Abstract
Thermodynamic properties of the two-sublattice Hubbard model with local anharmonicity and direct interaction between pseudospins are considered within the mean field approximation (MFA) and one-loop approximations (the first order of high-density expansion method). Such a model can be used for description of dielectric properties of YBaCuO-type superconductors along c-axis [D. Mihailovic, A.J. Heeger, Pyroelectric and piezoelectric effects in single crystals of YBa 2Cu 3O 7− δ , Solid State Commun. 75 (1990) 319–323; A.I. Grachev, I.V. Pleshkov, Pyroelectric voltage in YBCO thin films, Solid State Commun. 101 (7) (1997) 507–512] (pseudospins represent an anharmonic motion of apical oxygen O4). Unlike Mitsui model, the pseudospin part of the considered Hamiltonian includes interactions of pseudospins with electrons. As a result, the ( T c, h) diagrams change their shapes and the width of area with ferroelectric ordering depends on carriers concentration. Regions of existence of phase transitions deal with pseudospin ordering are established. Approach for correction of one-loop approximation results is proposed.
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