Thermal conductivity of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ below 1 K: Evidence for normal-carrier transport well below $T_c$

We report measurements of thermal conductivity at temperatures below 1 K on superconducting and insulating (oxygen-deficient) ceramic $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. For the superconductor, the temperature dependence of the thermal conductivity $\kappa$ 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 $\kappa \propto T^3$ 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 $\kappa$ 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-$T_c$ material.