Growth and structural characterization of large superconducting crystals of La$_{2-x}$Ca$_{1+x}$Cu$_2$O$_{6}$

TL;DR

This study reports the growth of large La-Ca-2126 crystals, their structural and magnetic characterization, and the identification of phases contributing to superconductivity with transition temperatures up to 60 K.

Contribution

It presents a method for growing large superconducting La-Ca-2126 crystals and characterizes their multi-phase structure related to superconductivity.

Findings

Superconducting crystals contain three coherent phases.

The dominant phase is La-Ca-2126 with Ca doping.

Ca segregation and phase formation support hole-doping and superconductivity.

Abstract

Large crystals of La$_{2-x}$Ca$_{1+x}$Cu$_2$O$_{6}$ (La-Ca-2126) with $x=0.10$ and 0.15 have been grown and converted to bulk superconductors by high-pressure oxygen annealing. The superconducting transition temperature, $T_c$, is as high as 55~K; this can be raised to 60~K by post-annealing in air. Here we present structural and magnetic characterizations of these crystals using neutron scattering and muon spin rotation techniques. While the as-grown, non-superconducting crystals are single phase, we find that the superconducting crystals contain 3 phases forming coherent domains stacked along the $c$ axis: the dominant La-Ca-2126 phase, very thin (1.5 unit-cell) intergrowths of La$_2$CuO$_4$, and an antiferromagnetic La$_8$Cu$_8$O$_{20}$ phase. We propose that the formation and segregation of the latter phases increases the Ca concentration of the La-Ca-2126, thus providing the hole-doping that supports superconductivity.