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

摘要

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 asCLBLCO) are considered through quantitative examination of data for electricalresistivity, 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 compensatingLa 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 optimumsuperconductivity occurs for 0.4 {\le} x {\le} 0.5, that the superconductivityand magnetism observed are nonsymbiotic phenomena, and thatcharge$-$compensated doping leaves the carrier density in the cuprate planesnearly invariant with x, implying that only a small fraction of superconductingcondensate resides therein. Applying a model of electronic interactions betweenphysically separated charges in adjacent layers, the mean in-plane spacingbetween interacting charges, {\ell} = 7.1206 {\AA}, and the distance betweeninteracting layers, {\zeta} = 2.1297 {\AA}, are determined for x = 0.45. Thetheoretical optimal T$_{C0}$ {\propto} {\ell}$^{-1}${\zeta}$^{-1}$ of 82.3 K isin excellent agreement with experiment ({\approx} 80.5 K), bringing the numberof compounds for which T$_{C0}$ is accurately predicted to 37 from sixdifferent superconductor families (overall accuracy of {\pm}1.35 K).