Enhancement of the critical current by surface irregularities in Fe-based superconductors

The critical current $I_c$ of single crystals of the iron pnictide superconductor BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$, has been studied through measurements of magnetic hysteresis cycles. We show that the introduction of surface irregularities in the $\mu$m scale significantly increase $I_c$, primarily near the irreversibility magnetic field $H_{irr}$, where the surface currents are the main contribution to $I_c$. Such an increase is consistent with a theoretical estimate for the maximum non-dissipative current that a rough surface can sustain, based on Mathieu-Simon continuum theory for the vortex state.