Low-temperature heat capacity ofα-uranium and its relation to the pressure dependence ofTc

The heat capacities of five samples of $\alpha$-uranium, including one single crystal, have been measured between approximately 0.1 and 2 K, at zero pressure. The four polycrystalline samples showed broad bulk superconducting transitions. The single crystal, for which susceptibility measurements showed a transition near 0.3 K, was not completely superconducting at the lowest temperature of our measurements, and we estimate ${\overline{T}}_c\le 0.1$ K. The shapes of the heat-capacity anomalies associated with the transitions to the superconducting state were those of broadened BCS transitions, thus showing that local moments and pair-breaking mechanisms are not involved in limiting the values of $T_c$. The values of $\gamma$, the coefficient of the electronic heat capacity, were significantly higher for the polycrystalline samples than for the single crystal, and there is evidence from other work of a similar trend in the lattice heat capacity. It is suggested that these trends are related to the pressure dependence of the same parameters, and that these pressure dependences and that of $T_c$ are all produced by an unusual and strong pressure dependence of the phonon spectrum.