Charge compensation and optimal stoichiometry in superconducting (CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3Oy

Abstract

The superconductive and magnetic properties of charge-compensated (Ca${}_{x}$La${}_{1\ensuremath{-}x}$)(Ba${}_{1.75\ensuremath{-}x}$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 \ensuremath{\le} $x$ \ensuremath{\le} 0.5 local maxima in ${T}_{C}$ occur for $y$ near 7.15. It is shown that optimum superconductivity occurs for 0.4 \ensuremath{\le} $x$ \ensuremath{\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, $\ensuremath{\ell}=7.1206$ \AA{}, and the distance between interacting layers, \ensuremath{\zeta} = 2.1297 \AA{}, are determined for $x$ = 0.45. The theoretical optimal ${T}_{C0}\ensuremath{\propto}{\ensuremath{\ell}}^{\ensuremath{-}1}{\ensuremath{\zeta}}^{\ensuremath{-}1}$ of 82.3 K is in excellent agreement with experiment (\ensuremath{\approx}80.5 K), bringing the number of compounds for which ${T}_{C}$${}_{0}$ is accurately predicted to 37 from six different superconductor families (overall accuracy of $\ifmmode\pm\else\textpm\fi{}$1.35 K).