Superconductivity at 3.5 K in BaPb0.75Sb0.25O3: why is Tc so low?

Abstract

SUPERCONDUCTING BaPb0.75Bi0.25O3, with a transition temperature (Tc ) of 12 K (ref. 1), is now widely viewed as the precursor to the recent appearance of copper oxide superconductors with record high Tc S. The observation of superconductivity near 30 K in Ba0.6K0.4BiO3 (refs 2,3) has raised serious questions as to whether conventional electron–phonon coupling is the primary pairing mechanism in the bismuth–lead oxide perovskites. Their Tc S are anomalously high, given their low densities of electronic states at the Fermi level4,5, but the debate as to the mechanism of superconductivity is far from settled (see, for example, refs 6–8). Here we report the occurrence of superconductivity at 3.5 K in a new member of the family, BaPb0.75Sb0.25O3. Its Tc , less than one-third that of BaPb0.75Bi0.25O3, is surprisingly low in view of the similarities between the two compounds. The discovery of superconductivity in BaPb0.75Sb0.25O3 establishes a series of isostructural compounds withTc S differing by an order of magnitude. This series provides the basis for further studies to elucidate the essential electronic characteristics that lead to high Tc in the bismuth oxide compounds.