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

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.