Superconductivity of [formula omitted] compounds

Abstract

The superconductivity of single phase La 1.5− x 2 Sr x Ba 1.5− x 2 Cu 3 O y, 0 ≤ x ≤ 0.80 (LSB) compounds with tetragonal triple-perovskite structure was studied. Samples with 0.10 ≤ x ≤ 0.80 are superconducting and T c(zero) is found between 28 and 48 K. With increasing x, both the hole concentration in the Cu2O layers (the copper oxygen layers in the center perovskite layer) and T c increase. It is interesting to find that although the hole concentration and oxygen stoichiometry of the LSB series are similar to those of the La 1.5− x 2 Ca x Ba 1.5− x 2 Cu 3 O y (LCB) series, the maximum T c of the LSB series is about 30 K lower than that of the LCB series. The difference in T c is probably a result of difference in locations of the Ca 2+ and Sr 2+ ions in the unit cell. The smaller divalent Ca 2+ ion prefers the smaller trivalent Y 3+ site and acts as a hole dopant for the superconducting Cu2O layers. On the contrary, the larger divalent Sr 2+ ion presumably prefers the larger divalent Ba 2+ site, and thus hole doping on the Cu2O layers is not as prominent as the Ca 2+ ion in the Y 3+ site.