Large oxygen-isotope effect in Sr0.4K0.6BiO3: Evidence for phonon-mediatedsuperconductivity

The oxygen-isotope effect has been investigated in a recently discovered su perconductor Sr0.4K0.6BiO3. This compound has a distorted perovskite struct ure and becomes superconducting at about 12 K. Upon replacing O-16 with 180 by 60-80%, the T-c of the sample is shifted down by 0.32-0.50 K, correspon ding to an isotope exponent of alpha (0)=0.40(5). This isotope exponent is very close to that for a similar bismuthate superconductor Ba1-xKxBiO3 with T-c=30 K. The very distinctive doping and T-c dependencies of alpha (0) ob served in bismuthates and cuprates suggest that bismuthates should belong t o conventional phonon-mediated superconductors while cuprates might be unco nventional superconductors.