N.M. Plakida

On the role of kinematic and exchange interactions in superconducting pairing of electrons in the Hubbard model

Abstract A system of electrons with strong intra-atomic repulsion U ≫ t in the lower Hubbard subband is considered on the basis of the effective exchange Hamiltonian. An excitation spectrum allowing for the superconducting pairing is obtained by employing the projection technique for the Green function in terms of Hubbard operators. An equation for the gap includes both the kinematic (≈ t ) and exchange ( J ≈ t 2 / U ) interactions, but only the latter contributes to the d-wave pairing allowed in the strong correlation limit.

Exchange and spin-fluctuation mechanisms of superconductivity in cuprates

We propose a microscopical theory of superconductivity in CuO

Superconducting pairing in the mean-field approximation for the t-J model: numerical analysis

_2

On d-wave pairing in one band Hubbard model

layer within the effective two-band Hubbard model in the strong correlation limit. By applying a projection technique for the matrix Green function in terms of the Hubbard operators, the Dyson equation is derived. It is proved that in the mean-field approximation d-wave superconducting pairing mediated by the conventional exchange interaction occurs. Allowing for the self-energy corrections due to kinematic interaction, a spin-fluctuation d-wave pairing is also obtained.

On the theory of superconductivity in a model of oxide metals

T\sb{c}

Superconductivity in the two-band singlet-hole model for copper-oxide plane

dependence on the hole concentration and

Isotope effect in a superconductor with strong anharmonicity

\bf k

Isotope effect and anharmonic models of high-Tc superconductors

-dependence of the gap function are derived. The results show that the exchange interaction (which stems from the interband hopping) prevails over the kinematic interaction (which stems from the intraband hopping).A microscopic theory of superconductivity is considered in the framework of the Hubbard p-d model for the CuO2 plane. The Dyson equation is derived in the nonintersecting diagram approximation using the projection technique for the matrix Green function of the Hubbard operator. The solution of the equation for the superconducting gap shows that interband transitions for Hubbard subbands lead to antiferromagnetic exchange pairing as in the t-J model, while intraband transitions additionally lead to spin-fluctuation pairing of the d-wave type. The calculated dependences of the superconducting transition temperature on the hole concentration and of the gap on the wave vector are in qualitative agreement with experiments.

On the theory of the central peak at structural phase transitions

Abstract Discussion and numerical analysis of the mean-field solutions for superconducting pairing in the t - J model obtained in the previous paper are presented. The mean-field theory is based on the projection technique for Green functions in terms of Hubbard operators. The dependences of the critical temperature T c for the s-wave pairing on the electron concentration are calculated. The ground state energy for normal and superconducting states is computed numerically. Some additional strong arguments against the possibility of the s-wave pairing in the t - J model are given as well.

Optical and dc conductivities in high-T c superconductors: Spin-fluctuation scattering in the Emery model

Abstract The strong correlated one band or t-J model is considered. An excitation spectrum allowing for the superconducting pairing is obtained by employing the projection technique for the Green function in terms of Hubbard operators. By taking into account the local constraint which eliminates double site occupancy an equation for d-wave gap is obtained.