Orbital and spin effects for the upper critical field in As-deficient disordered Fe pnictide superconductors

Abstract

We report B_c2 data for LaO_{0.9}F_{0.1}FeAs_{1-delta} in a wide T and field\nrange up to 60 Tesla. The large slope of B_c2 approx ~ -6 Tesla/K near an\nimproved T_c = 28.5 K of the in-plane B_c2(T) contrasts with a flattening\nstarting at 23 K above 30 Tesla we regard as the onset of Pauli-limited\nbehavior (PLB) with B_c2(0) about 65 Tesla. We interpret a similar hitherto\nunexplained flattening of the B_c2(T) curves reported for at least three other\ndisordered closely related systems as also as a manifestation of PLB. Their\nMaki parameters have been estimated analyzing their B_c2(T) data within the WHH\napproach. The pronounced PLB of (Ba,K)Fe_2As_2 single crystals from a tin-flux\nis attributed also to a significant As deficiency. Consequences of our results\nare discussed in terms of disorder effects within conventional (CSC) and\nunconventional superconductivity (USC). USC scenarios with nodes on individual\nFermi surface sheets (FSS), can be discarded for our samples. The increase of\ndB_c2/dT|_{T_c} by sizeable disorder provides evidence for an important\nintraband (intra-FSS) contribution to the orbital upper critical field. We\nsuggest that it can be ascribed either to an impurity driven transition from\ns_{+-} USC to CSC of an extended s_{++}-wave state or to a stabilized\ns_{+-}-state provided As-vacancies cause predominantly strong intraband\nscattering in the unitary limit. We compare our results with B_c2 data from the\nliterature with no PLB for fields below 60 to 70 Tesla probed so far. A novel\ndisorder related scenario of a complex interplay of SC with two different\ncompeting magnetic instabilities is suggested.\n