Variation ofTcand resistivity in charge-compensated (CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3Oy

Abstract

We report careful measurements of the transition temperature ${\mathit{T}}_{\mathit{c}}$ of (${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$) (${\mathrm{Ba}}_{1.75\mathrm{-}\mathit{x}}$ ${\mathrm{La}}_{0.25+\mathit{x}}$)${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$ as a function of oxygen content y, mainly for compositions x=0.1 and 0.4. This 1:2:3 superconductor family, that exists in the composition range 0≤x≤∼0.5, is tetragonal and has a maximal ${\mathit{T}}_{\mathit{c}}$ of ∼82 K. Through mutual charge-compensating cosubstitutions, it preserves a constant charge on the noncopper cations (Q=7.25), independent of x. We observed a monotonic decrease in ${\mathit{T}}_{\mathit{c}}$ as y varies from the initial value ∼7.15 to the insulator state roughly at y≃6.9. At the metal-insulator (MI) transition, the value of ${\mathit{y}}_{\mathrm{MI}}$ decreases with increasing x. Most unusual is the substantial increase in ${\mathit{T}}_{\mathit{c}}$ by 21 to 24 K when x changes from 0.1 to 0.4, under conditions where the charge density remains constant. It is accompanied by a twofold decrease in resistivity ρ. We attribute this to the change in band structure with composition x, which occurs due to increase in Ca content and to oxygen redistribution near La that substitutes for Ba. This induces a net electron transfer from the planes to the disordered chains. This assumption is supported by the decrease in ${\mathit{y}}_{\mathrm{MI}}$ and in lattice parameters with increasing x. It implies that changes in mobile hole density, which are responsible for the increase in ${\mathit{T}}_{\mathit{c}}$ and decrease in ρ, take place despite the total charge density being constant.