Phase diagram and critical properties of oxygen ordering in YBa2Cu3Oz

Using transfer-matrix finite-size scaling, the authors reinvestigate a square lattice-gas model Hamiltonian to describe the order-disorder transition in the Cu-O basal plane of YBa2Cu3Oz. The model contains strong nearest-neighbor repulsions and diagonal (2nd neighbor) interactions that alternate in sign. These interactions ensure the stability (at zero temperature) of two experimentally observed structures: a single-cell orthorhombic phase near z = 7 and a double-cell orthorhombic phase near z = 6.5. Critical properties can be understood by recasting the model as an Ashkin-Teller model; numerical results are consistent with this picture