Upper limit of metastability of the vortex-free state of a two-dimensional superconductor in a nonuniform magnetic field

摘要

We calculate the magnetic field above which vortices necessarily appear in a two-dimensional superconducting film in a nonuniform magnetic field applied near its center, as in two-coil measurements of superfluid density. Experiments suggest that free vortices appear when the Meissner screening current density is large enough that the free-energy barrier for separating tightly-bound vortex-antivortex (V-aV) pairs into free vortices is comparable to k_BT, not at the much lower thermodynamic critical field. Specifically, we calculate the applied magnetic field above which there exists no metastable state without vortices. There is a simple analogy with the appearance of phase-slip centers in a one-dimensional superconducting wire.