Upper Critical Field Measurements up to 60 T in Arsenic Deficient LaO_0.9F_0.1FeAs_1−δ: Pauli Limiting Behavior at High Fields vs. Improved Superconductivity at Low Fields

We report resistivity and upper critical field B_c2(T ) data for As-deficient LaO_0.9F_0.1FeAs_1−δ in a wide temperature and field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T_c = 29 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. This flattening is interpreted as Pauli limiting behavior (PLB) with B_c2(0) ≈ 63 T. We compare our results with B_c2(T )-data reported in the literature for clean and disordered samples. Whereas clean samples show no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of B_c2(T ) for applied fields H || ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity.