Effects of concentration and temperature dependent third-neighbor interaction on the oxygen ordering of YBa2Cu3O7-y

This paper reports on the mechanism of the oxygen ordering on the Cu-O plane of YBa2Cu3O7-y superconductor which is studied in terms of the consistent cluster variation method (CCVM). Based on the two-dimensional Ising model, which includes the symmetric nearest neighbor (NN) interaction J1, asymmetric next-nearest neighbor (NNN) interactions J2 and J3, third-neighbor (TN) interaction J4 and three-body (TB) interaction J5, an nonlinear Hamiltonian is obtained by introducing the concentration and temperature dependent effective TN interaction Jeff4 within mean-field approximation (MFA). The calculated results show that the Jeff4 is highly isotropic in order phases and isotropic in disorder phase, so that the long-range order parameter of the O(II) phase exhibits an asymmptotical behavior at high temperature, and the symmetry of the phase diagram is broken. The behavior of the oxygen pair correlation in the O-Cu-O chains gives a good explanation for the normal states resistivity and the two plateaus of Tc