First-order phase transition in cubic ice between the low-temperature phase VIII and the high-temperature phase VII is studied in an extended mean-field approximation. It is assumed that the arrangements of four protons around each oxygen satisfy the ice condition rigorously, i.e., there are...

The asymptotic behavior of the spin pair correlation function along an axis is investigated for the Ising model on a brickwork lattice with second neighbor interactions. It is suggested that some of long-range phase predicted by the mean-field appriximation arereflected in the asymptotic...

An approximation in the cluster variation method (CVM) is used to investigate the modulated phases as well as the critical temperatures of the 2D and the 3D ANNNI models. It is first found that it shows the existence and the non-existence of the Lifshitz points in the 3D and the 2D model,...