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 ${T}_{c}=(1.374\ifmmode\pm\else\textpm\fi{}0.001)$\ifmmode^\circ\else\textdegree\fi{}K, and has a value of $\frac{{C}_{\mathrm{es}}({T}_{c})}{\ensuremath{\gamma}{T}_{c}}=2.42$, in good agreement with BCS theory. (Here ${C}_{\mathrm{es}}$ is the superconducting electronic specific heat, and $\ensuremath{\gamma}$ is the temperature coefficient of the normal electronic specific heat.) The $\ensuremath{\gamma}$ and ${\ensuremath{\Theta}}_{D}$ for thorium were found to be (4.31\ifmmode\pm\else\textpm\fi{}0.05) mJ/mole ${\mathrm{deg}}^{2}$ and (163.3\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode^\circ\else\textdegree\fi{}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 $\ensuremath{\gamma}$ and ${\ensuremath{\Theta}}_{D}$ of the purer uranium sample were found to be (10.03\ifmmode\pm\else\textpm\fi{}0.02) mJ/mole ${\mathrm{deg}}^{2}$ and (207\ifmmode\pm\else\textpm\fi{}1)\ifmmode^\circ\else\textdegree\fi{}K, respectively.