Effects of two gaps and paramagnetic pair breaking on the upper critical field of SmFeAsO0.85 and SmFeAsO0.8F0.2 single crystals

Abstract

We investigated the temperature dependence of the upper critical field [H-c2(T)] of fluorine-free SmFeAsO0.85 and fluorine-doped SmFeAsO0.8F0.2 single crystals by measuring the resistive transition in low static magnetic fields and in pulsed fields up to 60 T. Both crystals show that H-c2(T)'s along the c axis [H-c2(c)(T)] and in an ab-planar direction [H-c2(ab)(T)] exhibit a linear and a sublinear increase, respectively, with decreasing temperature below the superconducting transition. H-c2(T)'s in both directions deviate from the conventional one-gap Werthamer-Helfand-Hohenberg theoretical prediction at low temperatures. A two-gap nature and the paramagnetic pair-breaking effect are shown to be responsible for the temperature-dependent behavior of H-c2(c) and H-c2(ab), respectively.