The cluster variation method is used to calculate a phase diagram for a two-dimensional Ising model representing the Cu2O plane of the high- T c superconductor YBa2Cu3Oz. Both first ( V 1 and second-neighbor ( V 2) interactions are considered, with V 2/ V 1= -1/2; At high temperatures, the transition from the disordered (tetragonal) to the ordered (orthorhombic) phase is second-order. A tricritical point is found below which phase separation occurs. Fractional site occupancy and second-neighbor pair correlations are calculated as a function of temperature. The relevance of the model to the thermodynamics of ordering in the high- T c compound is discussed.
- Two-dimensional Ising model
- cluster variation method