Thermal conductivity of YBa2Cu3O7- below 1 K: Evidence for normal-carrier transport well below Tc

We report measurements of thermal conductivity at temperatures below 1 K on superconducting and insulating (oxygen-deficient) ceramic YBa2Cu3O7-. For the superconductor, the temperature dependence of the thermal conductivity begins to weaken below 0.5 K, approaching a T-linear behavior. Below 1 K, the insulating material, produced by vacuum-annealing the superconductor, exhibits a T3 dependence characteristic of phonon boundary scattering. Reannealing samples in flowing oxygen restores superconductivity at 90 K and the T-linear behavior of the thermal conductivity at the lowest temperatures. Scanning electron micrographs reveal an identical grain structure before and after successive heat treatments, indicating that the anomaly in for the superconductor is not associated with the specific geometry of the microstructure. The data are consistent with the presence of a small number of normal carriers in the high-Tc material.