The anomalous thermal conductivity of La2-xSrxCuO4-yat very low temperatures

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Abstract
The authors report measurements of thermal conductivity ( kappa ) extending to 100 mK on a series of La2-xSrxCuO4-y specimens with 0.103 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 Tc in the superconductors. They suggest that a non-superconducting band of electrons is present in the highly doped materials.