Tunneling estimates of paramagnon suppression of superconductivity in transition metals

New experimental estimates are given of the separate electron-phonon (λ_ph) and paramagnon (λ_S) contributions to mass renormalization for the transition metals Nb and V, from recent tunneling results. Our analysis, which is the first of its kind, is based on the recent demonstration of Daams, Mitrovic, and Carbotte of a simple rescaling of the Eliashberg equations using an effective coupling parameter $\text{λ}{}_{\mathrm{ph}}\text{(1+λ}{}_{\mathrm{S}}\text{)}$. The results provide a consistent picture of superconductivity in these transition elements from tunneling, in substantially improved agreement with other measurements. We conclude that spin fluctuations reduce the electron-phonon T_c values in Nb and V by 25% and 45%, respectively.