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

摘要

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.