Suppression of the critical temperature of superconducting NdFeAs(OF) single crystals by Kondo-like defect sites induced by alpha-particle irradiation.

We report the effect of alpha-particle irradiation on the reduction of the critical temperature T{c} of a NdFeAs(OF) single crystal. Our data indicate that irradiation defects cause both nonmagnetic and magnetic scattering, resulting in the Kondo-like excess resistance Delta rho(T) proportional to lnT over 2 decades in temperatures above T{c}. The critical density of magnetic irradiation defects which suppresses T{c} is found to be much higher than those for cuprates and multiband BCS superconductors. We suggest that such anomalously weak pair breaking by irradiation defects indicates that magnetic scattering in pnictides is coupled with pairing interactions mediated by spin fluctuations.

Tunneling Measurements of Superconducting Quasi-Particle Density of States and Calculation of Phonon Spectra.

It is an unfortunate fact that the tunneling technique, which has proved incredibly successful in the study of superconductivity, has given little information about the normal state properties of metals and semiconductors. It will be shown that, in the determination of the superconducting quasi-particle density of states, it is the change in density induced by the onset of superconductivity which is measured rather than the total density. Returning to the problem of normal materials, a review of the limited achievements and failures of tunneling will be presented. This will include the influence of band edges on tunneling in p-n diodes and metal-semiconductor contacts, the structures observed in tunneling into bismuth and the negative results obtained in nickel and palladium. The dominant effect of the change in barrier shape in most of these tunneling characteristics will be illustrated.