Critical current and vortex properties in FeS_1-xSe_x (x = 0, 0.06)

We report the critical current, vortex pinning mechanism, and irreversibility field in FeS_1-xSe_x (x = 0, 0.06) single crystals. Critical current in FeS is comparable to that in K_xFe_2-ySe₂ with much higher T_c. Fitting reduced field dependence of pinning force with Dew-Hughes model indicates that pinning center is core-normal surfacelike. Reduced temperature dependence of critical current indicates that dominant interaction of vortex cores and pinning centers is via scattering of charge carriers with reduced mean free path (δl). Crossover behavior in field dependence of depinning energy U₀ was observed. The proximity of the irreversibility lines to the upper critical field (H_c2) is consistent with strong pinning in FeS and FeS_0.94Se_0.06. Large critical current and strong pinning suggest that FeS can serve as building blocks for new superconductors with high critical current.