Superconductivity of Re-Os, Re-Ru, Ru-Os, and Re-W hcp Alloy Systems and Slightly Doped Re

The superconducting transition temperature $T_c$ has been measured on the hcp Re-Os, Re-Ru, Ru-Os, and Re-W alloy systems and on slightly doped Re. The $T_c$ variation is analyzed and discussed in terms of McMillan's theory. A sharp peak is observed in the slope of the curve for $T_c$ vs Os concentration. This is attributed to the Fermi-surface-topology change of Re due to alloying. A small addition of impurities is observed always to increase the $T_c$ of Re. This is explained in terms of the effects of the band-structure smearing and the Fermi-surface-topology change. By invoking only the second effect near the Fermi level, we can also easily account for the unusual impurity effect on $T_c$ of slightly doped Tl observed by other workers.