Microscopic derivation of magnetic-flux-density profiles, magnetization hysteresis loops, and critical currents in strongly pinned superconductors.

Abstract

We present a microscopic derivation, without electrodynamical assumptions, of $B(x,y,H(t))$, $M(H(t))$, and $J_c(H(t))$, in agreement with experiments on strongly pinned superconductors, for a range of values of the density and strength of the pinning sites. We numerically solve the overdamped equations of motion % dynamics of these flux-gradient-driven vortices which can be temporarily trapped at pinning centers. The field is increased (decreased) by the addition (removal) of flux lines at the sample boundary, and complete hysteresis loops can be achieved by using flux lines with opposite orientation. The pinning force per unit volume we obtain for strongly-pinned vortices, $J_c B \sim n_p f_p^{1.6}$, interpolates between the following two extreme situations: very strongly-pinned independent vortices, where $J_c B \sim n_p f_p$, and the 2D Larkin-Ovchinikov collective-pinning theory for weakly-pinned straight vortices, where $J_c B \sim n_p f_p^{2}$. Here, $n_p$ and $f_p$ are the density and maximum force of the pinning sites.