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

摘要

We report resistivity and upper critical field B_c2(T) data for As deficientLaO_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and high field range up to60 T. These disordered samples exhibit a slightly enhanced superconductingtransition at T_c = 29 K and a significantly enlarged slope dB_(c2))/dT = -5.4T/K near T_c which contrasts with a flattening of B_(c2)(T) starting near 23 Kabove 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 reportedin the literature for clean and disordered samples. Whereas clean samples showno PLB for fields below 60 to 70 T, the hitherto unexplained flattening ofB_(c2)(T) for applied fields H || ab observed for several disordered closelyrelated systems is interpreted also as a manifestation of PLB. Consequences ofour results are discussed in terms of disorder effects within the frame ofconventional and unconventional superconductivity.