High-temperature superconductivity in La- and Lu-substituted YBa₂,Cu₃,O₇, and related oxides

The perovskites, Y0.75La0.25Ba2Cu3O7 and Y0.75Lu0.25Ba2Cu3O7, show high-T c superconductivity (with zero resistance at or above 80 K), just as the parent compound YBa2Cu3O7. The Lu-substituted oxide, with the smallest unit-cell parameters, shows the highest T c besides exhibiting a 100% Meissner effect. H c1, in these oxides is around 25 mT, but the H c2, is large. The thermopower of YBa2Cu3O7 shows a sharp transition to zero at the superconducting transition, reinforcing the bulk nature of the superconductivity. Preliminary studies show that ErBa2Cu3O7 and Er0.5Y0.5Ba2Cu3O7 are both high-temperature superconductors with zero resistance in the 82–90 K range.