, which are orthogonal and carry all its nonclassical features [1]. For applications it is necessary to be able to control and manipulate the properties and mode content of squeezed light. To solve this task, the scheme based on the sumfrequency generation process seeded by squeezed light was suggested [24]. The proposed scheme was able to block a certain temporal mode of nonclassical light by converting its photons into the sumfrequency mode with a narrow Gaussian spectral profile. In this work we develop further the idea of the sumfrequency generation with the squeezed light at the input and give detailed theoretical description of this process in the frame of Schmidt modes. We analyze the transformation of spectral properties of squeezed light and predict new effects. We describe quantumoptical gate which provides wide opportunities for managing the spectral signal of squeezed light and allows to control the Schmidt mode weights. The complete blocking of the signal in a certain Schmidt mode is shown to redistribute the weights of other modes and therefore gives the possibility of engineering the spectral and temporal properties of outgoing signal. In the full conversion regime the quantumoptical gate is demonstrated to transfer all the features of nonclassical squeezed vacuum state to the light generated in the sum frequency channel.
[1] P.R. Sharapova, O.V. Tikhonova, S. Lemieux et. al., Phys. Rev. A. 97, 053827 (2018) [2] A. Eckstein, B. Brecht and C. Silberhorn, Optics Express. 19, 13770 (2011). [3] B. Brecht, A. Eckstein, A. Christ et al, New J. Phys. 13, 065029 (2011). [4] V. Ansari, J. Donohue, B. Brecht and C. Silberhorn, Optica 5, 534550 (2018).
V.V. Sukharnikov, O.V. Tikhonova JETP Letters 109, issue 9 (2019)
In 2018, a number of experimental works [13] were published, in which it was shown that lanthanum hydrides at high pressures P = 150¸190 GPa are superconductors with very high critical temperatures T_{c} = 215¸260 K. The detected crystalline phase is considered to have FM3M symmetry and LaH_{10} stoichiometry. However, calculations of the phonon spectrum of this structure show that it is dynamically stable only for pressures of P>210 GPa, which is beyond the pressure range of experimental work. This paper presents the results of a search for new structures of lanthanum hydride, which could correspond to the experimental results [13] and would be dynamically stable at pressures in the range P = 150¸200 GPa. Based on quantum calculations in the framework of the density functional theory, a new structure of lanthanum hydride La_{2}H_{24} was predicted for the first time. This structure is dynamically stable up to pressures of the order of 150 GPa. It is a semimetal and has a low symmetry of crystal lattice P1. An important feature of the structure is the presence of quasimolecular hydrogen chains, which leads to the presence of frequencies of about 420 meV in the phonon spectrum, exceeding the maximum oscillation frequency of the metallic hydrogen FDDD phase (ω~360 meV). These properties allow us to expect to achieve a high superconducting critical temperature for lanthanum hydride La_{2}H_{24}. [1] A. P. Drozdov, V. S. Minkov, S. P. Besedin, P. P. Kong, M. A. Kuzovnikov, D. A. Knyazev, M. I. Eremets – Superconductivity at 215 K in lanthanum hydride at high pressures – arXiv:1808.07039. [2] M.Somayazulu, M.Ahart, A.Mishra, Z.M. Geballe, M.Baldini, Y.Meng, V.V. Struzhkin, and R.J.Hemley – Evidence for superconductivity above 260 K in lanthanum superhydride at megabar pressures – arXiv:1808.07695. [3] A. P. Drozdov, P. P. Kong, V. S. Minkov, S. P. Besedin, M. A. Kuzovnikov, S. Mozaffari, L. Balicas, F. Balakirev, D. Graf, V. B. Prakapenka, E. Greenberg, D. A. Knyazev, M. Tkacz, M. I. Eremets. Superconductivity at 250 K in lanthanum hydride under high pressures – arXiv:1812.01561.
Degtyarenko N.N., Grishakov K.S., Mazur E.A. JETP Letters 109, issue 6 (2019)
To date, a significant number of indirect observations indicating the existence of a superconducting state up to room temperature in some small regions of highly oriented pyrolytic graphite (HOPG) samples have been reported [1]. The main problem was that the superconducting regions included only a small amount of carbon material with an unknown structural nature and, consequently, showed poor reproducibility of the superconductivity effect for different samples of HOPG with the same macroscopic dimensions. Significant progress in controlling the effect of superconductivity was obtained by embedding multilayer multilayered graphene flakes into a polystyrene matrix, so that covalent bonds are formed between the multilayered graphene flakes and the polystyrene [2,3]. In those papers, we reported a current–voltage characteristic of Josephson type up to temperatures higher than room temperature. In the present paper, we show that for the resulting magnetic moment of the same composite a magnetic field dependence typical of superconductors is observed in the same temperature range where previously a Josephson currentvoltage characteristic was observed. In the experiment, we used a vibrating magnetometer of the PPMS9 series (Quantum Design) in the temperature range 2400 K and with magnetic fields of 0 – ± 10 T. The reason for the emergence of superconductivity in multilayered graphene, as was first discussed in [2,3], may be the formation of covalent bonds with the polystyrene, leading to deformation of the graphene. Such deformation can produce a shift or rotation at different angles, including the magic angle [4], of one layer of graphene relative to another in multilayered graphene flakes embedded in a polystyrene matrix. As a result, within the interface regions between the graphene layers, flat energy zones arise, which can lead to roomtemperature superconductivity [5]. [1] P. Esquinazi, N. García, J. BarzolaQuiquia, P. Rödiger, K. Schindler, J.L. Yao, M. Ziese, Indications for intrinsic superconductivity in highly oriented pyrolytic graph. Phys. Rev. B 78(1–8), 134516 (2008) [2] A.N. Ionov, Technical Physics Letters 41(7), 651 (2015) [3] A.N. Ionov, J Low Temp Phys, 185, 515 (2016). [4] Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, P. JarilloHerrero, Nature, 556, 43 (2018). [5] G. E. Volovik, JETP Lett. 107, 516 (2018). A.N.Ionov, M.P.Volkov, M.N.Nikolaeva JETP Letters 109, issue 3 (2019)
In the 2D developed hydrodynamic turbulence at high Reynolds numbers the formation of the Kraichnan direct cascade with a constant enstrophy flux is due to the appearance of the vorticity quasi  shocks, because of the compressibility of continuously distributed lines of the divorticity field ${\bf B}=\mbox{rot}\,\mathbf{\omega}$ [1]. This property follows directly from the frozenness equation for ${\bf B}$,
[1] E.A.Kuznetsov, V.Naulin, A.H.Nielsen, and J.J.Rasmussen, Phys. Fluids 19, 105110 (2007).
E.A. Kuznetsov, E.V. Sereshchenko, JETP Letters 109, issue 4 (2019). The quantum spin Hall insulator state (QSHI) is a twodimensional topological phase of matter with insulating 2D bulk state and a pair of spinpolarized gapless helical edge states. These edge states may have spintronic applications, which are made possible by the allnew demonstration of QSHI state at 100 K performed on a WTe2 monolayer [1]. However, device engineering involving monolayer materials is challenging, often because of structural or chemical instabilities. The realistic candidates for hightemperature QSHI in semiconductor heterostructures with mastered technological process are the threelayer InAs/Ga(In)Sb/InAs quantum wells (QWs) confined between widegap AlSb barriers [2]. Depending on their layer thicknesses, these QWs host trivial, QSHI and semimetal states. A major advantage of the threelayer QWs, compared to the widely studied HgTe QWs, is a temperatureinsensitive inverted bandgap [3], which under certain condition exceeds the value of 45 meV known for WTe_{2} monolayers [1]. This work reports experimental study of 2D semimetal state in InAs/GaSb/InAs QWs. Already observed in inverted HgTe QWs [4,5], these topologically nontrivial states are characterized by a nonlocal overlap between conduction and valence bands. To probe the bulk states of the grown sample, we carried out THz photoluminescence measurements and Landau spectroscopy. By analyzing experimental results, we have demonstrated the existence of a nonradiative recombination channel due to the overlap of the conduction and valence bands. [1] S. Wu, V. Fatemi, Q. D. Gibson et al. (Collaboration), Science 359, 76 (2018). [2] S. S. Krishtopenko and F. Teppe, Sci. Adv. 4, eaap7529 (2018). [3] S. S. Krishtopenko, S. Ruffenach, F. GonzalezPosada et al. (Collaboration), Phys. Rev. B 97, 245419 (2018). [4] Z. D. Kvon, E. B. Olshanetsky, D. A. Kozlov et al. (Collaboration), JETP Lett. 87, 502 (2008). [5] G. M. Gusev, E. B. Olshanetsky, Z.D. Kvon et al. (Collaboration), Phys. Rev. Lett. 104, 166401 (2010).
S.S.Krishtopenko, S. Ruffenach, F. GonzalezPosada et al. (Collaboration) JETP Letters 109, issue 2 (2019)
In connection with recent studies of extremely longliving cyclotron spinflip excitations [13] (CSFE)  actually magnetoexcitons in a quantum Hall electron gas, the contribution to light absorption related to such a magnetoexcitonic ensemble is discussed. The CSFE relaxation found experimentally in the unpolarized quantum Hall system created in a real GaAs/AlGaAs heterostructure reaches 100 $\mu$s [4] at finite temperature $T\!\simeq\!0.5\,$K,
S. Dickmann
JETP Letters 109, issue 1 (2019)
A gravitational wave signal, GW170817, from a binary neutron star merger has been recordedby the Advanced LIGO and Advanced Virgo observatories on August 17, 2017 [1]. The deep underwater neutrino telescope Baikal Gigaton Volume Detector (BaikalGVD) is currently under construction in Lake Baikal [2].In this work we present results of searches for highenergy neutrinos in coincidence with GW170817 by BaikalGVD. Two different time windowswere used for the search. First, a ±500 s time window around the merger was used to search for neutrinos associated with prompt and extended gammaray emission. Second, a 14day time window following the GW detection, to cover predictions of longerlived emission processes. Since background events from atmospheric muons and neutrinos can be significantly suppressed by requiring time and space coincidence with the GW signal, relatively weak cuts can be used for neutrino selection. For the search for neutrino events within a ±500 s window around the GW event, 731 events were selected, which comprise >5 hit light sensors at>2 hit strings. After applying cascade reconstruction procedures and dedicated quality cuts, two events were selected. Finally, requiring directional coincidence with GW170817y< 20° no neutrino candidates survived.The absence of neutrino candidates associated with GW170817 in the ±500 s window as well as in 14 day window allows to constrain the fluence of neutrinos from GW170817. Assuming an E^{2} spectrum singleflavor differential limits to the spectral fluence in bins of one decade in energy have been derived. In the range from 5 TeV to 10 PeV a 90% CL upper limit is 5.2×(E/GeV)^{2} GeV^{1}cm^{2}for ±500 s time window search. The corresponding upper limit to the spectral fluencefor 14 day search window is 9.0×(E/GeV)^{2} GeV^{1}cm^{2}over the same energy range.
A.D. Avrorin, A.V. Avrorin, V.M. Aynutdinov et.al. (Baikal Collaboration) JETP Letters 108, issue 12 (2018)
Transport phenomena in anisotropic porous media are widely discussed in the literature. We investigate the Knudsen regime diffusion in alumina aerogels~~high porosity materials composed of long cylindrical strands. The theory and experimental results for nematic aerogel with nearly parallel strands were reported earlier [1].
In the present paper we explore a different type of anisotropic aerogellike metamaterial, which we call the planar aerogel. Like nematic aerogel, it is a macroscopically uniform system with axial symmetry which consists of strands of diameter $10\,\text{nm}$. The directions of these strands, however, are uniformly distributed in a plane perpendicular to the symmetry axis (rather than parallel to it, as in nematic aerogel). Proposed theory is based on the assumption that elastic collisions with the strands is the most important scattering mechanism. We consider two opposite limits: specular and diffuse scattering (denoted by the subscripts $S$ and $D$). Axially symmetric diffusion tensor has two distinct principal values: $D^{xx}=D^{yy}$ for diffusion in the aerogel plane and $D^{zz}$ along the symmetry axis. From the theory it follows, somewhat surprisingly, that the diffusion anisotropy in the specular scattering model is smaller than that in the diffuse model: $D^{xx}_\text{S}/D^{zz}_\text{S}=1.97$ and $D^{xx}_\text{D}/D^{zz}_\text{D}=2.50$.
In the experiments we used the spin echo technique to investigate the spin diffusion in normal liquid $^3$He confined in the planar aerogel. At very low temperatures $T\sim 1\,\text{mK}$, where the Fermi quasiparticle population is small and the Knudsen regime is achieved, our experimental results are in a good agreement with the theory for the case of the specular scattering.
[1] V.V.Dmitriev, L.A.Melnikovsky, A.A.Senin, A.A.Soldatov, and A.N.Yudin, JETP Lett. 101, 808 (2015).
Dmitriev V.V., Kutuzov M.S., Melnikovsky L.A., Slavov B.D., Soldatov A.A.,Yudin A.N.
The non  dissipative transport effects have been widely discussed recent years. These effects are to be observed in the non  central heavy ion collisions [1]. They have also been considered for the Dirac and Weyl semimetals [2] and in $^3$HeA [3].
Among the other effects their family includes the chiral separation effect (CSE) [4], the chiral vortical effect (CVE) [5], the anomalous quantum Hall effect (AQHE) [2]. All those phenomena have the same origin  the chiral anomaly.
In the present paper we propose the new non  dissipative transport effect  the chiral torsional effect (CTE). Namely, we will discuss the emergence of axial current of thermal quasiparticles in the presence of torsion. It will be shown that this effect is intimately related to the chiral vortical effect [5], i.e. the latter may be considered as the particular case of the CTE. It is well  known that in conventional general relativity torsion vanishes identically, it appears only in its various extensions. However, the background (non  dynamical) gravity with torsion emerges in certain condensed matter systems. For example, elastic deformations in graphene and in Weyl semimetals induce the effective torsion experienced by the quasiparticles [6]. In $^3$HeA torsion appears dynamically when motion of the superfluid component is non  homogeneous.
[1] W. T. Deng and X. G. Huang, \Vorticity in HeavyIon Collisions," Phys. Rev. C 93, no. 6,
064907 (2016) [arXiv:1603.06117 [nuclth]]. [2] A. A. Zyuzin and A. A. Burkov, \Topological response in Weyl semimetals and the chiral anomaly," Phys. Rev. B 86 (2012) 115133 [arXiv:1206.1868 [condmat.meshall]]. [3] G.E. Volovik, The Universe in a Helium Droplet, Clarendon Press, Oxford (2003). [4] \Anomalous Axion Interactions and Topological Currents in Dense Matter",Max A. Metlitski and Ariel R. Zhitnitsky,Phys. Rev. D 72 (2005), 045011 [5] A. Vilenkin, Phys. Rev. D 22, 3080 (1980) [6] G.E.Volovik, M.A.Zubkov, Annals of Physics 340/1 (2014), pp. 352368, arXiv:1305.4665 [condmat.meshall].
Z.V.Khaidukov, M.A.Zubkov
JETP Letters 108, issue 10(2018)
Investigation of the superconductivity in novel ironbased superconductors is one of the main trends in modern condensed matter physics [1]. Some of iron chalcogenide superconductors [2] have qualitatively different electronic properties from other ironbased superconductors (e.g. iron pnictides) [3]. Among them, the K_{x}Fe_{2−y}Se_{2} compound and the FeSe monolayer on the SrTiO_{3} substrate take quite a special place. Early days angular resolved photoemission spectroscopy (ARPES) experiments practically could not resolve holelike Fermi surface sheets near the Γpoint of the Brillouin zone in contrast to the iron pnictides and some iron chalcogenides (e.g. bulk FeSe). Recently in the work [4] ARPES observation of a “hidden” holelike band approaching the Fermi level near the Γpoint for the K_{0.62}2Fe_{1.7}Se_{2} system and thus proposing a holelike Fermi surface near the Γpoint was reported. Inspired by the work [4] we show by LDA+DMFT [6] study that for K_{0.62}Fe_{1.7}Se_{2} system near the Γpoint there are two holelike bands crossing the Fermi level and forming the Fermi surface near the Γpoint. Its appearance can justify spinfluctuation mechanism of superconductivity in this class of systems [6] with a rather high critical temperature T_{c}∼30K. Good qualitative and even quantitative agreement of the calculated and ARPES Fermi surfaces is obtained. ^{1}M.V. Sadovskii. Usp. Fiz. Nauk 178, 1243 (2008). ^{2}M.V. Sadovskii. Usp. Fiz. Nauk 186, 1035 (2016). ^{3}M.V. Sadovskii, E.Z. Kuchinskii, I.A. Nekrasov, JMMM 324 3481, (2012). ^{4}M. Sunagawa et al., J. Phys. Soc. Jpn. 85, 073704 (2016). ^{5}K. Held et al. Int. J. Mod. Phys. B 15, 2611 (2001). ^{6}P.J. Hirshfeld, M.M. Korshunov, I.I. Mazin. Rep. Prog. Phys. 74, 124508 (2011). I.A.Nekrasov, N.S.Pavlov JETP Letters 108 , issue 9 (2018)
The discovery of solar and atmospheric neutrino oscillations means that at least two of the three mass neutrino states are nonzero. Certain values of the oscillation parameters together with restrictions on the sum of the light neutrino masses obtained from the Planck space telescope data limit the heaviest mass state (ν1, ν2, ν3) of three known types of neutrinos (νe, νμ, ντ) to 70 meV. The measured decay width of the Zboson indicates that the heavier neutrino mass states, if they exist, must be related to the sterile neutrino. The simplest mechanism of mass formation is ensured by the existence of righthanded, sterile neutrino interactions. Such neutrinos can be mixed with three active types of neutrinos. The mixing effect leads to neutrino oscillations, it can manifest itself in the processes of production of active neutrinos and lead to the decay of sterile neutrinos into particles of the Standard Model (SM). Sterile neutrinos, in one form or another, appear in many extensions of the SM, they are wellmotivated candidates for the role of dark matter particles. Although the search for sterile neutrinos has been conducted for many years, convincing results of their existence have not yet been obtained [1]. This paper is devoted to the search for the manifestations of massive neutrinos in the measured electron spectra arising from the decay of nuclei ^{144}Ce – ^{144}Pr. The source of electronic antineutrinos ^{144}Ce – ^{144}Pr is one of the most suitable for studying neutrino oscillations into a sterile state with a mass of about 1 eV. We decided to test the possibility of radiation in these beta transitions of heavy sterile neutrinos with a mass of from 1 keV to 3 MeV. The range of possible studied neutrino masses is determined by the resolution of the spectrometer used [2] and the boundary energy of beta decay of the ^{144}Pr nucleus. A spectrometer consisting of a Si(Li) fullabsorption detector and a transition Sidetector was used for precision measurements of the electron spectrum arising from the beta decays of ^{144}Ce – ^{144}Pr nuclei. The beta spectrum measured during 364 h is analyzed to find the contribution from heavy neutrinos with masses from 10 keV to 1 MeV. For neutrinos with a mass in the range (150–350) keV, new upper limits on the mixing parameter at the level U_{eH}^{2} ≤ 2×10^{–3}  5×10^{−3} for 90% confidence level have been obtained. The achieved sensitivity to U_{eH}^{2} can be increased several times after precision measurement of the response function when using a 4πgeometry spectrometer, in which the response function for monochromatic electrons practically coincides with the Gaussian function [3]. [1]. K.N. Abazajian, M.A. Acero, S.K. Agarwalla et al. (Collaboration), Light Sterile Neutrinos: A White Paper, arXiv:1204.5379v1 (2012). [2]. I. E. Alexeev, S.V. Bakhlanov, N.V. Bazlov, E. A. Chmel, A. V. Derbin, I. S. Drachnev, I.M. Kotina, V.N. Muratova, N.V. Pilipenko, D.A. Semyonov, E.V. Unzhakov, V.K. Yeremin, Nuclear Inst. And Methods in Physics Research A 890, 647 (2018). [3]. A.V. Derbin, A. I. Egorov, I.A. Mitropolskii, V. N. Muratova, S.V. Bakhlanov, and L.M. Tukhkonen, JETP Lett. 65, 605 (1997).
A.V. Derbin, I.S. Drachnev, I.S. Lomskaya, V.N. Muratova. N.V. Pilipenko, D.A. Semenov, L.M. Tykhkonen, E.V. Unzhakov, A.Kh. Khusainov JETP Letters 108, issue 8 (2018)
The possibility to create, manipulate and detect spinpolarized currents is at the very heart of semiconductor spintronics [1]. Stationary spin polarized currents were successfully generated in various semiconductor heterostructures and lowdimensional mesoscopic samples [2]. However, controllable manipulation of charge and spin states, applicable for ultra small size electronic devices design requires analysis of nonstationary effects and transient properties [35]. Consequently, the problem of nonstationary evolution of initially prepared spin and charge state in correlated nanostructures (quantum dots, impurity atoms, etc.) is really vital. In the present paper we analyze nonstationary spinpolarized currents flowing through the correlated singlelevel quantum dot localized between nonmagnetic leads in the presence of applied bias voltage and external magnetic field. We reveal, that spin polarization and direction of the nonstationary currents can be simultaneously inverted by sudden changing of applied bias voltage. We also analyze time evolution of the spin polarization degree and demonstrate the possibility of its sign changing following the applied bias polarity. This effect opens the possibility for the spinpolarization train pulses generation with the opposite degree of polarization. Application of external magnetic field allows to consider correlated singlelevel quantum dot as an effective nonstationary spin filter. [1] I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys., 76, 323 (2004) [2] M.E. Torio, K. Hallberg, S. Flach, A.E. Miroshnichenko, M. Titov, Eur. Phys. J. B37, 399 (2004) [3] N.S. Maslova, I. V. Rozhansky, V.N. Mantsevich, P.I. Arseyev, N.S. Averkiev, E. Lahderanta, Phys. Rev. B 97, 195445 (2018) [4] V.N. Mantsevich, N.S. Maslova, P.I. Arseyev, Physica E, 93,224 (2017) [5] N.S. Maslova, P.I. Arseyev, V.N. Mantsevich, Solid State Comm. 248, 21 (2016)
Mantsevich V.N., Maslova N.S., Arseyev P.I. JETP Letters 108, №7 (2017)
It is well known that YangMills theory possesses a nontrivial topological structure: it has an innite series of energetically degenerate but topologically distinct classical vacua. At nite temperature thermal uctuations of elds can lead to (sphaleron) transitions between various vacuums. Due to the chiral anomaly the rate of these transitions describes the evolution of the chiral charge in Quantum Chromodynamics or baryon charge in electroweak theory. [1] G. D. Moore and M. Tassler, JHEP 1102, 105 (2011) doi:10.1007/JHEP02(2011)105 [arXiv:1011.1167 [hepph]].
A.Yu.Kotov JETP Letters 108, issue 6 (2018)
At the birth of quantum mechanics, E. Schrödinger realized that a free relativistic electron, described by the Dirac Hamiltonian, exhibits oscillations in space resulting from the interference of the positive and the negativeenergy solutions of the Dirac equation [1]. Recently, it was suggested that Zitterbewegung is not limited to free electrons but is a common feature of systems with a gapped or levelsplit spectrum exhibiting a formal similarity to the Dirac Hamiltonian [2]. Here, we study the motion of electrons in a semiconductor system with spinorbit coupling and the Zeeman gap opened by an external magnetic field. It is shown that, in addition to the wellknown Brownian motion, electrons experience an inherent trembling motion of quantummechanical nature. The effect originates from the fact that the electron velocity is not a conserved quantity and contains an oscillating contribution. The Zitterbewegung occurs for all the electrons, also for electrons in thermal equilibrium. Experimental study of the electron Zitterbewegung in such conditions requires the use of noise spectroscopy. We show that the Zitterbewegung of individual electrons can be phasesynchronized by initializing the electrons in the same spin state. In this case, the coherent precession of the individual electron spins drives their backandforth motion in real space giving rise to a macroscopic highfrequency electric current. Such a coherent Zitterbewegung is maintained as long as the coherent spin precession of the electrons is not destroyed by the processes of spin dephasing. We develop a theory of the coherent Zitterwebegung for the cases of ballistic and diffusive electron transport, predict its enhancement at the plasmon resonance conditions, and discuss its relation to the spingalvanic effect [3,4]. [1] E. Schrödinger, Über die kräftefreie Bewegung in der relativistischen Quantenmechanik, Sitz. Press. Akad. Wiss.Phys.Math. 24, 418 (1930). [2] W. Zawadzki and T. M. Rusin, Zitterbewegung (trembling motion) of electrons in semiconductors: a review, J. Phys.: Condens. Matter 23, 143201 (2011). [3] E.L. Ivchenko, Yu.B. LyandaGeller, and G.E. Pikus, Current of thermalized spinoriented photocarriers, Sov. Phys. JETP 71, 550 (1990). [4] S.D. Ganichev, E.L. Ivchenko, V.V. Bel’kov, S.A. Tarasenko, M. Sollinger, D. Weiss, W. Wegscheider, and W. Prettl, Spingalvanic effect, Nature 417, 153 (2002).
S. A. Tarasenko, A. V. Poshakinskiy, E. L. Ivchenko, I. Stepanov, M. Ersfeld, M. Lepsa, and B. Beschoten JETP Letters 108, issue 5 (2018)
Cyclotron resonance photoconductivity (CRP) is one of the power tools for study of the interaction of twodimensional particles with electromagnetic radiation especially after the discovery of microwave induced magnetoresistance oscillations [1] that have created a lot of questions in the area, where, after the issue of the wellknown review [2], it seemed that everything was clear. In this work, we report on the observation of CRP of twodimensional (2D) electrons under very unusual conditions – in 2D semimetal in that their number (109 – 1010) cm2 is much (from one to three orders) less than number of holes. So for the first time the cyclotron resonance have been observed from the electrons moving through the hole liquid, which strongly screens an impurity scattering potential and an electronelectron interaction. At first glance, it is impossible to observe CRP in this situation because of a very small absorption rate; however it has been detected in our experiments. Moreover, at 432 µm wavelength no decreasing of the CRP amplitude was observed when electron density decreased from 1010 cm2 to 109 cm2 . The experiments demonstrate that interaction of 2D electrons in semiconductor structures with the high frequency electromagnetic field is not so simple problem. It is likely there is a strong field enhancement in 2D system due to many particle effects in the spirit of a recent theory work [3]. Anyway, the further study of this phenomenon is of undoubted interest. [1] I. A. Dmitriev, A. D. Mirlin, D. G. Polyakov, and M. A. Zudov, Rev. Mod. Phys. 84, 1709 (2012). [2] T. Ando, A. B. Fowler, and F. Stern, Rev. Mod. Phys. 54, 673 (1982). [3] A. D. Chepelianskii, D. L. Shepelyansky, Phys. Rev. B 97, 125415 (2018). Z.D. Kvon JETP Letters 108, issue 4 (2018) Investigation of hybrid structures containing superconductors and magnetic materials attracts great interest due to different interesting phenomena such as spintriplet superconducting pairing, anomalous superconducting and magnetic proximity effects and other ones that were reviewed in several articles [15]. In this work, the spindependent electron transport phenomena have been studied theoretically for doublebarrier structures SIF_{1}FIF_{2}N, where S is a superconductor, F is a ferromagnetic metal, N is a normal metal, IF is a spinactive barrier. It was predicted that under certain conditions the negative differential resistance may be realized in the structures SIF_{1}FIF_{2}N, if the polarization at least one of the barriers is not small: R_{b↑}  R_{b↓} is of the order of ( R_{b↑} + R_{b↓} ), where R_{b↑} , R_{b↓} are the contributions to the (normal state) resistance of the barrier related with spinup and spindown electrons, respectively. It was shown that the negative differential resistance is realized if the superconducting proximity effect is strong, the thickness of the F layer is short enough, the exchange field in this layer is not small with respect to the superconducting energy gap Δ, and the spinorbit relaxation time due to impurity scattering in the F layer is significantly greater than ħ/Δ. Another investigated features of the differential resistance of the SIF_{1}FIF_{2}N structures are its voltage asymmetric dependences and its strong dependence on the mutual orientations of the exchange fields in the barriers and in the F layer, that is the reason of the giant magnetoresistance effect.
Zaitsev A.V. JETP Letters 108, issue 3 (2018) Nonlinear magnetotransport in twodimensional (2D) electron systems reveals fascinating novel physical phenomena such as quantal Joule heating [1], zero differential resistance [2] or conductance [3] states, and Zener tunneling between Landau levels [4]. The later effect is related to a backscattering of 2D electrons colliding with a short range, sharp impurity potential. The effect is considered to be absent for a smooth, long range disorder. Surprisingly, this paper shows that a longrange, smooth periodic modulation of the electrostatic potential affects significantly the electron backscattering leading to an unexpected interference of the Zener and commensurability oscillations of the magnetoresistance [5]. The electrostatic modulation is obtained via a fabrication of a periodic array of nanoscaled metallic strips with a period a = 200nm located on top of the studied samples. The interference leads to a dramatic modification of the commensurability oscillations of the magnetoresistance reminiscent of a beating pattern. Due to the long range periodic electrostatic modulation the proposed model relates the observed interference to a modification of the electron spectrum, in particular, the electron lifetime. The model is in a good agreement with the experiment, indicating the relevance of the proposed explanation. The obtained results indicate that the quantization of the electron spectrum is of a paramount importance for nonlinear electron transport in low dimensional systems. 1. Jing Qiao Zhang, Sergey Vitkalov and A. A. Bykov, Phys. Rev. B 80, 045310 (2009). 2. A. A. Bykov, J.Q. Zhang, S. A. Vitkalov, A. K. Kalagin, and A. K. Bakarov, Phys. Rev. Lett. 99, 116801 (2007). 3. A. A. Bykov, Sean Byrnes, Scott Dietrich, and Sergey Vitkalov, Phys. Rev. B 87, 081409(R) (2013). 4. C. L. Yang, J. Zhang, R. R. Du, J. A. Simmons, J. L. Reno, Phys. Rev. Lett. 89, 076801 (2002). 5. D. Weiss, K. von Klitzing, K. Ploog, and G. Weimann, Europhys. Lett. 8, 179 (1989).
A. A. Bykov, I. S. Strygin, E. E. Rodyakina, S. A. Vitkalov JETP Letters 108, issue 2 (2018) Recent progress on novel twodimensional metalbased compounds [1,2] have encouraged us to pay attention to this underinvestigated and highly promising class of materials. Here we would like to present the prediction of a new CoC phase which is very intriguing by uncommon symmetry as well as electronic and mechanical properties. In particular, both the ab initio bending analysis and phonon calculations have shown that 2D CoC demonstrates stability of orthorhombic lattice structure in contrast to probably more expected hexagonal or square types. Moreover, from electronic structure analysis, it was obtained that the cobalt net and carbons dimers are connected through a combination of covalent, ionic and metallic bonding. The estimated mechanical elastic modulus for 2D CoC are comparable to those for hBN and only 30% lower than for the “worldrecord” graphene, whereas Poisson’s ratios and flexural rigidity are higher (or equal) than for the wellknown 2D structures. The predicted metallic states of 2D CoC and promising mechanical properties might be of practical importance for future CoCbased heterostructure synthesis, whereas thorough description of potentially interesting magnetic and optical properties have to motivate further studies. [1] Kano, E.; Kvashnin, D. G.; Sakai, S.; Chernozatonskii, L. A.; Sorokin, P. B.; Hashimoto, A.; Takeguchi, M. OneAtomThick 2D Copper Oxide Clusters on Graphene. Nanoscale 2017, 9 (11), 3980–3985. [2] Zhao, J.; Deng, Q.; Bachmatiuk, A.; Sandeep, G.; Popov, A.; Eckert, J.; Rümmeli, M. H. FreeStanding SingleAtomThick Iron Membranes Suspended in Graphene Pores. Science 2014, 343 (6176), 1228–1232.
Larionov K.V., Popov Z.I., Vysotin M.A., Kvashnin D.G., Sorokin P.B. JETP Letters, 108, issue 1, 2018 Successful exfoliation of oneatomthick graphene layer from the graphite crystal in 2004 [1] stimulated the search for new twodimensional carbon nanostructures. In graphene each carbon atom is bonded to its three nearest neighbors, so that CC bonds form a pattern of hexagons, while pentagons are considered as topological defects. Recently, a new carbon allotrope, pentagraphene, composed entirely of pentagons, has been proposed [2]. Later, however, it was argued that pentagraphene cannot be made experimentally because, first, it is thermodynamically unstable and rapidly restructures toward graphene [3] and, second, intrinsic mechanical stress created by two mutually orthogonal sublattices of carbon dimers results in the growth of strongly curved rather than planar pentagraphene layers [4]. We draw attention to another weak point of pentagrafene, its thermal stability. Tightbinding molecular dynamics simulation showed that after the formation of a single defect of the StoneWales type, the disordered region does not remain localized, but rapidly spreads over the entire sample. The lifetime of the pentagrafene sample until complete disordering of its structure decreases exponentially with increasing temperature and is inversely proportional to the sample area. At room temperature, mesoscopic samples of pentagrafene may have rather high thermal stability. 1. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, and A.A. Firsov, Science 306, 666 (2004). 2. S. Zhang, J. Zhou, Q. Wang, X. Chen, Y. Kawazoe, and P. Jena, Proc. Nat. Acad. Sci. 112, 2372 (2015). 3. C.P. Ewels, X. Rocquefelte, H,W. Kroto, M.J. Rayson, P.R. Briddon, and M.I. Heggie, Proc Nat. Acad Sci. U S A. 112, 15609 (2015). 4. P. Avramov, V. Demin, M. Luo, C.H. Choi, P.B. Sorokin, B. Yakobson, and L. Chernozatonskii, J. Phys. Chem. Lett. 6, 4525 (2015).
Openov L.A., Podlivaev A.I. JETP Letters 107, issue 11 (2018) Until recently, the electromagnetic field has been considered as being quantum one with few photons and classical one with quite a few of them. Then a macroscopic quantum state of a field with many photons  a squeezed field  was discovered. In addition, the reverse case was also made possible: a onephoton wave packet may not prove to be a quantum one. An effect that is very sensitive to the state of the "onequantum" object, allowing us to distinguish between the classical and quantum states of a onephoton field was found in the present work. The effect is due to the possibility of complete suppression of collective decay of an ensemble of identical excited atoms localized within the area far smaller than that of the characteristic wavelength [1]. The wellknown Dicke model is generalized for accounting the interaction with a vacuum electromagnetic field of zero photon density up to the second  order algebraic perturbation theory [1,2]. Then the effects of quantum interference of various radiation processes are correctly described, and the dynamics of the atomic ensemble is characterized as nonWiener dynamics [1]. In this work, the joint effect of a broadband onephoton wave packet and a vacuum electromagnetic field on the atomic ensemble is investigated. The master equations of nonWiener dynamics are obtained in [3]. The state of onephoton field can both be prepared in two different ways and presented in different states. If such a field interacts with a localized excited atomic ensemble under suppression of collective decay, then a strong effect is observed. The case of semiexcited atomic ensemble is calculated analytically, which shows diametrically opposite difference in the type of radiation. The quantum onephoton source produces a pulse of superradiation (collective decay), whose intensity is proportional to the square of the number of atoms of the ensemble. On the other hand, in the case of a classical onephoton source an incoherent radiation is generated, similar to that of the one generated by the emission of independent atoms. 1. A.M. Basharov, Phys. Rev. A 84, 013801 (2011). 2. A.I. Maimistov, A.M. Basharov, Nonlinear optical waves, Dordrecht: Kluwer Academic, 1999. 3. A.I. Trubilko, A.M. Basharov. JETP, 2018 (in press)
A.I. Trubilko, A.M. Basharov JETP Letters 107 , issue 9 (2018) Experimental observation of the magnetic topological states  magnetic skyrmions in chiral magnets [1] caused the rising interest to them. Such attention is motivated both by the hopes to use their unique properties (such as high mobility in electric current) in novel spintronic devices and by their topologically caused attributes interesting to the fundamental condensed matter physics, topological Hall effect for example [2]. In the chiral magnets the magnetic skyrmions are naturally stabilized by weak relativistic Dzyaloshinskii–Moriya interaction and thus, the skyrmions can exist only within a narrow temperaturefield region which hinders their application. So the search of the possibilities of the skyrmion stabilization in the common magnetic materials at room temperature is the actual problem. The idea of our work is spatially modulate the energy of the domain wall surrounding skyrmion core by nanostructurisation of the film and so artificially create the potential well (or the lattice of such wells) for the skyrmionic state. This well will prevent skyrmion transformation to the labyrinth domain structure. The first possible way to the goal is to spatially modulate the material parameters of the magnetic film [3]. In this presented work we experimentally studied the alternative way of the nanostructurisation, namely the spatial modulation of the thickness of the CoPt multilayered film with the perpendicular anisotropy. The structure is the regular lattice (period is 300 nm) of the stubs (diameters is 150 nm) etched on the surface of the film. The magnetic force microscopy allows to observe skyrmion formation in the system during the magnetizing in the uniform perpendicular field. The skyrmons stay stable even after reducing the field to zero. The magnetization curve of the system is studied both by Hall magnetometry and by magnetooptical methods. The experimentally observed topological magnetic configurations and hysteresis loops are verified by micromagnetic simulations. [1] U. K. Rossler, N. Bogdanov, and C. Pleiderer, Spontaneous skyrmion ground states in magnetic metals, Nature (London) 442, 797 (2006). [2] N. Nagaosa and Y. Tokura, Topological properties and dynamics of magnetic skyrmions, Nat. Nanotech. 8, 899 (2013). [3] M.V. Sapozhnikov, S.N. Vdovichev, O.L. Ermolaeva, N.S. Gusev, A.A. Fraerman, S.A. Gusev, Yu.V. Petrov, Artificial dense lattice of magnetic bubbles, Appl. Phys. Lett. 109, 042406 (2016).
M. V. Sapozhnikov, O. L. Ermolaeva, E.V. Skorohodov, M.N. Drozdov JETP Letters 107, issue 6 (2017)
In the bulk of a superfluid, besides wellknown and experimentally observed quantum vortex rings, theoretically there can exist (developing in time) also solitary topologically nontrivial excitations as vortex knots [13]. The simplest of them are torus knots ${\cal T}_{p,q}$, where $p$ and $q$ are coprime integers, while parameters of torus are the toroidal (large) radius $R_0$ and the poloidal (small) radius $r_0$, both sizes being large in comparison with a width of quantum vortex core $\xi$. It was believed on the basis of previously obtained numerical results that such knots are unstable and they reconnect during just a few typical times, traveling a distance of several $R_0$ (the lifetime is somewhat longer for smaller ratios $B_0=r_0/R_0$). The mentioned results were obtained for not too large ratios $R_0/\xi\lesssim 20$, and with a very coarse step (about 0.1) on parameter $B_0$. JETP Letters 107, issue 5 (2018). For the first time the magnetic phase transition in DyF_{3} at low temperatures was observed by ^{3}He NMR. The spin kinetics of liquid ^{3}He in contact with a mixture of microsized powders LaF_{3} (99.67%) and DyF_{3} (0.33%) at temperatures 1.53 K was studied by pulse NMR technique. The DyF_{3} is a dipole dielectric ferromagnet with a phase transition temperature T_{c} = 2.55 K, while as the diamagnetic fluoride LaF_{3} used as a diluent for optimal conditions for observation of ^{3}He NMR. The phase transition in DyF_{3} is accompanied by a significant changes in the magnetic fluctuation spectrum of the dysprosium ions. The spin kinetics of ^{3}He in contact with the substrate is sensitive to this fluctuations. An significant change in the rates of the longitudinal and transverse nuclear magnetization of ^{3}He in the region of magnetic ordering of solid matrix was observed. A technique is proposed for studying the static and fluctuating magnetic fields of a solid matrix at the low temperatures using liquid ^{3}He as a probe. .. lakshin, .I. Kondratyeva, V.V. Kuzmin, .R. Safiullin, .. Stanislavovas, .V. Savinkov, .V. Klochkov, .S. Tagirov JETP Letters 107 issue 2, 2018 Microspheres at the surface of liquid are widely used now for visualization of wave and vortex motion [1, 2]. The experiments of this kind had been performed recently to study of turbulence at the surface of liquid helium [3]. That’s why it is of interest to consider the corrections to a classic Archimedes' principle, because while the size of a particle floating at the surface decreases, the forces of surface tension and molecular interaction start to play a significant role. We study the deviations from Archimedes' principle for spherical particles made of molecule hydrogen near the surface of liquid He^{4}. Classic Archimedes' principle takes place if particle radius $R_0$ is greater than capillary length of helium $L_{k} \approx $ 500 µm and the height $h_+$ of the part of the particle above He is proportional to $R_0$ . Over the range of $30 <R_0 <500$ µm Archimedes' force is suppressed by the force of surface tension and $h_{+} \sim R^{3}_{0} / L^{2}_{k}$. When $R_0<30$µm, the particle is situated under the surface of liquid helium. In this case Archimedes' force competes with Casimir force which repels the particle from the surface to the depth of liquid. The distance from the particle to the surface $h_{} \sim R^{5/3}_c / R^{2/3}_0$ if $R_0>R_c...R_c$ can be expressed as $R_c \approx (\frac {\hbar c}{\rho g}) \approx $ 1µm, $\hbar $ is Planck's constant, c is speed of light, $\rho $ is helium density. For the very small particles ( $R_0<R_c)$ $h_{}$does not depend on their size: $h_{}$=$R_c$. 1. S. V. Filatov, S. A. Aliev, A. A. Levchenko, and D. A. Khramov, JETP Letters, , 104(10), 702 (2016). 2. S. V. Filatov, D. A. Khramov, A. A. Levchenko, JETP Letters, 106(5), 330 (2017). 3. A. A. Levchenko, L. P. MezhovDeglin, A. A. Pel’menev, JETP Letters, 106(4), 252 (2017). 4. E. V. Lebedeva, A. M. Dyugaev , and P. D. Grigoriev, JETP, 110(4), 693 (2010). 5. A. M. Dyugaev, P. D. Grigoriev, and E. V. Lebedeva, JETP Letters, 89(3), 145 (2009).
A.M. Dyugaev, E.V. Lebedeva, JETP Letters, 106 issue 12, 2017 One of the frontiers of quantum condensed matter physics seeks to analyze and classify scenarios of the superconductorinsulator quantum phase transition (SIT). Fermionic scenario [1] rules that disorder, when strong enough, breaks down Cooper pairs thus transforming a superconductor into a metal. The further cranking up disorder strength localizes quasiparticles turning the metal into an insulator. According to Bosonic scenario [2,3] disorder localizes Cooper pairs which survive on the insulating side of the SIT and provide an insulating gap. In the Fermionic scenario, the disorderdriven SIT is a twostage transition through the intermediate state that exhibits finite resistance R_{□} and is ordinarily referred to as quantum metal. In Bosonic scenario, the SIT this intermediate state shrinks into a single point in which the resistance assumes the universal quantum resistance per square R_{c} = 6.45 kΩ/□ [3]. The disorderdriven SIT was reported in films of InO_{x} [4, 5], Be [6], TiN [7]. However, the resistance R_{c }that separates superconducting and insulating states in these films is not universal. The access and detailed study of the phases in the critical vicinity of the SIT in different materials remains one of the major challenges. Here we observe the direct disorderdriven superconductorinsulator transition in NbTiN films with R_{c} = 2.7 kΩ/□ at room temperature. We show that the increasing the film's resistance suppresses the superconducting critical temperature T_{c} in accord with the Fermion model. We find that incrementally increasing R_{□} suppresses the BerezinskiiKosterlitzThouless temperature down to zero, while the critical temperature T_{c} remains finite, which complies with the Bosonic model. Upon further increase of R_{□}, the ground state of system becomes insulating. Finally, we demonstrate that the temperature dependence of the resistance of insulating films follows the Arrhenius law. [1] A. M. Finkel'stein, Superconducting transition temperature in amorphous films, JETP Lett. 45, 46 (1987). [2] A. Gold, Dielectric properties of disordered Bose condensate, Phys. Rev. A 33, 652 (1986). [3] M.P. A. Fisher, G. Grinstein, S. Grivin, Presence of quantum diffusion in two dimensions: Universal resistance at the superconductorinsulator transition, Phys. Rev. Lett. 64, 587 (1990). [4] A. F. Hebard, M. A. Paalanen, Magneticfieldtuned superconductorinsulator transition in twodimensional films, Phys. Rev. Lett. 65, 927 (1990). [5] D. Shahar, Z. Ovadyahu, Superconductivity near the mobility edge, Phys. Rev. B 46, 10917 (1992). [6] E. Bielejec, J. Ruan, W. Wu, Anisotropic magnetoconductance in quenchcondensed ultrathin beryllium films, Phys. Rev. B 63, 1005021 (2001). [7] T. I. Baturina et al., Localized superconductivity in the quantumcritical region of the disorderdriven superconductorinsulator transition in TiN thin films, Phys. Rev. Lett. 99, 257003 (2007).
M. V. Burdastyh, S. V. Postolova, T. I. Baturina, T. Proslier, V. M. Vinokur, A.Yu. Mironov JETP Letters 106 (11) (2017)
We demonstrate that nonequilibrium spin excitations drift to macroscopically large distances in
1. Yu.A. Bychkov, S.V. Iordanskii, and G.M. Eliashberg, Twodimensional electrons in a strong
Gorbunov A.V., Kulik L.V., Kuznetsov V.A., Zhuravlev .S., JETP Letters 106, issue 10 (2017) Twodimensional topological insulators are have attracted much recent interest since they feature helical edge states inside their band gap [1,2]. In the absence of timereversal symmetry breaking, spinmomentum locking prohibits elastic backscattering of these helical states, i.e., the helical edge is a realization of an ideal transport channel with conductance equal to e^{2}/h. However, this theoretical prediction was not confirmed by experiments on HgTe/CdTe [36] and InAs/GaSb [7,8] quantum wells. The timesymmetric interaction of the helical states with a "quantum magnetic impurity'' (an impurity which has its own quantum dynamics) is a leading candidate for explaining these experiments. In spite of recent theoretical studies of this problem [914], several key questions has not been addressed in details. We study theoretically the modification of the ideal currentvoltage characteristics of the helical edge in a twodimensional topological insulator by weak scattering off a single magnetic impurity. As a physical realization of such a system we have in mind the (001) CdTe/HgTe/CdTe quantum well (QW) with a Mn impurity that possesses spin S=5/2. Contrary to previous works, we allow for a general structure of the matrix describing exchange interaction between the edge states and the magnetic impurity. For S=1/2 we find an analytical expression for the backscattering current at arbitrary voltage. For larger spin, S>1/2, we derive analytical expressions for the backscattering current at low and high voltages. We demonstrate that the differential conductance may exhibit a nonmonotonous dependence on the voltage with several extrema. [1] X.L. Qi, S.C. Zhang, Topological insulators and superconductors, Rev. Mod. Phys. 83, 1057 (2011). [2] M. Z. Hasan, C. L. Kane, Colloquium: Topological insulators, Rev. Mod. Phys. 82, 3045 (2010). [3] M. Konig, S. Wiedmann, C. Brune, A. Roth, H. Buhmann, L. W. Molenkamp, X.L. Qi, S.C. Zhang, Quantum spin Hall insulator state in HgTe quantum wells, Science 318, 766 (2007) [4] K. C. Nowack, E. M. Spanton, M. Baenninger, M. Konig, J. R. Kirtley, B. Kalisky, C. Ames, P. Leubner, C. Brune, H. Buhmann, L. W. Molenkamp, D. GoldhaberGordon, K. A. Moler, Imaging currents in HgTe quantum wells in the quantum spin Hall regime, Nat. Mater. 12, 787 (2013). [5] G. Grabecki, J. Wrobel, M. Czapkiewicz, L. Cywinski, S. Gieratowska, E. Guziewicz, M. Zholudev, V. Gavrilenko, N. N. Mikhailov, S. A. Dvoretski, F. Teppe, W. Knap, T. Dietl, Nonlocal resistance and its fluctuations in microstructures of bandinverted HgTe/(Hg,Cd)Te quantum wells, Phys. Rev. B 88, 165309 (2013). [6] G. M. Gusev, Z. D. Kvon, E. B. Olshanetsky, A. D. Levin, Y. Krupko, J. C. Portal, N. N. Mikhailov, S. A. Dvoretsky, Temperature dependence of the resistance of a twodimensional topological insulator in a HgTe quantum well, Phys. Rev. B 89, 125305 (2014). [7] E. M. Spanton, K. C. Nowack, L. Du, G. Sullivan, R.R. Du, K. A. Moler, Images of edge current in InAs/GaSb quantum wells, Phys. Rev. Lett. 113, 026804 (2014). [8] L. Du, I. Knez, G. Sullivan, R.R. Du, Observation of quantum spin Hall states in InAs/GaSb bilayers under broken timereversal symmetry, Phys. Rev. Lett. 114, 096802 (2015). [9] J. Maciejko, Ch. Liu, Y. Oreg, X.L. Qi, C. Wu, S.C. Zhang, Kondo effect in the helical edge liquid of the quantum spin Hall state, Phys. Rev. Lett. 102, 256803 (2009). [10] Y. Tanaka, A. Furusaki, K. A. Matveev, Conductance of a helical edge liquid coupled to a magnetic impurity, Phys. Rev. Lett. 106, 236402 (2011). [11] J. I. Vayrynen, M. Goldstein, L. I. Glazman, Helical edge resistance introduced by charge puddles, Phys. Rev. Lett. 110, 216402 (2013). [12] J. I. Vayrynen, M. Goldstein, Y. Gefen, L. I. Glazman, Resistance of helical edges formed in a semiconductor heterostructure, Phys. Rev. B 90, 115309 (2014). [13] V. Cheianov, L. I. Glazman, Mesoscopic fluctuations of conductance of a helical edge contaminated by magnetic impurities, Phys. Rev. Lett. 110, 206803 (2013). [14] L. Kimme, B. Rosenow, A. Brataas, Backscattering in helical edge states from a magnetic impurity and Rashba disorder, Phys. Rev. B 93, 081301 (2016).
Kurilovich P.D. , Kurilovich V.D., Burmistrov I.S. , Goldstein M. JETP Letters 106 (9) (2017)
Chimera is, according to Greek mythology, a monstrous creature combining the parts of different animals (a lion with a head of a goat and a tail of a snake). Physicists recently adopted this name for complex states in nonlinear dynamical systems, where instead of an expected symmetric synchronous state one observes coexistence of synchronous and asynchronous elements [1]. Since the discovery of chimeras by Kuramoto and Battogtokh in 2002 [2], these states have been reported in numerous theoretical studies and experiments. [1] M. J. Panaggio, D. M. Abrams, Chimera states: coexistence of coherence and incoherence in networks of coupled oscillators, Nonlinearity 28 , R67 (2015). [2] Y. Kuramoto, D. Battogtokh, Coexistence of Coherence and Incoherence in Nonlocally Coupled Phase Oscillators, Nonlinear Phenom. Complex Syst. 5 , 380 (2002). [3] M. Rosenblum, A. Pikovsky, SelfOrganized Quasiperiodicity in Oscillator Ensembles with Global Nonlinear Coupling, Phys. Rev. Lett. 98 , 064101 (2007). [4] L. A. Smirnov, G. V. Osipov, A. Pikovsky, Chimera patterns in the KuramotoBattogtokh model, J. Phys. A: Math. Theor. 50 , 08LT01 (2017).
Bolotov M.I., Smirnov L.A., Osipov G.V., Pikovsky A. JETP Letters 106, issue 6 (2017) Wellknown Faraday waves can be parametrically generated on a free surface of ordinary (classical) fluids such as water or on superfluid helium HeII surface when a sample cell is vibrated vertically. Standingwave patterns appear on the surface, and their frequencies are onehalf the driving frequency. The acceleration threshold for the parametric excitation of Faraday waves on the surface of water is near an order of magnitude higher than on the surface of HeII at the same frequencies [1]. Generation of vorticity by interacting nonlinear surface waves has been predicted theoretically in a number of papers [2, 3] and generation of vortices by noncollinear gravity waves on a water surface has been observed experimentally [4].Our study has shown that classical 2D vortices can be generated by Faraday waves on the surface of superfluid HeII also, more over one can observe formation of the vortex lattice in addition to the wave lattice on the surface of HeII in a rectangular cell. Combined with predictions [5] that the sharpest features (about nm sizes) in the cell walls can induce nucleation of quantum vortex filaments and coils on the interface and formation a dense turbulent layer of quantum vortices near the solid walls with a nonclassical average velocity profile which continually sheds small vortex rings into the bulk of vibrating HeII, this opens up new prospects for studying the properties of a quantum liquid and turbulent phenomena on the surface and in bulk of supefluid liquids.
[1] Haruka Abe, Tetsuto Ueda, Michihiro Morikawa, Yu Saitoh, Ryuji Nomura, Yuichi Okuda, Faraday instability of superfluid surface, Phys. Rev. E 76, 046305 (2007).
Levchenko A.A., MezhovDeglin L. P., Pel’menev A.A. JETP Letters 106, issue 4 (2017)
Nanoscale integration of organic and metallic particles is expected to open up new opportunities for the design highperformance nanoscale devices. Optimization of heterostructures requires experimental and theoretical analysis of their specific physical properties. Nanosystem consisting in gold It was found that observed effects can be explained by means of the feedback caused by the nonlinear interaction of polarizations with their total reflection in the metallic core. At low temperatures Bloch vectors related with twolevel systems form an analog of a ferromagnetic state. With increasing fluctuations, antiferromagnetic states are formed along with the desynchronization of ferromagnetic one. These properties allows us to explain the observed changes in the intensity of the and line form of laser generation with temperature. Experimental and numerical results of the work demonstrate that the synchronization of the polarization of dye molecules caused by inverse nonlinear coupling yields an analog of plasmonpolariton superradiance. 1. D.J. Bergman and M.I. Stockman, Phys.Rev.Lett. 90, 027401 (2003). 2. M. Haridas et al, J. Appl. Phys.114, 064305 (2013). 3. M. Praveena et al, Phys. Rev. B 92, 235403 (2015). A. S. Kuchyanov, A.A. Zabolotskii, Plekhanov A.I. JETP Letters 106 (2) (2017) Recently Sr_{2}FeSi_{2}O_{7} comes into focus as a possible compound with unusual magnetoelectric coupling or, in other words, as a novel potential multiferroic [1,2]. Results of terahertz spectroscopy in the paramagnetic state show that the multiplet Fe^{+2}(S=2) of the ground state splits due to the spinorbit coupling. However the energy intervals between the lowlying singlet state and excited states are quite small so that all spin states are populated at the temperature of about 100 K. The Fe^{+2} ion occupies the center of a tetragonally distorted tetrahedron. In the present communication the origin of the magnetoelectric coupling is described as follows. The odd crystal field from the tetrahedral environment induces the coupling of the orbital momentum of the Fe^{+2}( ^{5}D) state with the external electric field. On the other hand, the orbital momentum is coupled with spin via the spin –orbit interaction. Both angular momenta are coupled with the external magnetic field, which is enhanced due to the presence of the superexchange interaction between neighboring Fe^{+2 }ions. Combining all these couplings, the author derived the affective spin Hamiltonian for the magnetoelectric coupling, which made it possible to calculate relative intensities of the electric dipole transitions between spin states and estimate the magnetization caused by the external electric field as well as the electric polarization induced by the magnetic field.
M.V. Eremin JETP Letters 105 (11) (2017) It is well known the conductivity of hightemperature superconductors (HTSCs) with T_{C} ~100 K (YBaCuO, BiSrCaCuO, etc.) is provided at T~300 K by hole (h) fermions [1]. It is also known the superconducting transition in such cuprates is accomplished by means of the Cooper pairing, while the fluctuating Cooper pairs with charge 2e exist even at T=T_{C}+(~30 K) [2]. Hence it inevitably follows in the interval T_{C}<T<300 K the hole Fermi surface (FS) of these HTSCs transforms into an electron one as a result of a topological transformation (the Lifshitz transition (LT) [3]. There is one of the central questions in the problem of the pseudogap state [1] of copperoxide highT_{C} superconductors: how and at what temperatures this transformation occurs. To evidence the charge carrier conversion the Hall effect is used usually. As for the BiSrCaCuO and YBaCuO, their Hall coefficients (R_{H}) have several features in the temperature range T_{C}…300 K [4,5]. The most significant of them is observed before the T_{C} in the region of fluctuation conductivity and can be interpreted as a manifestation of a scale holeelectron (he) conversion in a system of charge carriers, i.e. as the LT. However, this point of view is not universally accepted. As for the data on the transformation of the FS obtained by the ARPES (Angle Resolved Photoemission Spectroscopy) method [7], they, like [4,5], support several rearrangements of the FS, including those occurring near T_{C}. Meanwhile, it is the possibility to evidence the he conversion in a hole HTSC (the last condition is sure), which does not require either electric or magnetic fields to create the Hall potential difference. The technique developed by us [7,8] is based on the phenomenon of rearrangement of the spectrum of charge carriers in the nearsurface layer of a hole HTSC being in contact with a normal metal (Me). This phenomenon is a consequence of the annihilation of "aboriginal" hole fermions in the HTSC/Me interface with electrons penetrated from Me. The essence of this technique is the registration of changes in the resistance of the HTSC/Me interface r_{}, which is characterized by a small number of hole carriers. The appearance of the temperature singularities of r_{C} and the sign of r_{C } variation (dr_{}) make it possible to obtain an idea of the character of the changes in the system of charge carriers of the HTSC array. The dependences r_{C}(T) of the Bi(Pb)SrCaCuO/Pb and YBaCuO/In interfaces have been studied and anomalies near the temperature of the pseudogap opening and before the superconducting transition have been observed. We are shown that in Bi(Pb)SrCaCuO and YBaCuO, when the temperature T=T_{C}+(~10 K) is reached, that do not concerns to fluctuating Cooper pairs condensation. So, there is due to changing the topology of the FS. As a result, significant piece of FS becomes electronic. The most probable reason for the topological transition is the achievement of the temperature of the 2D3D crossover (the temperature of the threedimensionality of HTSC), which is a consequence of a modification in the electronic subsystem that leads to a change in the interaction mechanisms of the fluctuation Cooper pairs [9, 10]. 1. The Physics of Superconductors, Vol.1. Conventional and HighT_{C} Superconductors. Ed. by K.H. Bennemann and J.B. Katterson, Berlin, Springer, (2003). 2. K. Kawabata, S. Tsukui, Y. Shono, O. Michikami, H. Sasakura, K. Yoshiara, Y. Kakehi, T. Yotsuya, Phys. Rev. B58, 2458 (1998). 3. I.M. Lifshits, JETP 38, 1569 (1960) (in Russian). 4. Q. Zhang, J. Xia, M. Fang, Z. He, S. Wang, Z. Chen, Physica C 162164, 999 (1989). 5. A.L. Solovjov, FNT 24, 215 (1998) (in Russian). 6. T. Kondo, A.D. Palczewski, Y. Hamaya, T. Takeuchi, J.S. Wen, Z.J. Xu, G. Gu, A. Kaminski, arXive: 1208.3448v1 (2012). 7. V.I. Sokolenko, V.A. Frolov, FN 39, 134 (2013) (in Russian). 8. V.A. Frolov, VAN, Ser.: Vacuum, pure materials, superconductors, 1, 176 (2016) (in Russian). 9. Y.B. Xie, Phys. Rev., B46, 13997 (1992). 10. A.L. Solovjov, V.M. Dmitriev, FNT 35, 227 (2009) (in Russian).
Sokolenko V.I., Frolov V.A. JETP Letters 105, issue 10 (2017)
Correct allowing for the interparticle interaction in manybody systems faces considerable mathematical difficulties. The most frequently used approximation in such problems is the mean field approximation (MFA) which neglects fluctuations and the particles are considered as a continuous medium of inhomogeneous density. If , moreover, the system is described by the classical distribution function ( the statistics can be a quantum one) we obtain the well known Thomas  Fermi approach .However there are situations when at least some of the degrees of freedom of the system have to be treated in accord with quantum mechanics. Such examples are electrons in quantum wells or dipolar excitons in an electrostatic trap. In such cases the density of particles appearing in MFA is to be expressed via wave functions of a particle in the effective potential. The latter, in its turn, depends on the wave functions and occupation numbers, so one has to solve a selfconsistent problem. In case of a shortrange interparticle pair potential (2D gas of dipolar excitons) a nonlinear wave equation arises while for the longrange ( Coulomb) pair interaction the corresponding equation becomes integrodifferential (nonlocal effects). Two different systems are considered: bose  gas of dipolar excitons in a ring shape trap and fermigas of electrons in a quantum well of a MOSstructure. The trapped excitons are described by the GrossPitaevskyi nonlinear equation and for the very simple case of the rectangular potential of the “empty” trap the exact analytical solution is found. The most interesting result of this problem is criterion for existence of bound state in the effective potential ( in the one particle problem a 1D symmetric potential well always contains at least one bound state) . Methodologically instructive is the way of obtaining the eigenvalue of the GrossPitaevskyi equation: the ground state energy is found from the normalization condition. In case of electrons in a quantum well one deals with nonlinear integrodifferential equation for which the exact solution is unknown. The direct variational method was used to find the frequency of the intersubband transition. This frequency turned out to be scaled with the electron concentration N as $N^{2/3}$.
Chaplik A.V. JETP Letters 105 (9) (2017)
A model of fermion condensation, advanced more than 25 years ago, still remains the subject of hot debates, due to the fact that within its frameworks, nonFermiliquid (NFL) behavior, ubiquitously exhibited by strongly correlated Fermi systems, including electron systems of solids, is properly elucidated. The model is derived with the aid of the same Landau postulate that the ground state energy $E$ is a functional of its quasiparticle momentum distribution $n$, giving rise to the conventional Landau state. However, the model discussed deals with completely different solutions, emergent beyond a critical point, at which the topological stability of the Landau state breaks down, and therefore relevant solutions of the problem are found from the wellknown variational condition of mathematical physics $\delta E(n)/\delta n({\bf p})=\mu$ where $\mu$ is the chemical potential. Since the left side of this condition is nothing but the quasiparticle energy $\epsilon({\bf p})$, the variational condition does imply formation of the flat band or, in different words, a fermion condensate (FC). In fact, variational condition furnishes an opportunity to find solely the FC quasiparticle momentum distribution $n_*({\bf p}\in \Omega)$. JETP Lett. 105 (8) (2017). Materials harder than diamond are always attract great attention from the scientists all over the world. Many attempts were made towards the synthesis especially of carbon material harder than diamond, which is the hardest possible material nowadays. A special interest belongs to materials called as fullerites. There are several experimental and theoretical works, where the synthesis and investigation of superhard fullerite were carried out. [1]–[4] Such materials reveal outstanding mechanical properties with the bulk modulus of several times higher than that of diamond. In this case the computational approaches and methods allow the theoretical investigations and prediction of a new materials with desired properties without using very expensive experimental equipment. Here we used the stateoftheart theoretical methods of computational predictions to predict new carbon phases based on the fullerene molecules of different sizes (C_{60} and C_{20}). Using the evolutionary algorithm, implemented in USPEX package, [5] we considered more than 3000 possible crystal structures to find the most stable ones. The important point, that predicted phases are based on the polymerized fullerites, displaying the superior mechanical properties. We defined the crystal structure of predicted 4 stable allotropes by simulating the XRD patterns. All predicted structures are highly symmetric. The mechanical properties were studied in details in terms of elastic tensor, bulk and shear moduli and velocities of acoustic waves. All predicted structures display elastic constants and bulk modulus very close to diamond, which allows to say that we indeed predict new superhard phases. The possible way of synthesis of such phases was proposed consisting in the cold compression of a mixture of graphite and C_{60} fullerenes. The important feature of predicted phases (besides the mechanical properties) is that they have relatively small band gap ~2.5 eV, while the cI24 phase has the direct gap of 0.53 eV. All obtained data allows the conclusion that predicted superhard semiconducting phases based on the polymerized fullerenes reveal necessary properties for applications in the electronic as basic elements.
[1] V.D. Blank, S.G. Buga, G.A. Dubitsky, N. R Serebryanaya, M.Y. Popov, and B. Sundqvist, Carbon 36, 319 (1998). [2] M. Popov, V. Mordkovich, S. Perfilov, A. Kirichenko, B. Kulnitskiy, I. Perezhogin, and V. Blank, Carbon 76, 250 (2014). [3] Y.A. Kvashnina, A.G. Kvashnin, M.Y. Popov, B.A. Kulnitskiy, I.A. Perezhogin, E.V. Tyukalova, L.A. Chernozatonskii, P.B. Sorokin, and V.D. Blank, J. Phys. Chem. Lett. 6, 2147 (2015). [4] Y.A. Kvashnina, A.G. Kvashnin, L.A. Chernozatonskii, and P.B. Sorokin, Carbon 115, 546 (2017). [5] C.W. Glass, A.R. Oganov, and N. Hansen, Comput. Phys. Commun. 175, 713 (2006).
Kvashnina Yu.A., Kvashnin D.G., Kvashnin A.G., Sorokin P.B. JETP Letters 105 ( 7) (2017)
Recently stochastic clustering with statistical selfsimilarity (fractality) has been found on material surface exposed under extreme plasma thermal loads in fusion devices (see [1]). In such devices, multiple processes of erosion and redeposition of the eroded material, surface melting and motion of the surface layers lead to a stochastic surface growth on the scales from tens of nanometers to hundreds of micrometers. The moving of eroded material species during redeposition from plasma and agglomeration on the surface is governed by stochastic electric fields generated by the hightemperature plasma. The specific property of the nearwall plasma in fusion device is the nonGaussian statistics of electric field fluctuations with longrange correlations [2]. It leads to the stochastic agglomerate growth with a selfsimilar structure (hierarchical granularity  fractality) of nonGaussian statistics contrary to a trivial roughness observed in ordinary processes of stochastic agglomeration. The dominant factor in such process in fusion device is the collective effect during stochastic clustering rather than the chemical element composition and physical characteristics of the solid material. In support of this view it is reported in this Letter, that such similar stochastic fractal structure with hierarchical granularity and selfsimilarity is formed on various materials, such as tungsten, carbon materials and stainless steel exposed to hightemperature plasma in fusion devices. In the literature it is discussed hypotheses of universal scalings of stochastic objects and processes with multiscale invariance property (statistical selfsimilarity), see e.g. [3]. The kinetic models propose the describing of the stochastic clustering with a selfsimilar structure and considering the power law solutions for the number N of agglomerating clusters with mass m (see e.g. [4]), N(m)=Cm^{(3+}^{h}^{)/2}, where h is a selfsimilarity exponent of the agglomeration kinetic model, C is a constant factor. It is surprisingly found in this Letter that such the power laws (with power exponents from 2.4 to 2.8) describing the roughness of the test specimens from fusion devices are strictly deviated from that of the reference samples formed in a trivial agglomeration process forming Brownianlike rough surface (such as samples exposed to lowtemperature glow discharge plasma and rough steel casting with the power law exponent in the range of 1.97 to 2.2). Statistics of stochastic clustering samples from fusion devices is typically nonGaussian and has a "heavy" tails of probability distribution functions (PDF) of stochastic surface heights (of the Hurst exponent from 0.68 to 0.86). It is contrary to the Gaussian PDF of the reference samples with trivial stochastic surface. Stochastic clustering of materials from fusion devices is characterised by multifractal statistics. Quantitative characteristics of statistical inhomogeneity of such material structure, including multifractal spectrum with broadening of 0.5 ¾ 1.2, are in the range observed for typical multifractal objects and processes in nature. This may indicate a universal mechanism of stochastic clustering of materials under the influence of hightemperature plasma.
1. V.P. Budaev et al., JETP Letters vol. 95, 2, 78 (2012). 2. V.P. Budaev, S.P. Savin, L.M. Zelenyi, PhysicsUspekhi 54 (9), 875 (2011) 3. A. L. Barabasi and H. E. Stanley, Fractal Concepts in Surface Growth (Cambridge Univ. Press, Cambridge, 1995). 4. C. Connaughton, R. Rajesh, O. Zaboronski, PRL 94 (19), 194503 (2005).
V.P. Budaev, JETP Letters vol. 105, issue 5 (2017)
Modern physics of liquid crystals is much younger than its traditional condensed matter material counterparts. Therefore the field is not yet completely elaborated and exhausted, and one may still expect discoveries of new mesogen materials exhibiting of new types of liquidcrystalline ordering. A few years ago such a discovery of socalled bentcore or dimer mesogens which can form short pitch heliconical nematic state (also known as twistbend nematics, $N_{TB}$) [1, 2], attracted a lot of interest to this new state of matter with nanoscale orientational modulation. First, to understand the nature of the phase, basically different from conventional uniform nematics and from modulated in mass density smectics (see e.g., Landau theory approach, [3,4]). Second, to exploit potentially very perspective applications of the $N_{TB}$ liquid crystals. Along this way, very recently S.M.Saliti, M.G.Tamba, S.N. Sprunt, C.Welch, G.H.Mehl, A.Jakli, J.T.Gleeson [5] observed of the unprecedentedly large magnetic field induced shift $\Delta T_c(H)$ of the nematic  isotropic transition temperature. What is even more surprising $\Delta T_c(H)$ does not follow the thermodynamics textbook wisdom prediction $H^2$ scaling. Our interpretation of such a behavior is based on singular longitudinal fluctuations of the nematic order parameter. Since these fluctuations are governed by the Goldstone director fluctuations they exist only in the nematic state. External magnetic field suppresses the singular longitudinal fluctuations of the order parameter. The reduction of the fluctuations changes the equilibrium value of the modulus of the order parameter in the nematic state, and leads to additional (with respect to the mean field contribution) fluctuational shift of the nematic  isotropic transition temperature. The mechanism works for any nematic liquid crystals, however the magnitude of the fluctuational shift increases with decrease of the Frank elastic moduli. Since some of these moduli supposed to be anomalously small for the bentcore or dimer mesogen formed nematic liquid crystals, just these liquid crystals are promising candidates for the observation of the predicted fluctuational shift of the phase transition temperature.
JETP Letters 105 (4) (2017) In a recent letter A. Danan et al. [A. Danan, D. Farfurnik, S. BarAd et al., Phys. Rev. Lett. 111, 240402 (2013)] have experimentally demonstrated an intriguing behavior of photons in an interferometer. Simplified layout of the experimental setup represents a nested MachZehnder interferometer (MZI) and is shown below. The surprising result is obtained when the inner MZI is tuned to destructive interference of the light propagating toward mirror F. In that case the power spectrum shows not only peak at the frequency of mirror C but two more peaks at the frequencies of mirrors A and B, and no peaks at the frequencies of mirrors E and F. From these results authors conclude that the path of the photons is not represented by connected trajectories, because the photons are registered inside the inner MZI and not registered outside it. These unusual results have raised an active discussion. Nevertheless, until now there was no comprehensive and clear analysis of the experiment within the framework of the classical electromagnetic waves approach. In this letter, we calculate the signal power spectrum at the output of the nested MZI, based on traditional concept of the classical electromagnetic waves (or quantum mechanics). This concept imply the continuity of the wave (photon) trajectories. We give intuitive clear and comprehensive explanation of paradoxical results. So, there is no necessity for a new concept of disconnected trajectories.
Simplified experimental setup with two nested MachZehnder interferometers. A, B, C, E, and F stands for mirrors; BS1 and BS2, and PBS1 and PBS2 stands for ordinary and polarized beam splitters respectively. The elements BS1, A, B, and BS2 form an inner MZI whereas the elements PBS1, C, E, F and PBS2 form an outer MZI. Various mirrors inside the MZI vibrate with different frequencies. The rotation of a mirror causes a vertical shift of the light beam reflected off that mirror. The shift is measured by a quadcell photodetector QCD. When the vibration frequency of a certain mirror appears in the power spectrum, authors conclude that photons have been near that particular mirror
G.N.Nikolaev JETP Letters 105 (3) (2017) The dynamics of the quantum vacuum is one of the major unsolved problems of relativistic quantum field theory and cosmology. The reason is that relativistic quantum field theory and general relativity describe processes well below the Planck energy scale, while the deep ultraviolet quantum vacuum at or above the Planck energy scale remains unknown. Following the condensed matter experience we develop a special macroscopic approach called qtheory, which incorporates the ultraviolet degrees of freedom of the quantum vacuum into an effective theory and allows us to study the dynamics of the quantum vacuum and its influence on the evolution of the Universe. The vacuum in our approach is considered as the Lorentzinvariant analog of a condensedmatter system (liquid or solid) which is stable in free space. The variable q is the Lorentzinvariant analog of the particle number density, whose conservation regulates the thermodynamics and dynamics of manybody systems. This approach is universal in the sense that the same results are obtained using different formulations of the qfield. In the paper, we choose the qfield in terms of a 4form field strength, which has, in particular, been used by Hawking for discussion of the main cosmological constant problem  why is the observed value of the cosmological constant many orders of magnitude smaller than follows from naive estimates of the vacuum energy as the energy of zeropoint motion. In qtheory, the huge zeropoint energy is naturally cancelled by the microscopic (transPlanckian) degrees of freedom, as follows from the GibbsDuhem identity, which is applicable to any equilibrium ground state including the one of the physical vacuum. In the paper, we consider a further extension of qtheory. We demonstrate that, in an expanding Universe, the variable effectively splits into two components. The smooth part of the relaxing vacuum field is responsible for dark energy, while the rapidly oscillating component behaves as cold dark matter. In this way, qtheory provides a combined solution to the missingmass problem and the cosmological constant problem. If this scenario is correct, the implication would be that direct searches for darkmatter particles remain unsuccessful in the foreseeable future. F.R. Klinkhamer and G.E. Volovik, JETP Letters 105, issue 2 (2017)
The ability to detect nonequilibrium spin accumulation (imbalance) by all electrical means is one of the key ingredients in spintronics . Transport detection typically relies on a nonlocal measurement of a contact potential difference induced by the spin imbalance by means of ferromagnetic contacts or spin resolving detectors . A drawback of these approaches lies in a difficulty to extract the absolute value of the spin imbalance without an independent calibration. An alternative concept of a spintocharge conversion via nonequilibrium shot noise was introduced and investigated in experiment recently . Here, the basic idea is that a nonequilibrium spin imbalance generates spontaneous current fluctuations, even in the absence of a net electric current. Being a primary approach , the shot noise based detection is potentially suitable for the absolute measurement of the spin imbalance. In addition, the noise measurement can be used for a local noninvasive sensing. In this letter, we calculate the impact of a spin relaxation on the spin imbalance generated shot noise in the absence of inelastic processes. We find that the spin relaxation increases the noise up to a factor of two, depending on the ratio of the conductor length and the spin relaxation length. The design of the system. A diffusive normal wire of the length L is attached to normal islands on both ends. Nonequilibrium energy distribution on the left hand side of the wire generates the shot noise at a zero net current. The spin imbalance on the lefthand side of the wire is due to the electric current flowing from one ferromagnetic lead (red) to another one with opposite magnetization (blue).
V.S. Khrapai and K.E. Nagaev JETP Letters 105, №1 (2017)
