Chain-Fragment Doping and the Phase Diagram ofYBa2Cu3O7−x

Abstract

We show, using a proposed oxygen-defect structure for ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ and a tight-binding model based on density-functional calculations, that the distribution of holes between planes and (fragmented) chains causes jumps and plateaus for the hole counts in the planes as a function of the nonstoichiometry, $x$. These features can be correlated with the occurrence of the antiferromagnetic as well as the 60- and 90-K superconducting phases. Our findings are supportive of the view that the hole count is the controlling parameter for the superconductivity.