Phonon properties of A-15 superconductors obtained from heat-capacity measurements

The heat capacities of Nb3Al, Nb3Sn, and Nb3Sb were measured between 2 and 400 K. Their superconducting transition temperatures Tc are 18.5, 17.9, and 0.2 K, respectively. The higher-temperature entropy was analyzed, and it was found that phonon-mode softening occurs on cooling for the two high-Tc compounds, but not for Nb3Sb. Geometric mean phonon frequencies ωg are also evaluated for these compounds and for V3X, where X=Si, Ga, Ga0.5Sn0.5, and Sn. As is shown, ωg is an appropriate phonon average for evaluating McMillan's expression for the electron-phonon mass enhancement. Average phonon properties cannot reliably be used to calculate Tc values for these A-15 materials. It is suggested that, for the higher electronic density-of-states materials, select phonon modes strongly couple to the electronic system and influence the magnitude of Tc to a greater extent than average-phonon correlations would indicate. This view is supported by the observation that the higher-density-of-states materials exhibit the more pronounced phonon-mode softening on cooling. © The American Physical Society