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

TL;DR

This study investigates charge compensation and optimal doping in a tetragonal cuprate superconductor, revealing that optimal superconductivity occurs at specific doping levels with nearly invariant carrier density, and confirms a theoretical model predicting T$_C$ accurately.

Contribution

The paper introduces a detailed analysis of charge compensation effects in CLBLCO, demonstrating the invariance of carrier density and validating a model that accurately predicts T$_C$ across multiple superconductor families.

Findings

Optimal T$_C$ occurs at x between 0.4 and 0.5.

Charge compensation leaves carrier density nearly unchanged.

Theoretical T$_C$ prediction matches experimental data within 1.35 K.

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 {\AA}, and the distance between interacting layers, {\zeta} = 2.1297 {\AA}, are determined for x = 0.45. The theoretical optimal T$_{C0}$ {\propto} {\ell}$^{-1}${\zeta}$^{-1}$ 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).