Nonconstant electronic density of states tunneling inversion for A15 superconductors: Nb3Sn

We re-examine the tunneling data on A15 superconductors by performing a generalized McMillan-Rowell tunneling inversion that incorporates a nonconstant electronic density of states obtained from band-structure calculations. For Nb3Sn, we find that the fit to the experimental data can be slightly improved by taking into account the sharp structure in the density of states, but it is likely that such an analysis alone is not enough to completely explain the superconducting tunneling characteristics of this material. Nevertheless, the extracted Eliashberg function displays a number of features expected to be present for the highest quality Nb3Sn samples.Comment: 11 pages, 11 figure

Nonequilibrium Transport through a Kondo Dot in a Magnetic Field: Perturbation Theory

Using nonequilibrium perturbation theory, we investigate the nonlinear transport through a quantum dot in the Kondo regime in the presence of a magnetic field. We calculate the leading logarithmic corrections to the local magnetization and the differential conductance, which are characteristic of the Kondo effect out of equilibrium. By solving a quantum Boltzmann equation, we determine the nonequilibrium magnetization on the dot and show that the application of both a finite bias voltage and a magnetic field induces a novel structure of logarithmic corrections not present in equilibrium. These corrections lead to more pronounced features in the conductance, and their form calls for a modification of the perturbative renormalization group.Comment: 16 pages, 7 figure

Evidence for Effective Weakening of Electron-Phonon Interaction in Superconducting Tantalum, Niobium, Lead and Aluminum

It is sporadically documented in the literature that the electron-phonon in- teraction is much weaker than expected in the superconducting state in metals. Owing to a tendency to explain it away this observation has so far gone under-recognized. By applying a straightforward numerical procedure to normal state resistivity and superconducting tunneling conductance experimental data the atomic potentials in tantalum, niobium, lead and aluminum are revealed to be consistently weaker in the superconducting state than in the normal state.<br/