Nematicity-induced changes in the Raman response in FeSe

We use polarization-resolved electronic Raman spectroscopy to study quadrupolar charge dynamics in non-magnetic FeSe_1-xS_x superconductor. We observe two types of long-wavelength XY symmetry excitations: (i) a low-energy quasi-elastic scattering (QEP) that exhibits critical enhancement upon cooling towards nematic phase transition temperature T_S(x), and (ii) a broad electronic continuum with a maximum at 55 meV. We report a surprising discovery of clear pseudo-gap suppression in the low frequency spectra of electronic long-wavelength charge quadrupole excitation below T_S(x), with temperature dependence of the gap’s magnitude reminiscent of the nematic order parameter. We relate the pseudo-gap development to significant reconstruction of the bands’ orbital composition near the Fermi pockets. We demonstrate that the intensity of the QEP grows with increasing sulfur concentration x and maximizes at a critical doping x_cr ≈ 16% where the nematic transition vanish, while the pseudo-gap size decreases with the suppression of T_S(x). 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 where the Fermi liquid properties recover.
In the superconducting phase, the Pomeranchuk fluctuations acquire coherence and undergo a metamorphosis into sharp in-gap collective mode feature, similar to the observations for several other families of the pnictide superconductors.