Vortex dynamics and upper critical fields in ultrathin Bi films - art. no.214519

Current-voltage (I-V) characteristics of quench condensed, superconducting, ultrathin Bi films in a magnetic field are reported. These I-V's show hyst eresis for all films, grown both with and without thin Ge underlayers. Film s on Ge underlayers, close to superconductor-insulator transition, show a p eak in the critical current, indicating a structural transformation of the vortex solid. These underlayers, used to make the films more homogeneous, a re found to be more effective in pinning the vortices. The upper critical f ields (B-c2) of these films are determined from the resistive transitions i n perpendicular magnetic field. The temperature dependence of the upper cri tical field is found to differ significantly from Ginzburg-Landau theory, a fter modifications for disorder.