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-xP_x)_2, has been studied through measurements of magnetic hysteresis cycles. We show that the introduction of surface irregularities in the μm 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.