
PHASE TRANSITIONS IN STRONGLY CORRELATED ELECTRON SYSTEMS. EXACTLY SOLVABLE MODELSIhor Stasyuk (Personal webpage)Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine
Some problems of the theory of strongly correlated electron systems are discussed in the lecture. A brief review of the history of the main ideas and model development (from the Bogoliubov polar model of the metal, Hubbard model and its extensions to the FalicovKimbaJl and pseudospinelectron models) is given. The dynamical mean field theory (DMFT) approach, which is exact hi the limit of the infinite dimension of space, is presented on the example of the binary alloy lattice model. It provides a derivation of equations for the coherent potential and electron Greenâ€™s function in an analytic form as well as expressions for the grand canonical potential and static susceptibilities in the cases of the exactly solvable models. Besides the binary alloy model the pseudospinelectron model (PEM) and FalicovKimball (FK) one belong to the models of this kind. The results of recent investigations of the FK model performed by various groups are discussed. The main features of the energy spectrum and thermodynamics of the model as well as phase transitions into modulated or segregated phases are considered. Special attention is paid in the lecture to the pseudospinelectron model which appeared in the last few years in connection with the investigation of the highTc superconductors and systems with hydrogen bonds (the model is closely related to the FK model but differs by the regime of the thermodynamical averaging procedure). The results of investigation of the equilibrium states of PEM (using its various versions) within the DMFT scheme and by means of the generalized random phase approximation are analyzed and compared. The possibilities of applicai ion of the PEM to description of the inhomogeneous states and structure instabilities in the highTc superconducting systems are discussed. 