The anomalous thermal conductivity of la_2-xsr_xcuo_4-y at very low temperatures

The authors report measurements of thermal conductivity (kappa) extending to 100 mK on a series of La_2-xSr_xCuO_4-y specimens with 0.10<or=x<or=0.25. For two superconducting samples (x=0.20, x=0.25) the temperature dependence of kappa weakens below about 0.5 K, approaching a variation linear in T near 0.1 K. A non-superconducting sample (x=0.10) exhibits a kappa varies as T³ dependence below 1 K, characteristic of phonon boundary scattering. After annealing in a vacuum, the x=0.20 specimen became insulating and had a thermal conductivity similar in magnitude and temperature dependence to that of the x=0.10 sample. Thus superconducting and non-superconducting samples are clearly distinguished by the behaviour of their thermal conductivities at sub-kelvin temperatures. These results are shown to be consistent with the presence of a small number of free carriers well below T_c in the superconductors. They suggest that a non-superconducting band of electrons is present in the highly doped materials.