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

We report upper critical field Bc2(T) data for LaO0.9F0.1FeAs1-δ in a wide temperature and field range up to 60 T. The large slope of Bc2 ≈ -5.4 to -6.6 TK-1 near an improved Tc ≈ 28.5 K of the in-plane Bc2(T) contrasts with a flattening starting near 23 K above 30 T we regard as the onset of Paulilimited behaviour (PLB) with Bc2(0) ≈ 63-68 T. We interpret a similar hitherto unexplained flattening of the Bc2(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 ∥ (a, b), also as a manifestation of PLB. Their Maki parameters have been estimated by analysing their Bc2 (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 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, 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. [ABSTRACT FROM AUTHOR]