As vacancies, local moments, and Pauli limiting in LaFeAs1−δO0.9F0.1 superconductors

Abstract

We report magnetization measurements of As-deficient LaO0.9F0.1FeAs1–δ (δ≈0.06) samples with improved superconducting properties as compared with As-stoichiometric optimally doped La-1111 samples [i.e., a slightly higher Tc (enhanced by about 2 K) and nearly a twice as large slope of −dHc2/dT at Tc]. 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 μB per AV or 0.8 μB/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 LaO0.9F0.1FeAs1–δ. In contrast, no Pauli limiting behavior is found for the optimally doped, As-stoichiometric LaO0.9F0.1FeAs superconductor in accord with its low spin susceptibility.

Source-reported materials — not catalogue approval

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