Upper critical field measurements up to 60 T in arsenic-deficient LaO_0.9F_0.1 Fe As: Pauli limiting at high fields vs. improved superconductivity at low fields

We report resistivity, Hall and upper critical field Bc2(T) data for arsenic deficient LaO0.9F0.1 FeAs1-δ in a wide temperature and high field range up to 60 T. These disordered samples exhibit a slightly enhanced transition temperature of T_c = 29.0 K and a significantly enlarge slope dB_c2/dT = –5.4 T/K near T_c. The high-field B_c2(T) data obtained from resistance measurements in pulsed magnetic fields follow up to about 30 T the WHH (Werthamer-Helfand-Hohenberg) curve for the orbital limited upper critical field, but show a clear flattening above 30 T. This flattening is interpreted as the onset of Pauli-limited behavior with B_c2(0) ~ 63 to 68 T. We compare our results with B_c2(T) data reported in the literature for clean and disordered samples. Whereas clean samples show no Pauli-limiting behavior for fields below 60 to 70 T as measured so far, the hitherto unexplained flattening of B_c2(T) for applied fields H||ab observed for several disordered closely related systems as Co-doped BaFe₂As₂ or (Ba,K)Fe₂As₂ (obtained from a Sn-flux) is interpreted as a manifestation of Pauli-limiting behavior. The corresponding enhanced Maki parameters point to significant paramagnetic effects in these disordered FeAs-based superconductors. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity. The enhancement of the upper critical field slope near Tc by strong disorder provides evidence for an important attractive intraband contribution to s-wave pairing of Cooper pairs in disordered Fe pnictides at relatively high transition temperatures.