Specific heat of thin-filmA15 superconductors: An anomalous inhomogeneity discovered

Thin films of metallurgically stable A15 superconductors, primarily Nb-Sn, V-Ga, and V-Si, have been prepared by electron-beam coevaporation onto heated sapphire substrates. The specific heat of these films at low temperature reveals that the width of the transition from the normal to the superconducting state varies from a low of 0.3 K for stoichiometric ${\mathrm{Nb}}_3$Sn to a high of 6 K for A15 Nb-Sn prepared off-stoichiometry. The systematics of this transition width were probed by varying the deposition parameters. A strong dependence is found on deposition temperature, composition, and microstructure. The source appears to be a compositional inhomogeneity which develops at the film surface during growth. A model is proposed for a new type of surface segregation applicable in a growing film where the surface mobility is orders of magnitude greater than the bulk mobility.