Orbital and spin effects for the upper critical field in strongly disordered iron pnictide superconductors
We report upper critical field, resistivity, and Hall data for disordered (As deficient) LaO_0.9F0.1FeAs_1-\delta in high fields up to 60 T. Due to the large linear initial slope of Bc2 near T_c the in-plane Bc2(T) shows a flattening near 23 K above 30 T which points to the onset of a Pauli-limited (PL) behavior with Bc2(0) about 65 T. We interpret the hitherto unexplained flattening of the Bc2(T) curves reported for several other disordered related systems as the Co-doped 122 or the (Ba,K)Fe2As2 (single crystals) for applied fields H || (a,b) also as evidence for PL. Their Maki parameters alpha about 0.9 -1.5 and similarly alpha about 3.5 for NdO_0.7F_0.3FeAs to be compared with alpha about 1.3 for our As deficient La-1111 sample have been estimated analyzing the Bc2(T) data within the WHH model. The significant paramagnetic effect observed for several 122 single crystals obtained from a Sn-flux is attributed also to a pronounced As deficiency based on wave length dispersive x-ray spectroscopy data reported by Ni et al. (2008 (PRB, v.78 014507). Consequences of our results are discussed in terms of disorder effects within conventional and unconventional superconductivity (SC). Proposed unconventional p- and 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 Pauli-limiting behaviour for fields below 60 to 70 Tesla measured so far. A novel disorder related scenario of SC with 2 different competing magnetic instabilities is proposed.