Superconductivity in impurity systems with a lower density of charge carriers and with strong electron correlations

The influence of a paramagnetic impurity on the superconducting transition temperature T-c, energy gap Omega(g), and order parameter Delta at T=0 is investigated in systems with a lower density of charge carriers and with strong electron correlations. It is shown that, since the Migdal theorem is violated in these systems, taking the vertex and "crossing" functions into account in the weak-coupling approximation explains the substantial change in the calculated quantities, in particular, the increase of T-c and of the critical impurity concentrations. The decrease of T-c, Delta, and Omega(g) with increasing impurity concentration is slowed considerably in comparison with the case of conventional superconductors on account of nonadiabaticity effects. (C) 2002 American Institute of Physics.