The superconducting transition temperature Tc has been measured on the hcp Re-Os, Re-Ru, Ru-Os, and Re-W alloy systems and on slightly doped Re. The Tc variation is analyzed and discussed in terms of McMillan's theory. A sharp peak is observed in the slope of the curve for Tc 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 Tc 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 Tc of slightly doped Tl observed by other workers.
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