Superconductivity at 60 K in La_2-xSr_xCaCu₂O₆: the simplest double-layer cuprate

Abstract

STARTING with the pioneering work of Bednorz and Müller¹, many copper-oxide-based superconductors with high transition temperatures (T_c) have been discovered. All contain layers of copper-oxygen squares, pyramids or octahedra as their electronically active structural components^2,3. One structure type, first reported for La₂SrCu₂O₆ and La₂CaCu₂O₆ (ref. 4), has stood as an enigma since the beginning of high-T_c research. This crystal structure^4-7 (Fig. 1) is the least complex of all the structures with the double layers of copper oxide pyramids common to the compounds with highest T_c, yet despite considerable effort, both published^8,9 and unpublished, it has not until now been made superconducting. Here we report the successful synthesis and preliminary physical characterization of superconducting (La, Sr)₂CaCu₂O₆. The highest transition temperature observed is 60 K at the composition La_1.6Sr_0.4CaCu₂O₆. This is a uniquely simple double-layer superconductor, which, like its single-layer analogue (La, Sr)₂CuO₄, becomes superconducting through the introduction of carriers in an unambiguous manner—by straightforward atomic substitution without the intervention of charge reservoir layers with flexible valence states.