Tunneling α2F(ω) and heat-capacity measurements in high-T/sub c/ Nb3Ge

Improved film synthesis has allowed us to prepare Nb3Ge/SiO/sub x//Pb tunnel junctions on samples with high T/sub c/ (up to 21.2 K) and large energy gap (Δ3Ge up to 3.85 meV). These junctions have satisfactory features for taking derivative measurements. The data were reduced by the modified McMillan-Rowell proximity gap inversion analysis developed by Arnold and Wolf to generate α2F(ω) and related microscopic parameters. The trend previously seen of a movement of the lowest phonon branch to lower energies as the T/sub c/ and gap increase is continued, resulting in the lowest-energy phonon mode being well defined and enhanced in strength. Theoretical functional derivatives for T/sub c/ (by Bergmann and Rainer) and the energy gap (by Mitrovic et al.) qualitatively explain the rise in T/sub c/, energy gap, and 2Δ/k/sub B/T/sub c/. Heat-capacity measurements have been performed on various samples to give bulk Nb3Ge properties, including one sample which was analyzed by tunneling α2F(ω) and heat capacity. γ = 34 +- 1.5 mJ/mole K2 and the bare density of states, N(0) = 1.5 +- 0.1 states/eV atom, suggests that a high density of states is inadequate to explain the high T/sub c/ in Nb3Ge. Values for 2Δ/k/sub B/T/sub c/ = 4.2 +- 0.1 and ΔC/γT/sub c/approx.1.9 indicate a strong-coupled superconductor, in agreement with tunneling results on this sample