L. A. Maksimov

Quantum diffusion of atoms in a crystal localization and phonon-stimulated delocalization

Abstract Localization phenomena in the subsystem of interacting particles which diffuse by tunneling in a crystal is considered. The temperature dependence of the diffusion coefficient is found for the regime of phonon-stimulated delocalization. The quantum diffusion in 3 He- 4 He solid state solution is analysed especially, and comparison is made with the results of Micheev et. al., who first observed this phenomenon.

On the superconductivity in 2D systems with nesting

Abstract Superconducting properties of La 2 CuO 4 are studied within the model of weak coupling for the Emery hamiltonian. The weak coupling model for the 2D CuO 2 lattice is obtained by means of the canonical transformation from the strong coupling model. The elementary excitations within the Emery model are the Zhang-Rice magnetic polarons whose motion depends essentially on the spin state of the Cu-sublattice. The effective spectrum of magnetic polarons for the RVB ground state is shown to satisfy the nesting condition near the Fermi surface in some narrow range of doping level. Several mechanisms are found to contribute to effective interaction, i.e. the usual Hubbard repulsion and sign changing momentum-dependent correlation contributions. Superconducting d-pairing emerges provided repulsion interaction dominates, and s-pairing can be realized if the sign-changing interaction gives the main contribution. In both cases the superconductivity arises owing to nesting which is responsible for the strong anisotropy of the pairing interaction. Nesting conditions are not satisfied for the Neel state and hence superconductivity does not arise in this case.

Hole spectrum of adjacent CuO2 planes in the two-band model and the non-monotonic Tc dependence on the external parameters

Abstract The single hole spectrum ϵ( k ) of adjacent CuO 2 planes is studied within a framework of a two-band model for small-radius magnetic polaron. The Fermi level position relative to the magnetic Brillouin zone boundary depends strongly on the value of the interplane hole hopping parameter h and the hole concentration x at small x . Within this approach a qualitative explanation of the non-monotonic T c behaviour as a function of concentration, pressure and number of adjacent CuO 2 layers in HTSC is given.

On the peculiarities of the magnetic polaron hole spectrum in high-Tc superconductors

Abstract The one hole spectrum ϵ( k ) in the CuO 2 plane is studied in the framework of the magnetic polaron approach on the basis of the two-band Hubbard model. The manifested strong temperature dependence of the spectrum ϵ( k ) is connected with the temperature behavior of the spin correlation functions. It is also shown that for an accurate description of the lowest band bottom position it is important to take into account the dependence of the hopping amplitude on the hopping trajectory and to extend the simplest Zhang-Rice site basis.

Key features of the hole spectrum in three-band Hubbard model for High-Tc superconductors

Abstract In the framework of the hole effective hamiltonian for the three-band model of the CuO2 plane in high-Tc superconductors the hole spectrum e( k ) is investigated on the base of the spin polaron concept. Key features of the experimentally observed dispersion are described: (1) extended saddle point and (2) isotropic band bottom. The important role of direct OO hoppings and frustration in magnetic system is revealed.

Theory of relaxation phenomena in glasses and doped semiconductors at low temperatures

Abstract Low-temperature relaxation properties of dielectric glass and doped disordered semiconductors, also called Fermi glasses, are studied. The isomorphism in the description of both types of material is established. In particular, it is proved that the flip-flop relaxation mechanism, which is a characteristic of dielectric glasses, is also specific for Fermi glasses. The low-temperatures behavior for the thermal conductivity, electrical conductivity and nuclear relaxation in the glassy systems considered is investigated. The relation to experiment is analyzed, which allows us to clarify the key role of the flip-flop processes in the formation of the universal properties of disordered dielectrics. This assertion is still a point of much controversy.