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Superconductivity in iron selenide (FeSe) arises from a nematic phase. This study reveals anisotropic spin excitations in the normal and superconducting states, suggesting a highly anisotropic superconducting gap driven by spin fluctuations.

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Area of Science:

  • Condensed Matter Physics
  • Materials Science
  • Quantum Materials

Background:

  • Superconductivity in iron selenide (FeSe) is linked to a preceding nematic phase.
  • This nematic phase breaks the four-fold rotational symmetry of the iron plane.
  • Potential origins include orbital ordering, spin fluctuations, or magnetic quadrupolar order.

Purpose of the Study:

  • To investigate the nature of spin excitations in the nematic phase of FeSe.
  • To understand the relationship between nematicity and superconductivity in FeSe.
  • To determine the role of spin fluctuations in the anisotropic superconducting gap.

Main Methods:

  • Inelastic neutron scattering was performed on detwinned single crystals of FeSe.
  • Crystals were mounted on a BaFe2As2 substrate to achieve detwinning.
  • Spin excitations were measured at various energy transfers and wave vectors.

Main Results:

  • Spin excitations showed two-fold (C2) anisotropy at low energies (6-11 meV) in the normal state, concentrated at antiferromagnetic wave vectors QAF = (±1, 0).
  • A gapped four-fold (C4) mode was indicated by reduced anisotropy at lower energies (3-5 meV).
  • In the superconducting state, a spin resonance (3.6 meV) at QAF exhibited incommensurate scattering (5-6 meV), reflecting strong nematic anisotropy.

Conclusions:

  • The results highlight the extreme electronic anisotropy inherent in the nematic phase of FeSe.
  • The observed spin dynamics are consistent with a highly anisotropic superconducting gap.
  • Spin fluctuations are strongly implicated as the driving mechanism for this anisotropy.