Depression of the superconducting transition temperature by magnetic impurities: Effect of Kondo resonance in thefdensity of states

Abstract

A new formulation is presented for the calculation of superconducting $T_c$ depression by dilute Ce impurities. The treatment is based on the finite-temperature large-degeneracy expansion for the degenerate-orbital Anderson model. The results may be simply interpreted in terms of resonant structure in the one- and two-body scattering amplitudes; the resonance scale, $T_0$, may, in principle, be measured directly by inverse-photoemission studies of the f density of states. The initial slope of the depression curve (-${\mathrm{dT}}_c$/dC)${\Vert }_{C=0\mathrm{}}$ depends solely on the ratio $T_0$/$T_{c0}$, with $T_{c0}$ the pure-metal transition temperature. The maximum initial slope is anomalously large in comparison with typical values for nonresonant magnetic impurity scattering. For an orbital degeneracy N=6, maximal depression occurs for $T_0$≃5$T_{c0}$. .AE