Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

We investigated the temperature dependence of the upper critical field [Hc2(T)] of fluorine-free SmFeAsO_0.85 and fluorine-doped SmFeAsO_0.8F_0.2 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.