Evidence for Pauli-limiting behaviour at high fields and enhanced upper critical fields near T-c in several disordered FeAs based superconductors

We report resistivity and upper critical field B_c2 (T) data for disordered (As-deficient) LaO_0.9F_0.1 FeAs_1-delta in a wide temperature and high field range up to 60 T. These samples exhibit a slightly enhanced superconducting transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4 T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K above 30 T. The latter evidences Pauli-limiting behaviour (PLB) with B_c2(0) approximate to 63 T. We compare our results with B_c2(T)-data from the literature for clean and disordered samples. Whereas clean samples show almost no PLB for fields below 60-70 T, the hitherto unexplained pronounced flattening of B_c2(T) for applied fields H vertical bar ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences are discussed in terms of disorder effects within the frames of (un)conventional superconductivity, respectively.