Topological Defects in the Flux-Line Lattice and Their Relationship to the Critical Current of a Type-II Superconductor

We report on a detailed magnetic decoration study of the real-space structure of the flux-line lattice (FLL) in NbS${\mathrm{e}}_2$. We find two types of topological defects with different behaviors as a function of applied current and history. Isolated dislocations anneal with increasing current but peak at the critical current $J_c$, being related to the microscopic disorder in the sample related to $J_c$. The density of FLL grain boundaries is independent of current below $J_c$ but drops abruptly at $J_c$, being the result of FLL nucleation effects and not affecting $J_c$. We also report on current-induced hexatic order in the FLL.