Alloy Concentration Limitations for Ideal Superconductivity Transitions

For mean free paths less than 440±100 Å, the dc resistance, magnetic susceptibility, and low-frequency surface impedance data on a series of indium-base alloys (Bi, Pb, Sn, Cd, Tl, Hg) indicate negative interfacial energies between superconducting and normal material. The experiments demonstrate the presence of an intermediate state, normal and superconducting, at magnetic fields much less than $H_c$ as well as the usual filamentary superconductivity at high magnetic fields. Thus, quantitatively, the mean-free-path effect and, qualitatively, the intermediate state effect at low fields are in agreement with the theory of Abrikosov and Gorkov. Data on severely cold-worked high-purity tantalum and on a cold-worked In alloy are compared to the effect of alloying alone. Cold work has only a minor effect in increasing the filamentary critical magnetic field in comparison to the mean-free-path effects in indium alloys. The implications for high-field superconductivity are discussed.