As vacancies, local moments, and Pauli limiting in LaFeAs _1-δO_0.9F_0.1 superconductors

We report magnetization measurements of As-deficient LaO _0.9F_0.1FeAs_1-δ (δ0.06) samples with improved superconducting properties as compared with As-stoichiometric optimally doped La-1111 samples [i.e., a slightly higher T_c (enhanced by about 2 K) and nearly a twice as large slope of -dH_c2/dT at T_c]. In this As-deficient system with almost homogeneously distributed As-vacancies (AV), as suggested by the 75As-nuclear quadrupole resonance measurements, we observe a strong enhancement of the spin susceptibility by a factor of 3-7. This observation is attributed to the presence of an electronically localized state around each AV, carrying a magnetic moment of about 3.2 μ_B per AV or 0.8 μ_B/Fe atom. From theoretical considerations, we find that the formation of a local moment on neighboring iron sites of an AV sets in when the local Coulomb interaction exceeds a critical value of ∼1 eV in the dilute limit. Its estimated value amounts to ∼2.5 eV and implies an upper bound of ∼2 eV for the Coulomb repulsion at Fe sites beyond the first neighbor shell of an AV. Electronic correlations are thus moderate or weak in doped La-1111. The strongly enhanced spin susceptibility is responsible for the Pauli limiting behavior of the superconductivity that we observe in As-deficient LaO_0.9F _0.1FeAs_1-δ. In contrast, no Pauli limiting behavior is found for the optimally doped, As-stoichiometric LaO_0.9F _0.1FeAs superconductor in accord with its low spin susceptibility.