Temperature dependence of the lower critical field and strong pinning in high-temperature superconductors

We show, within the framework of the Ginzburg-Landau theory, that both the conventional and the anomalous temperature dependence of the lower critical field observed in high-temperature superconductors may result from the flux penetration through a set of separated microdefects. Microdefects modeled by normal layers with proximity-induced superconductivity can produce drastic enhancement of the lower critical field at low temperatures and can provide strong-pinning centers. The pinning interaction between an isolated vortex and the normal layer is primarily magnetic at high temperatures. At low temperatures, magnetic interaction is reduced, due to the increase of the normal-layer coherence length.