Orbital and spin effects for the upper critical field in strongly disordered iron pnictide superconductors

We report upper critical field Bc2(T ), resistivity, and Hall data for disordered (arsenic deficient) LaO0.9F0.1FeAs1−δ in a wide temperature and high field range up to 60 T. Due to the large linear initial slope of Bc2 ≈ -5.4 to -6.6T/K near Tc ≈ 28.5 K the T -dependence of the in-plane Bc2(T ) shows a clear flattening already near 23 K above 30 T which is intepreted as the onset of a Pauli-limited behavior with Bc2(0) ≈ 63 to 68 T. We interpret the hitherto unexplained flattening of the Bc2(T ) curves reported for several other disordered closely related systems as the Codoped BaFe2As2 or the (Ba,K)Fe2As2 (single crystals) for applied fields H ‖ (a, b) also as a manifestation of Pauli limiting behaviour. The corresponding Maki parameters α ≈ 0.9− 1.5 and similarly α ≈ 3.5 for NdO0.7F0.3FeAs to be compared with α ≈ 1.3 for our arsenic deficient LaO0.9F0.1FeAs1−δ sample have been estimated analyzing the Bc2(T ) data within the WHH (Werthamer-Helfand Hohenberg) model. The significant paramagnetic effect observed for several (Ba,K)Fe2As2 single crystals obtained from a Sn-flux is attributed also to a pronounced arsenic deficiency based on wave length dispersive x-ray spectroscopy data reported by Ni et al. (2008 Phys. Rev. B 78 014507 ). Consequences of our results are discussed in terms of disorder effects within conventional and unconventional superconductivity (SC). Proposed unconventional pand d-wave scenarios of SC can be discarded for our samples. The enhancement of the upper critical field slope near Tc by strong disorder provides evidence for an important attractive intraband contribution to pairing of Cooper pairs in Fe pnictides. We compare our results with Bc2 data in the literature which show often no Paulilimiting behaviour for fields below 60 to 70 Tesla measured so far. A novel disorder related scenario of a complex interplay of SC with two different competing magnetic instabilities is proposed. PACS numbers: 74.25.-q, 74.25.Ha, 74.25.Op, 74.70.Dd Submitted to: New J. Phys. Orbital and spin effects for the upper critical field 2