Monte Carlo simulation of low temperature phase diagrams of YBa₂Cu₃O_6+x

Abstract

We report the results of Monte Carlo simulation of oxygen ordering in the oxygen deficient portion (x<0.5) of YBa₂Cu₃O_6+x at low temperatures. We find qualitative agreement among cluster - variation, Monte Carlo and transfer matrix methods. However, low temperature and ground state simulations clearly indicate the presence of a tetragonal phase. There is also evidence for two second order phase transition lines separating the tetragonal and the "double cell" ortho II phase. The effect of decreasing the inter-chain repulsion on oxygen ordering has also been investigated.