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

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.