THERMODYNAMICS OF A PSEUDOSPIN-ELECTRON MODEL
Institute for Condensed Matter Physics, National Academy of Sciences of UkrainePseudospin-electron (PE) model is one of theoretical models which considers the interaction of electrons with local lattice vibrations where an anharmonic variables are represented by pseudospins. The theoretical investigation of the PE model is an enduring subject of interest at the quantum statistics department. The model is used to describe the strongly correlated electrons of CuO2 sheets coupled with the vibrational states of apex oxygen ions OIV (which move in the double-well potential) in YBaCuO type high-T super conductors (HTSC) . Recently a similar model has been applied for investigation of the proton-electron interaction in molecular and crystalline systems with hydrogen bonds .
The purpose is to present the thermodynamics of the PE model in the case of the different type interactions between pseudospins. First, we provide an overview of the results of works which deal with the theoretical investigation of the PE model with the inclusion of the direct pseudospin-pseudospin interaction (but without the electron transfer (tij = 0)). Second, we present the results of the investigation of the model in the case of the absence of the direct pseudospin-pseudospin interaction and Hubbard correlation (Jij = 0, U = 0), when interaction between pseudospins via conducting electron is done.
 K.A. Muller. On the oxygen isotope effect and apex anharmonicity in high-Tc cuprates. Z. Phys. B, 80 (1990) 193.
 E. Matsushita. Model of electron-proton correlation in quasi-one-dimensional halogen-bridged mixed-valence complexes: role of proton motion. Phys. Rev. B, 51, No. 24 (1995) 17332.