Charge compensation and optimal stoichiometry in superconducting (Ca$_x$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_3$O$_y$

Abstract

The superconductive and magnetic properties of charge$-$compensated (Ca$_x$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_3$O$_y$ (normally denoted as CLBLCO) are considered through quantitative examination of data for electrical resistivity, magnetic susceptibility, transition width, muon$-$spin rotation, x$-$ray absorption, and crystal structure. A derivative of LaBa$_2$Cu$_3$O$_y$, cation doping of this unique tetragonal cuprate is constrained by compensating La substitution for Ba with Ca substitution for La, where for 0 {\le} x {\le} 0.5 local maxima in T$_C$ occur for y near 7.15. It is shown that optimum superconductivity occurs for 0.4 {\le} x {\le} 0.5, that the superconductivity and magnetism observed are nonsymbiotic phenomena, and that charge$-$compensated doping leaves the carrier density in the cuprate planes nearly invariant with x, implying that only a small fraction of superconducting condensate resides therein. Applying a model of electronic interactions between physically separated charges in adjacent layers, the mean in-plane spacing between interacting charges, {\ell} = 7.1206 Å, and the distance between interacting layers, ζ = 2.1297 Å, are determined for x = 0.45. The theoretical optimal T$_{C0}$ {\propto} {\ell}$^{-1}$ζ$^{-1}$ of 82.3 K is in excellent agreement with experiment ({\approx} 80.5 K), bringing the number of compounds for which T$_{C0}$ is accurately predicted to 37 from six different superconductor families (overall accuracy of {\pm}1.35 K).