Spring-defect model of structural phase transition in YBa₂Cu₃O_7-δ

Abstract

A "spring-defect model" has been developed for describing the connection between elastic distortion and oxygen ordering and the concomitant structural phase transition (from tetragonal to orthorhombic at 900 K) in high T_c (≤ 90 K) superconductor YBa₂Cu₃O_7-δ. The lattice statics of the CuO basal plane is treated in the harmonic approximation in which the mean distance between two copper atoms is assumed to take two different values depending on whether or not there is an intervening oxygen defect. The presence or the absence of the latter is modelled in terms of a pseudo spin ρ =+1 or -1. Elimination of the lattice degrees of freedom leads to Ising interactions between oxygen atoms in a two-dimensional lattice, which turn out to be long-ranged, repulsive for first-neighbours and competing as well as anisotropic for second-neighbours, for certain choices of the spring constants (K, K') and the distortion parameter (α). The model thus provides a mechanism for the structural transition from the tetragonal phase to the orthorhombic phase. Also, the coupling between the spin and lattice degrees of freedom allows for a systematic study of ferroelasticity and phonon properties of this system.