Dynamics of orbital degrees of freedom probed via isotope Sb121,123 nuclear quadrupole moments in Sb-substituted iron pnictide superconductors

Abstract

Isotope Sb121,123 nuclei with large electric quadrupole moments are applied to investigate the dynamics of orbital degrees of freedom in Sb-substituted iron(Fe)-based compounds. In the parent compound LaFe(As0.6Sb0.4)O, the nuclear-spin relaxation rate (T1−1)121,123 at Sb121,123 sites was enhanced at structural transition temperature (Ts∼ 135 K), which is higher than Néel temperature (TN∼125 K). The isotope ratio (T1−1)123/(T1−1)121 indicates that the electric quadrupole relaxation due to the dynamical electric field gradient at the Sb site increases significantly toward Ts. It is attributed to the critically enhanced nematic fluctuations of stripe-type arrangement of Fe-3dxz (or 3dyz) orbitals. In the lightly electron-doped superconducting (SC) compound LaFe(As0.7Sb0.3)(O0.9F0.1), the nematic fluctuations are largely suppressed in comparison with the case of the parent compound; however, it remains a small enhancement below 80 K down to the Tc (∼20 K). The results indicate that the fluctuations from both the spin and orbital degrees of freedom on the 3dxz(or 3dyz) orbitals can be seen in lightly electron-doped SC state of LaFeAsO-based compounds. We emphasize that isotope Sb121,123 quadrupole moments are a sensitive local probe to identify the dynamics of orbital degrees of freedom in Fe pnictides, which provides an opportunity to discuss the microscopic correlation between the superconductivity and both nematic and spin fluctuations simultaneously, even in the polycrystalline samples.

Source-reported materials — not catalogue approval

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