Superconductivity of Thorium and Uranium

Abstract

The low-temperature susceptibility and specific heat of thorium and uranium have been measured. It is found that thorium becomes a superconductor at $T_c=(1.374\mathrm{}\pm 0.001)$°K, and has a value of $\frac{C_{\mathrm{es}}\left(T_c\right)}{\gamma T_c}=2.42\mathrm{}$, in good agreement with BCS theory. (Here $C_{\mathrm{es}}$ is the superconducting electronic specific heat, and $\gamma$ is the temperature coefficient of the normal electronic specific heat.) The $\gamma$ and ${\Theta }_D$ for thorium were found to be (4.31±0.05) mJ/mole ${\mathrm{deg}}^2$ and (163.3±0.7)°K, respectively. Both uranium samples appeared to undergo superconducting transitions when observed magnetically, yet both exhibited only normal-state behavior in their specific heat. Hence it seems likely that the apparent superconductivity of alpha uranium is not characteristic of the bulk metal. The $\gamma$ and ${\Theta }_D$ of the purer uranium sample were found to be (10.03±0.02) mJ/mole ${\mathrm{deg}}^2$ and (207±1)°K, respectively.