The oxygen isotope effect in Ba0.625K0.375BiO3

Abstract

The nature of the superconducting pairing mechanism in the high-temperature superconducting oxides is still an open question. Shortly after the discovery of these materials, several groups began an intensive study of the oxygen isotope effect. For a monatomic conventional (BCS) superconductor, the transition temperature Tc is proportional to M–α, where M is the mass of the atom and α=0.5, assuming no anharmonicity for the phonon modes and no Coulomb interactions. For a multicomponent material, with an αi, defined for each ion with mass Mi. Thus, each element contributes to the overall α (which would still equal 0.5) with a weight dependent on the structure of the phonon modes that are important for superconductivity. Because the 16O/18O mass ratio is much smaller than the available mass ratios for the isotopes of the other atoms in these materials, the oxygen contribution to α (αox) should be the largest and thus in principle the easiest to measure. A zero or very small oxygen isotope effect would thus imply an unconventional pairing mechanism, although the suppression of in normal phonon-mediated superconductors is common. A large value of α (>0.3) indicates conventional (BCS) superconductivity. Ba1–xKxBiO3 was recently found to be superconducting1, with Tc ≈30 K. Unlike the planar cuprate supercon-dors, this material is cubic. Here we report on the 18O isotope effect for Ba0.625K0.375BiO3. We find that this material has a large isotope effect, indicating that the pairing mechanism is conventional (phonon-mediated).