Thermal conductivity of Ba-K-Bi-O: A contrast to copper oxide superconductors

We report measurements of the thermal conductivity, κ(T), of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$ between temperatures of 70 mK and 300 K. We find that, below the superconducting transition temperature, the behavior of κ(T) is in sharp contrast to that observed in copper-oxide-based superconductors. Measurements in a 6-T magnetic field indicate that, in this material, the thermal conductivity is lower in the superconducting state than it is in the normal state at the same temperature. The information this provides about the strength of the electron-phonon coupling is discussed. Our data show no evidence of the T-linear term in the thermal conductivity below 0.5 K observed in sintered Y- and La-based superconductors.