Orbital and spin effects for the upper critical field in As deficient disordered Fe pnictide superconductors
Abstract
We report Bc2 data for LaO0.9F0.1FeAs1-delta in a wide T and field range up to 60 Tesla. The large slope of Bc2 approx ~ -6 Tesla/K near an improved Tc = 28.5 K of the in-plane Bc2(T) contrasts with a flattening starting at 23 K above 30 Tesla we regard as the onset of Pauli-limited behavior (PLB) with Bc2(0) about 65 Tesla. We interpret a similar hitherto unexplained flattening of the Bc2(T) curves reported for at least three other disordered closely related systems as also as a manifestation of PLB. Their Maki parameters have been estimated analyzing their Bc2(T) data within the WHH approach. The pronounced PLB of (Ba,K)Fe2As2 single crystals from a tin-flux is attributed also to a significant As deficiency. Consequences of our results are discussed in terms of disorder effects within conventional (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), can be discarded for our samples. The increase of dBc2/dT|Tc by sizeable disorder provides evidence for an important intraband (intra-FSS) contribution to the orbital upper critical field. We suggest that it can be ascribed either to an impurity driven transition from s+- USC to CSC of an extended s++-wave state or to a stabilized s+--state provided As-vacancies cause predominantly strong intraband scattering in the unitary limit. We compare our results with Bc2 data from the literature with no PLB for fields below 60 to 70 Tesla probed so far. A novel disorder related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.