arXiv subjects

A. M. Shvaika

Publications and source records attributed to A. M. Shvaika.

41 records · Page 3Linked to original sources

arXiv2000

Strong-coupling approach for strongly correlated electron systems

A perturbation theory scheme in terms of electron hopping, which is based on the Wick theorem for Hubbard operators, is developed. Diagrammatic series contain single-site vertices connected by hopping lines and it is shown that for each vertex the problem splits into the subspaces with ``vacuum states'' determined by the diagonal Hubbard operators and only excitations around these vacuum states are allowed. The rules to construct diagrams are proposed. In the limit of infinite spatial dimensions the total auxiliary single-site problem exactly splits into subspaces that allows to build an analytical thermodynamically consistent approach for a Hubbard model. Some analytical results are given for the simple approximations when the two-pole (alloy-analogy solution) and four-pole (Hartree-Fock approximation) structure for Green's function is obtained. Two poles describe contribution from the Fermi-liquid component, which is dominant for small electron and hole concentrations (``overdoped case'' of high-$T_c$'s), whereas other two describe contribution from the non-Fermi liquid and are dominant close to half-filling (``underdoped case'').

cond-mat.str-el↗

arXiv2000

Self-consistent description of simplified pseudospin-electron model

A method of the self-consistent calculation of the thermodynamical and correlation functions is presented. This approach is based on the GRPA (generalized random phase approximation) scheme with the inclusion of the mean field corrections. The numerical research shows that interaction between the electron and pseudospin subsystems leads to the possibility of the dipole (pseudospin) and charge-density instabilities as well as phase separation into the uniform and/or chess-board phases.

cond-mat.supr-con↗

arXiv2000

Thermodynamics of a pseudospin-electron model without correlations

Thermodynamics of a pseudospin-electron model without correlations is investigated. The correlation functions, the mean values of pseudospin and particle number, as well as the thermodynamic potential are calculated. The calculation is performed by a diagrammatic method in the mean field approximation. Single-particle Green functions are taken in the Hubbard-I approximation. The numerical research shows that an interaction between the electron and pseudospin subsystems leads in the $μ={}$const regime to the possibility of the first order phase transition at the temperature change with the jump of the pseudospin mean value $ $ and reconstruction of the electron spectrum. In the regime $n={}$const, an instability with respect to phase separation in the electron subsystem can take place for certain values of the model parameters.

cond-mat.supr-con↗

arXiv1998

Dynamics and thermodynamics of the pseudospin-electron model in the case of absence of the electron Hubbard correlation. I. The analytical consideration

Dynamics and thermodynamics of the model with local anharmonism in the case of absence of the electron (Hubbard) correlation is investigated in the present work. The correlation functions, mean values of pseudospin and particle number as well as the thermodynamical potential are calculated. The calculation is performed by diagrammatic method in the mean field approximation. Single--particle Green functions are taken in the Hubbard--I approximation.

cond-mat↗

arXiv1998

Pseudospin-Electron Model in Large Dimensions

Energy spectrum and thermodynamics of the pseudospin-electron model introduced at the consideration of the anharmonicity effects in high-$T_c$ superconductors are investigated in the dynamical mean field approximation ($d=\infty$ limit). In the limit of zero electron correlation $U\to 0$ this model is analytically exactly soluble within this approach: in the $μ=const$ regime the first order phase transition with the jump of the pseudospin mean value $ $ and reconstruction of the electron spectrum can realize, while in the $n=const$ regime the phase separation in electron subsystem can take place for certain values of the model parameters. On the basis of the obtained results the applicability of the approximate schemes previously used for the investigation of the pseudospin-electron model are discussed.

cond-mat.str-el↗