Vortices and the mixed state of ultrathin Bi films

TL;DR

This study investigates vortex behavior and the mixed state in ultrathin Bi superconducting films, revealing hysteresis, vortex pinning effects, and deviations from Ginzburg-Landau theory due to disorder.

Contribution

It provides new insights into vortex dynamics, critical current behavior, and the impact of Ge underlayers on vortex pinning in ultrathin Bi films.

Findings

Hysteresis observed in I-V characteristics across all films.

Ge underlayers enhance vortex pinning and cause a peak in critical current.

Upper critical fields deviate from Ginzburg-Landau predictions due to disorder.

Abstract

Current-voltage (I-V) characteristics of quench condensed, superconducting, ultrathin Bi films in a magnetic field are reported. These show hysteresis for all films, grown both with and without thin Ge underlayers. Films on Ge underlayers, close to superconductor-insulator transition, show a peak in the critical current, indicating a structural transformation of the vortex solid. These underlayers, used to make the films more homogeneous, are found to be more effective in pinning the vortices. The upper critical fields ($B_{c2}$) of these films are determined from the resistive transitions in perpendicular magnetic field. The temperature dependence of the upper critical field is found to differ significantly from Ginzburg-Landau theory, after modifications for disorder.