Upper critical field measurements up to 60 T in arsenic-deficient LaO(0.9)F(0.1)FeAs(1-delta): Pauli limiting behaviour at high fields vs improved superconductivity at low fields

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We report resistivity and upper critical field Bc2(T) data for As deficient LaO(0.9)F(0.1)FeAs(1-delta) in a wide temperature and high field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at Tc = 29 K and a significantly enlarged slope dB(c2))/dT = -5.4 T/K near Tc which contrasts with a flattening of B(c2)(T) starting near 23 K above 30 T. This flattening is interpreted as Pauli limiting behaviour (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.