Modeling the Critical Current of Polycrystalline Superconducting Films in High Magnetic Fields

We have performed simulations using time-dependent Ginzburg-Landau theory on a two-dimensional (2-D) polycrystalline system, where grain boundaries are modeled as narrow regions with a locally reduced critical temperature (T
_c
). For the small system sizes investigated, we find that the critical current density (J
_c
) is not sensitive to changes in grain size until the grain size is sufficiently small that it limits the average superparticle density in the system through the proximity effect. Furthermore, once T
_c
in the boundary regions is sufficiently low relative to the surrounding superconductor that grain boundary regions act as preferred channels for flux flow, further reductions in the boundary T
_c
only weakly reduce J
_c
across the superconductor.