Unconventional superconductivity in low density electron systems and conventional superconductivity in hydrogen metallic alloys
M. Yu. Kagan
Kapitza Institute for Physical Problems of the RAS, 119334 Moscow, Russia
National Research University Higher School of Economics, 109028 Moscow, Russia
Abstract
In a short review-article we first discuss the results, which are mainly devoted to the
generalizations of the famous Kohn-Luttinger mechanism of superconductivity in
purely repulsive fermion systems at low electron densities. In the context of
repulsive-U Hubbard model and Shubin-Vonsovsky model we consider briefly the
superconducting phase diagrams and the symmetries of the order parameter in novel
strongly correlated electron systems including idealized monolayer and bilayer
graphene. We stress that purely repulsive fermion systems are mainly the subject of
unconventional low-temperature superconductivity. To get the high temperature
superconductivity in cuprates (with TC of the order of 100 K) we should proceed to
the t-J model with the van der Waals interaction potential and the competition
between short-range repulsion and long-range attraction. Finally we note that to
describe superconductivity in metallic hydrogen alloys under pressure (with TC of the
order of 200 K) it is reasonable to reexamine more conventional mechanisms
connected with electron-phonon interaction. These mechanisms arise in the attractive-U
Hubbard model with static onsite or intersite attractive potential or in more realistic
theories (which include retardation effects) such as Migdal-Eliashberg strong
coupling theory or even Fermi-Bose mixture theory of Ranninger et al. and its
generalizations.