В рамках (псевдо)спинового S = 1 формализма показано, что структура антифазных доменных границ в фазе зарядового упорядочения системы со смешанной валентностью типа " триплета" Cu 1+,2+,3+ в купратах на двумерной квадратной решетке существенно зависит от параметра одноцентровых корреляций U. Компьютерное моделирование на больших квадратных решетках иллюстрирует изменение структуры границ от однородной моновалентной непроводящей типа Cu 2+ до нитевидной сверхпроводящей при относительно небольшом изменении положительных значений U.

The classical Monte Carlo method is used for the study of properties of the ground state and phase transitions of the spin-pseudospin model describing a two-dimensional Ising magnet with competing charge and spin interactions. This competition causes ground state degeneracy and frustration. It is shown that the ground state degeneracy is observed in the frustration area with nonzero probabilities of the formation of two different ordered states. Based on histogram analysis of Monte-Carlo data, the type of phase transitions is analyzed. It is found that first order phase transitions are observed near the frustration point, depending on the relationship between the spin s = 1/2 and pseudospin S = 1 interactions.

More than 35 years of experience in the study of cuprate superconductors shows that the main characteristics of the phase diagram can only be obtained by taking into account mesoscopic static/dynamic phase inhomogeneity as a key property of these materials. Within a minimal model for the CuO2 planes with the on-site Hilbert space reduced to only three effective valence centers [CuO4]7-,6-,5- (nominally Cu1+,2+,3+) with different conventional spin and different orbital symmetry we propose a unified non-BCS model that allows one to describe the main features of the phase diagrams of doped cuprates within the framework of a simple effective field theory. Using Maxwell's construction, the global nature of the electronic phase separation in the CuO2 planes of HTSC cuprates is established, which makes it possible to understand and explain many fundamental features of the physics of the normal and superconducting state of cuprates, including the mechanism of formation of the HTSC and pseudogap phase. The features of phase-inhomogeneous states and their evolution with temperature and doping degree, including the special role of the impurity potential in cuprates/nickelates with nonisovalent substitution, are considered for particular examples of the charge triplet model in the framework of the classical Monte Carlo method.