Recovery of superconductivity and the critical field in layered superconductors

The presence of magnetic impurities greatly affects the temperature dependence of the critical field ${\mathit{H}}_{\mathit{c}2}$. We focus, mainly, on layered superconductors. As a result of correlations among magnetic impurities, the scattering amplitude ${\mathrm{\Gamma }}_{\mathit{s}}$ decreased with T→0 K. This temperature dependence leads to a weakening of the pair-breaking effect and consequently to a positive curvature of ${\mathit{H}}_{\mathit{c}2}$(T) and a drastic increase in ${\mathit{H}}_{\mathit{c}2}$(0). The calculations are in excellent agreement with recent experimental data. It is also shown that the presence of inhomogeneities of a certain size also modifies the temperature dependence of ${\mathit{H}}_{\mathit{c}2}$. © 1996 The American Physical Society.