Upper Critical Field Measurements up to 60 T in Arsenic-Deficient LaO_(0.9)F_(0.1)FeAs_(1-delta): 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_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T_(c) velence 29 K and a significantly enlarged slope dB_(c2)/dT velence -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) approx= 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.