Quadrupolar charge dynamics in the nonmagnetic FeSe_1-xS_xsuperconductors

We use polarization-resolved electronic Raman spectroscopy to study quadrupolar charge dynamics in a nonmagnetic FeSe_1-xS_xsuperconductor. 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 T_S(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 x_cr≈0.16, while the pseudogap size decreases with the suppression of T_S(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.