Критические температуры модели локальных бозонов на квадратной решетке в приближении Бете

We consider the inclusion of short-range correlations for a two-dimensional model of local bosons on a square lattice in the framework of the Bethe approximation for clusters of 2 and 4 sites. Explicit equations are obtained for determining the critical temperatures of charge and superfluid ordering and their solutions are considered for various ratios of the charge-charge correlation parameter and the transfer integral. It is shown that taking into account short-range correlations for temperatures of charge ordering leads to the appearance of a critical concentration of bosons, limiting the region of existence of solutions like charge ordering. For superfluid ordering, when short-range correlations are taken into account, the critical temperature is reduced down to zero values at half-filling. The phase diagram of the model of local bosons is constructed with allowance for phase separation within the framework of Maxwell's construction, and it is shown that taking into account short-range correlations in the Bethe approximation quantitatively approximates the form of the phase diagram to the results of the quantum Monte Carlo method.

Приближение Бете для двумерной спин-псевдоспиновой системы

A two-dimensional spin-pseudospin model is considered, which generalizes a diluted antiferromagnetic Ising model with charged nonmagnetic impurities in the case of two types of charges. The analytical results in the Bethe approximation are compared with the results of numerical simulation using the classical Monte Carlo method for various parameters.