Effects of two gaps and paramagnetic pair breaking on the upper critical field of SmFeAsO 0.85 and SmFeAsO 0.8 F 0.2 single crystals

We investigated the temperature dependence of the upper critical field $[{H}_{c2}(T)]$ of fluorine-free ${\text{SmFeAsO}}_{0.85}$ and fluorine-doped ${\text{SmFeAsO}}_{0.8}{\text{F}}_{0.2}$ 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.