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 $\mu={}$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.Comment: 23 pages, 54 figures and diagram

Magnetic susceptibility of a CuO2 plane in the La2CuO4 system: I. RPA treatment of the Dzyaloshinskii-Moriya Interactions

Motivated by recent experiments on undoped La2CuO4, which found pronounced temperature-dependent anisotropies in the low-field magnetic susceptibility, we have investigated a two-dimensional square lattice of S=1/2 spins that interact via Heisenberg exchange plus the symmetric and anti-symmetric Dzyaloshinskii-Moriya anisotropies. We describe the transition to a state with long-ranged order, and find the spin-wave excitations, with a mean-field theory, linear spin-wave analysis, and using Tyablikov's RPA decoupling scheme. We find the different components of the susceptibility within all of these approximations, both below and above the N'eel temperature, and obtain evidence of strong quantum fluctuations and spin-wave interactions in a broad temperature region near the transition.Comment: 20 pages, 2 column format, 22 figure

Thermodynamics of Pseudospin-Electron Model in the U=0 Limit

Analytical consideration of a pseudospin-electron model in the absence of electron correlations is presented. Pseudospin and electron number mean values, thermodynamic potential, pair correlation functions are obtained in the same self-consistent approximation. The possibility of either first or second order phase transitions between different uniform phases (bistability) as well as between the uniform and the chessboard one is shown. In the regime n=const, an instability with respect to phase separation can take place