Superconductivity Mediated by Nematic Fluctuations in Tetragonal $\text{Fe}{\text{Se}}_{1-x}{\text{S}}_x$

Abstract

Nematic phases, where electrons in a solid spontaneously break rotational symmetry while preserving the translational symmetry, exist in several families of unconventional superconductors [1, 2]. Although superconductivity mediated by nematic fluctuations is well established theoretically [3-7], it has yet to be unambiguously identified experimentally [8, 9]. A major challenge is that nematicity is often intertwined with other degrees of freedom, such as magnetism and charge order. The FeSe${}_{1-x}$S${}_x$ family of iron based superconductors provides a unique opportunity to explore this concept, as it features an isolated nematic phase that can be suppressed by sulfur substitution at a quantum critical point (QCP) near $x_c=0.17$, where nematic fluctuations are the largest [10-12]. Here, we performed scanning tunneling spectroscopy measurements to visualize Boguliubov quasiparticle interference patterns, from which we determined the momentum structure of the superconducting gap near the Brillouin zone $\Gamma$ point of FeSe${}_{0.81}$S${}_{0.19}$. The results reveal an anisotropic, near nodal gap with minima that are $45^{\circ }$ rotated with respect to the Fe-Fe direction, characteristic of a nematic pairing interaction, contrary to the usual isotropic gaps due to spin mediated pairing in other tetragonal Fe-based superconductors. The results are also in contrast with pristine FeSe, where the pairing is mediated by spin fluctuations and the gap minima are aligned with the Fe-Fe direction. Therefore, the measured gap structure demonstrates not only a fundamental change of the pairing mechanism across the phase diagram of FeSe${}_{1-x}$S${}_x$, but it also indicates the existence of superconductivity mediated by nematic fluctuations in FeSe${}_{0.81}$S${}_{0.19}$.

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Cited by in corpus (6)

• Friedel oscillations and chiral superconductivity in monolayer NbSe${}_2$
• Microscopic origin of the nemato-elastic coupling and dynamics of hybridized collective nematic-phonon excitations
• Crossover of Superconductivity across the antiferromagnetic end point in FeSe${}_{1-\mathrm{x}}$S${}_{\mathrm{x}}$ under pressure
• Quasiparticle interaction originating from Bogoliubov Fermi Surfaces under pressure in 18%-S substituted FeSe studied via NMR
• Intermediate band analysis in Green's functions calculations of quasiparticle interference
• Superconductivity near an Ising nematic quantum critical point in two dimensions