Low-Temperature Lattice Thermal Conductivity of Potassium-Cesium Alloys

The thermal conductivity of five potassium-cesium alloys containing up to 10% cesium and one pure potassium sample which had a resistance ratio $\frac{{\rho }_{300}}{{\rho }_4}=2800\mathrm{}$ has been measured in the 4 to 0.5°K temperature range. In the alloys the lattice component of the thermal conductivity did not show the $T^2$ dependence usually found in the noble-metal alloys. Pippard's theory on the ultrasonic attenuation in metals was used to interpret the lattice conductivity results. It is shown that good agreement can be obtained if phonon scattering by the cesium impurities is considered in addition to the scattering by the conduction electrons. The agreement is much better than that found for the noble-metal alloys. This suggests that in the noble-metal alloys, Fermi-surface deformation effects are not negligible in determining the strength of the phonon-electron coupling. The results suggest that in a potassium sample with a residual resistance of 1 μΩ cm or less the low-temperature heat transport by the lattice is almost entirely via the tranverse modes.