Quadrupolar charge dynamics in the nonmagnetic FeSe1−xSx superconductors

Abstract

Significance The family of iron selenide (FeSe) superconductors is of great interest because it harbors an unusual nonmagnetic nematic state, exhibits a peculiar orbital-selective Cooper pairing, and has a strong potential for high-temperature superconductivity. Here, for FeSe1−xSx nonmagnetic metal, we report discovery of a pseudogap in the spectra of electronic long-wavelength charge quadrupole excitation below the nematic phase transition with temperature dependence of the gap’s magnitude reminiscent of a nematic order parameter. We argue that the intense continuum of excitations in the high-temperature phase with tetragonal symmetry arises due to non-Fermi liquid dynamics governed by Pomeranchuk fluctuations and that these fluctuations are suppressed in the symmetry-broken orthorhombic low-temperature phase, enabling the recovery of Fermi liquid properties. We use polarization-resolved electronic Raman spectroscopy to study quadrupolar charge dynamics in a nonmagnetic FeSe1−xSx superconductor. We observe two types of long-wavelength XY symmetry excitations: 1) a low-energy quasi-elastic scattering peak (QEP) and 2) a broad electronic continuum with a maximum at 55 meV. Below the tetragonal-to-orthorhombic structural transition at TS(x), a pseudogap suppression with temperature dependence reminiscent of the nematic order parameter develops in the XY symmetry spectra of the electronic excitation continuum. The QEP exhibits critical enhancement upon cooling toward TS(x). The intensity of the QEP grows with increasing sulfur concentration x and maximizes near critical concentration xcr≈0.16, while the pseudogap size decreases with the suppression of TS(x). We interpret the development of the pseudogap in the quadrupole scattering channel as a manifestation of transition from the non-Fermi liquid regime, dominated by strong Pomeranchuk-like fluctuations giving rise to intense electronic continuum of excitations in the fourfold symmetric high-temperature phase, to the Fermi liquid regime in the broken-symmetry nematic phase where the quadrupole fluctuations are suppressed.