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

We report upper critical field B-c2(T) data for LaO0.9F0.1FeAs1-delta in a wide temperature and field range up to 60 T. The large slope of B-c2 approximate to -5.4 to -6.6 T K-1 near an improved T-c approximate to 28.5 K of the in-plane B-c2(T) contrasts with a flattening starting near 23 K above 30 T we regard as the onset of Pauli-limited behaviour (PLB) with B-c2(0) approximate to 63-68 T. We interpret a similar hitherto unexplained flattening of the B-c2(T) curves reported for at least three other disordered closely related systems, Co-doped BaFe2As2, (Ba, K) Fe2As2 and NdO0.7F0.3FeAs (all single crystals), for applied fields H parallel to (a, b), also as a manifestation of PLB. Their Maki parameters have been estimated by analysing their B-c2(T) data within the Werthamer-Helfand-Hohenberg approach. The pronounced PLB of (Ba, K) Fe2As2 single crystals obtained from an Sn flux is attributed also to a significant As deficiency detected by wavelength dispersive x-ray spectroscopy as reported by Ni et al (2008 Phys. Rev. B 78 014507). Consequences of our results are discussed in terms of disorder effects within conventional superconductivity (CSC) and unconventional superconductivity (USC). USC scenarios with nodes on individual Fermi surface sheets (FSS), e.g. p- and d-wave SC, can be discarded for our samples. The increase of dB(c2)/dT vertical bar T-c 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 B-c2 data from the literature, which often show no PLB for fields below 60-70 T probed so far. A novel disorder-related scenario of a complex interplay of SC with two different competing magnetic instabilities is suggested.