Thermal Conductivity of Superconducting and Normal Lanthanum

The thermal conductivity of superconducting and normal f. c. c. lanthanum was measured at the temperature range 1.7 to 7.0°K. The thermal conductivity of yttrium was also measured to test the cryostat. The ratio of the energy gap of lanthanum at absolute zero to the transition temperature, 2 Δ (0)/ k T c , is 2.9 to 3.0, where T c =6.04°K. The ratio obtained from the temperature dependence of the thermal conductivity in the superconducting state is in agreement with the value deduced from the ratio of the thermal conductivities of the superconducting to the normal state. Although the specimen employed in this experiment has the resistivity ratio of 44, the magnetization measurements indicate that the specimen is a second kind superconductor due to a trace of impurity contained. High critical fields and the wide range of the field transition can be interpreted in terms of a second kind superconductor.