A crossover in the temperature behavior of the perpendicular upper critical magnetic field of layered superconductors and thin films

Abstract

A mechanism which relates the upturn of the perpendicular upper critical magnetic field Hc2⊥(T) in layered superconductors and thin films with the structural inhomogeneity in the bulk of the sample, provided that the local critical temperature Tc* inside the inhomogeneity is higher than in the rest of the sample (Tc) is proposed. Within the Ginzburg–Landau approach an equation which describes two types of experimentally observed nonlinearities in Hc2⊥(T) near Tc for ISN (insulator-superconductor-normal metal) and NSN layer configurations, is found. In the NSN case a crossover from the linear branch Hc2⊥(T)∝(Tc−T), for fields H⩽Hm, to the nonlinear branch with the upturn, if H>Hm, takes place. The crossover field Hm is inversely proportional to the local enhancement of the critical temperature (Tc*−Tc) and the distance R to the surface (the nearest surface, in case of a thin film). In the ISN case the upturn holds for H<Hm, whereas for higher fields Hc2⊥(T) crosses over to the linear branch. In the ISI case the Hc2⊥(T) is a linear function.