The order disorder phenomenon in YBa2Cu3O7_ δ for varying δ and the possible existence of two ordering temperatures

Abstract

We examine a model of long range atomic ordering in YBa2Cu 3O7—δ (0 ≤ δ ≤ 0.5 ), when the temperature T varies, in particular below the tetragonal-orthorhombic transition temperature To. A simple mean field description with two order parameters and interactions between first and second neighbours, is proposed for the ordering in the oxygen deficient basal CuO planes. It is the two dimensional analogue of the one given in the past for b.c.c. Fe1-xAl x, alloys and seems appropriate here according to recent experimental results. According to this scheme, a second ordering temperature T' o is expected to occur below To and to increase with δ while T o decreases. The observation of T'o may have been missed in fast cooling experiments so far, since equilibrium requires a long time to be reached at ordering temperatures. Recently experiments performing slow annealings appear to indeed support the existence of that second transition, all the more important that it is experimentally accompanied by an increase of the superconducting temperature Tc.