Grigori Volovik - 80

Warmest congratulations to our colleague Grigori Yefimovich Volovik, born 07/09/1946, member of the Editorial Board of JETP Letters and a Deputy Editor since 1991, on the occasion of his 80th birthday.

Grigori’s ascent to the elite of theoretical physics has been pretty straightforward and swift: in 1964 he became a student of the Moscow Institute of Physics and Technology (MIPT); upon having passed a needle eye of close to 20 applications per place he was selected by none but P.L. Kapitsa as a student to his chair of low temperature physics. He was subsequently one of the top students in the Class of 1970 at the MIPT Faculty of General and Applied Physics and continued as PhD student at the Landau Institute of Theoretical Physics (Landau ITP); ever since 1973 he is a staff member of Landau ITP, a principal researcher since 1994. Grigori defended his PhD thesis in 1973 and habilitated in 1982.

Grigori’s moment of glory came in 1975 when he, in collaboration with V.P. Mineyev, turned his eye on the rich topological properties of the superfluid He-3, and subsequently extended the developed technique to superconductivity. Eventually, he became a world renowned guru in this field of physics. A

longside his work at the Landau Institute, in 1993, Grigory became a professor at the Low Temperature Laboratory of the Helsinki University of Technology, and in 2001 became a foreign member of the Finnish Academy of Science and Letters, followed by election in 2007 to the German National Academy of Sciences Leopoldina.

The results of this series of studies are summarized in the monograph Exotic properties of superfluid ³He (World Scientific, 1992). Also in 1992. Grigory became, together with V.P. Mineev, winner of the Landau gold medal, and in 2014, Grigori, jointly with V.P. Mineev, was awarded the Lars Onsager prize of the American Physical Society for «their contribution to a comprehensive classification of topological defects in condensed matter phases with broken symmetry, culminating in the prediction of half-quantum vortices in superfluid He-3 and related systems.”

An important part of Gregory’s scientific biography is the famous book The Universe in a Helium Droplet (Clarendon Press, Oxford 2003, 2nd edition 2009, 4000+ citations). It is based on his new series of works extending concepts that proved so successful in the study of superfluidity and superconductivity to problems concerning phase transitions and topological defects in the Universe, as well as to the Standard Model of particle physics.

For this series of studies, Grigory was awarded the 2004 Simon Memorial Prize, with the citation «for his pioneering research on the effects of symmetry in superfluids and superconductors and for extending these concepts to quantum field theory, cosmology, quantum gravity and particle physics».

Grigory continues to work tirelessly in the field of theoretical physics, constantly expanding his list of publications (which has by now exceeded 500), and on the journal’s editorial board—as deputy editor-in-chief and a “guru” in the field of condensed matter theory.

The Editorial Board of JETP Letters wishes the honoree robust health, longevity, and we look forward to more inspiration from his studies and continuous advantage from his expertise in our Board.

The extraction of the proton beam circulating in a ring accelerator using a bent single crystal  was carried out in a number of research centers (JINR, IHEP, CERN, FNAL). The proton beam  was directed strictly perpendicular to the surface of the single crystal at a small angle  (smaller than the Lindhard angle) relative to the curved crystallographic planes. This geometry  of particle capture in the planar channeling mode is called end‑face capture. This paper presents the results of extracting a proton beam from an accelerator using the phenomenon of volume particle capture in the channeling mode. In this case, the proton beam is directed at the crystal at an angle many times greater than the Lindhard angle. As the proton moves in the bent crystal, the angle between the particle veloсity and the plane decreases over a short distance, and the particle moves almost parallel to the plane.In this region, due to multiple scattering on atoms, a particle has a probability of being captured into the channeling mode (i.e., transitioning from an over‑barrier state to an under‑barrier one). The figure illustrates both types of particle capture into channeling.

The extraction of a proton beam using particle capture into channeling was implemented for the first time in the world’s accelerator practice at the SPASCHARM facility at the Institute for High Energy Physics. When pointing a primary beam with an intensity of $3\times 10^{11}$ protons/cycle at a crystal in the ring of the U-70 accelerator  at the SPASCHARM installation, an intensity of up to $6\times 10^6$ protons/cycle was observed. Despite the lower intensity of the output beam compared to the traditional output based on the face-end capture of particles, the output using the phenomenon of volume capture provides more stable parameters of the output beams.

Figure 1: Scheme of particle extraction at volume capture

S.F.Reshetnikov et al.
JETP Letters 124, issue 5 (2026)

The unusual ground state doublet of the ²²⁹Th nucleus with the low energy M1(8.4 eV) transition between the nuclear states and the electron shell of the thorium ions with the similar electron transition represent two qubits spatially inserted one within the other. In the case of approximate equality of the energies of these transitions, weakly damped oscillations can be excited between these qubits, namely, multiple coherent energy transfer from the electron shell to the nucleus and vice versa. Due to the relatively high interaction energy of the electron and nuclear currents this process in the ²²⁹Th ions does not require resonant coincidence of the transition energies. The electron shell, transitioning between two electron states, “breathes”, periodically decreasing and increasing in size and plays the role of a kind of resonator for the virtual photon emitted by the nucleus.

Fig. Multiple exchange of excitation energy between the nucleus and its electron shell.

E.V. Tkalya
JETP Letters 124, issue 5 (2026)

       The formation of impurity-induced bound states of a particle hopping over lattice sites is one of the fundamental effects of quantum physics described in numerous textbooks. In the case of the nearest-neighbor hopping, a bound state arises for any strength of the attractive on-site impurity potential W in one- and two-dimensional cases (1D and 2D) and if the strength exceeds a critical value in the three-dimensional (3D) case (this value is determined by the ratio of W/t, where t is the hopping amplitude). The quantum statistics play no role in this single-particle problem. In particular, a similar behavior exhibits a composite boson, formed by two strongly bound fermion species if it is considered as a structureless hard-core particle with an effective hopping amplitude t_eff = t²/U (U >> t is the strong on-site attraction of the different species) in the presence of an impurity with a non-zero potential acting on the boson as a whole.

         Remarkably, that when placed near a single-site "magnetic" impurity—which exerts an opposite effect on its constituent fermions— the composite boson exhibits interesting differences from the well-known single-particle case.

 In this case the “classical” potential energy of the bound fermion pair does not depend on the lattice site and this infinite degeneracy is eliminated only through quantum virtual processes of dissociation and recombination. In addition to the band dispersion, these processes also generate an effective impurity potential that extends to the impurity site and its nearest neighbors. This emergent potential gives rise to a rich spectrum of localized states. Besides the usual symmetric bound state, the composite boson supports additional bound states of different orbital symmetry. We determine the energies of these states and the critical impurity strengths required for their formation in one-, two-, and three-dimensional lattices. Notably, the critical conditions are universal: they depend only on the ratio W/U and are independent of the hopping amplitude. Thus, the internal structure of a composite quantum particle can qualitatively transform even the simplest impurity problem. The predicted bound states — analogous to Yu–Shiba–Rusinov states in the weak coupling (BCS) regime — may be relevant to strongly correlated lattice systems, including ultracold atoms in optical lattices, excitonic systems, and other platforms supporting tightly bound quantum pairs.

The dependence of the energy of a bound state (counted from the bottom of the band) on the effectiveimpurity strength parameter 𝛾 for a 2D square lattice.

L.I.Knyazeva, V.I.Yudson
JETP Letters 124, issue 4 (2026)

Weak absorption of light by two-dimensional systems, including monolayer materials and some quantum wells, is one of the critical challenges for contemporary optoelectronics. The problem becomes even more pronounced in 2d systems with back gates used to control the carrier density, where the destructive interference of incident and reflected waves enforces ultralow absorbance. We show that both challenges are resolved in the metal-contacted 2d systems, with the metal sections being much wider than 2d sections. Absorption coefficient is raised to 50% in such structures without a back reflector, and up to 100% in the presence of the back reflector. Surprisingly, the distance to the back reflector can be well below the quarter of incident wavelength, contrasting the conventional matching condition in wave optics. The reason for perfect absorption with ultra-proximate reflector lies in large light phase shift acquired upon passage through metal-based grating.

Kirill Kapralov, Vladislav Atlasov, Alina Khisameeva, Viacheslav Muravev, Dmitry Svintsov,
JETP Letters 124, issue 4 (2026)

Surface water waves constitute an important example of so-called dynamic turbulence. Natural evolution of wave spectra results in development of the Kolmogorov cascade that transfer wave energy from low to high frequencies (Zakharov--Filonenko solution). This solution may be obtained from the basic
principles of resonant interaction of waves.

The natural wave spectra are anisotropic. This begs a question: how this anisotropy affects the Zakharov--Filonenko (ZF) spectrum. An anisotropic solution, also obtained from the principles of wave resonance, was found by Kats and Kontorovich. But a general study of anisotropy requires the detailed examination of the kinetic equation for waves. This is the main topic of the present paper.

The evolution of the generic powerlike-cosine anisotropic additions to ZF spectrum was determined analytically via the quadruplet integrals. These integrals were calculated numerically (the example graph here shows the exponent coefficient for the first cosine mode). The calculation has shown the existence of some new solutions.

The pure cosine mode has proved to be special in that it decays much slower than other ones. As a result, general wave spectra in the inertial interval usually includes ZF spectrum with the addition of the pure cosine and virtually no other harmonics.

V.Geogjaev
JETP Letters 124, issue 5 (2026)
 

New efficient mechanism of laser-driven ion acceleration based on the previously described regime of relativistic self-trapping (RST) of a high-power ultrashort laser pulse in near-critical-density plasma has been presented. Upon reaching the rear target surface, a charged up to 10–30 nC cavity, which is formed under RST due to cavitated electrons, experiences a directed Coulomb explosion (DCE).  This is the dominant process in ion acceleration. DCE is also accompanied by a boost in ion energy due to the charge-separation field from the cavity-accelerated electrons leaving the target.  The latter is manifested in somewhat increase in the maximum energy of ions.  As a result, DCE produces protons with the energies up to 30–35 MeV for a 2.2 J laser pulse. This corresponds to a record maximum proton energy per joule of incoming laser energy. The total laser energy conversion efficiency into MeV protons is approximately 2%. The mechanism does not require ultrahigh laser contrast, which is an important advantage over the competitive ion acceleration from ultrathin foils.

Figure.  Illustration of a directed Coulomb explosion during relativistic self-trapping of a laser pulse.

A.J. Castillo, S.G. Bochkarev, M.G. Lobok, V.Yu. Bychenkov,
JETP Letters 124, issue 4 (2026)

Abrikosov vortices are among the most fundamental manifestations of superconductivity. Their internal structure contains valuable information about the microscopic properties of the superconducting state and can now be directly visualized by scanning tunneling microscopy (STM) with atomic-scale resolution.

In this work, we investigate how the structure of a vortex is modified when it is trapped inside a mesoscopic superconducting grain connected to its surroundings through a finite-transparency interface. Using the quasiclassical Usadel theory, we show that the pair potential, supercurrent density, and local density of states exhibit abrupt changes at the grain boundary. Remarkably, in the experimentally relevant limit of small grains, these interface effects are governed by a single dimensionless parameter that combines the grain size and interface transparency.

Our theory predicts a characteristic jump in the local density of states at the grain boundary, which can be directly measured by STM. This provides a practical route for extracting the intergranular boundary resistance from spectroscopic data. We also derive an analytical criterion determining the minimum grain size required to support a vortex state.

The results establish a simple framework for interpreting vortex-core spectroscopy in granular superconductors and reveal how local superconducting properties can be inferred from the electronic structure of individual vortices.

A.A.Golubov, M.M.Khapaev, V. S. Stolyarov, M.Yu.Kupriyanov
JETP Letters 124, issue 1 (2026)

Magnons are excitation quasiparticles of magnetically ordered crystals. Due to their weak interactions and low mass, they can form magnon Bose-Einstein condensates at sufficient density. Quantum transport of magnons over distances of several centimeters has been observed in yttrium iron garnet films [1].

Magnons obey Bose statistics, so external RF pumping excites only magnons in the quantum state of pre-existing magnon BEC (Boson amplification). We used this effect to create a continuously existing BEC by receiving the RF signal from the magnons, amplifying it, and feeding it back to the sample [2].

The natural frequency of the BEC is determined by the balance of magnon relaxation and excitation energies and the phase relationship in the feedback loop. As a result, we obtained a nonlinear quantum object with unique properties that functions even at room temperature [3].

When the magnetic field changes, the phase relationship in the feedback line abruptly changes by a multiple of 2 pi, leading to a new stable state. This switching of quantum states is in many ways similar to the properties of a phase-locked quantum qubit in superconductors. [3].

Fig. 1. Switching of magnon BEC states

[1]. G. A. Knyazev, A. N. Kuzmichev, P. E. Petrov, I. V. Savochkin, P. M. Vetoshko, V. I. Belotelov, and Yu.M. Bunkov,  “Direct observation of the transition from spin waves to the magnon Bose condensate» Optics Express 32, 13761 (2024)
      [2]. Yu. M. Bunkov, P. M. Vetoshko, G. A. Knyazev, A. N. Kuzmichev, D. A. Samodelkin, “Permanent Bose–Einstein Condensate”, JETP Letters 123, 119, (2026).
      [3]. This article.

Yu.M.Bunkov et al.,
JETP Letters 123, issue 12 (2026)

Single spin asymmetries A_N in the inclusive production of charged pions in the reactions $\pi^{-}p^{\uparrow}\rightarrow\pi^{\pm}X$ were measured by the SPASCHARM experiment [1] at the U-70 accelerator complex in Protvino. The single-spin asymmetry in the reaction $\pi^{-}p^{\uparrow}\rightarrow\pi^{\pm}X$ was measured for the first time.

The single-spin asymmetries in both reactions, averaged over the transverse momentum range from 0.2 to 1.5 GeV/c, have the same positive sign. Taking into account systematic errors they are equal to each other in magnitude of ~20% and differ from zero by 3σ. Previously, asymmetries of similar sign and magnitude were measured in inclusive π⁰-meson production in various reactions [2].

Such a possible behavior was discussed earlier and predicted by the model of chromomagnetic polarization of quarks [3] and twist-3 model of asymmetry [4].

Dependence of A_𝑁 on x_F (left) and 𝑝_𝑇 (right) in the reactions $\pi^{-}p^{\uparrow}\rightarrow\pi^{\pm}X$ at 26.3 GeV (only statistic errors are shown, systematic errors are estimated as 5÷7 %).

1. V.V. Abramov et al, Phys. Part. Nucl. 54(1), 69 (2023).

2. V.D. Apokin et al.  Sov. J. Nucl. Phys. 49, 103 (1989); V.V. Mochalov et al, Phys. Atom. Nucl. 73(12), 2017 (2010); V.V. Abramov et al, Phys. Atom. Nucl. 77(5), 595 (2014).

3. V.V. Abramov, Phys. Atom. Nucl. 72(11), 1872 (2009).

4. A.V. Efremov, V.M. Korotkiyan, and O.V. Teryaev, Phys. Lett. B 348, 577 (1995).

V.V. Moiseev, V.V. Abramov, I.G. Alekseev et al.
JETP Letters 123, issue 11 (2026)