Two-band and pauli-limiting effects on the upper critical field of 112-type iron pnictide superconductors

Abstract

Abstract The temperature dependence of upper critical field μ 0 H c2 of Ca 0.83 La 0.17 FeAs 2 and Ca 0.8 La 0.2 Fe 0.98 Co 0.02 As 2 single crystals are investigated by measuring the resistivity for the inter-plane ( H // c ) and in-plane ( H // ab ) directions in magnetic fields up to 60 T. It is found that μ 0 H c2 ( T ) of both crystals for H // c presents a sublinear temperature dependence with decreasing temperature, whereas the curve of μ 0 H c2 ( T ) for H // ab has a convex curvature and gradually tends to saturate at low temperatures. μ 0 H c2 ( T ) in both crystals deviates from the conventional Werthamer-Helfand-Hohenberg (WHH) theoretical model without considering spin paramagnetic effect for H // c and H // ab directions. Detailed analyses show that the behavior of μ 0 H c2 ( T ) in 112-type Iron-based superconductors (IBSs) is similar to that of most IBSs. Two-band model is required to fully reproduce the behavior of μ 0 H c2 ( T ) for H // c , while the effect of spin paramagnetic effect is responsible for the behavior of μ 0 H c2 ( T ) for H // ab .