Model for the high-temperature oxygen-ordering thermodynamics in YBa2Cu3O6+x: Inclusion of electron spin and charge degrees of freedom.

Abstract

A lattice-gas model for the high-temperature oxygen-ordering thermodynamics in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ is presented, which assumes constant effective pair interactions between oxygen atoms and includes in a simple fashion the effect of the electron spin and charge degrees of freedom. This is done using a commonly utilized picture relating the creation of mobile electron holes and unpaired spins to the insertion of oxygen into the basal plane. The model is solved using the nearest-neighbor square approximation of the cluster-variation method. In addition, preliminary Monte Carlo results using next-nearest-neighbor interactions are presented. The model is compared to experimental results for the thermodynamic response function, kT(\ensuremath{\partial}x/\ensuremath{\partial}\ensuremath{\mu}${)}_{\mathit{T}}$ (\ensuremath{\mu} is the chemical potential), the number of monovalent copper atoms, and the fractional site occupancies. The model drastically improves the agreement with measured values of kT(\ensuremath{\partial}x/\ensuremath{\partial}\ensuremath{\mu}${)}_{\mathit{T}}$ as compared to bare lattice-gas models. Additionally, the monovalent copper count, in contrast to the standard lattice-gas models, is determined self-consistently and agrees qualitatively with experiment.