Superconductivity in Pure La and La-Gd

Superconducting La doped with the magnetic impurity Gd has been investigated to determine whether the thermodynamic properties show evidence for "gapless" superconductivity. The measurements show that the specific heat of pure hcp La closely follows the predictions of the BCS theory. The addition of small amounts of Gd does not appreciably change the normal-state electronic specific heat, but it drastically alters the superconducting characteristics. An increase in the Gd content decreases the rate at which the normalized superconducting specific heat $\frac{C_{\mathrm{es}}}{\gamma T_c}$ approaches zero. This may be due to a diminishing ratio of the energy of the energy gap to the critical temperature, or it may be due to ordering of the Gd ions. The measurements do not extend to low enough temperatures to distinguish between these two possibilities. The magnitude of the specific-heat jump at $T_c$ is depressed by the Gd somewhat more rapidly than $1.43\gamma T_c$, but less rapidly than is suggested by the electron-tunneling energy-gap measurements. These alloys can support current densities greater than 5×${10}^4$ A/${\mathrm{cm}}^2$, and they show no latent heat at the transition in a magnetic field. The latter two characteristics are qualitatively similar to behavior of type II superconductors.