Quadrupolar charge dynamics in the nonmagnetic FeSe _{1− x} S _x superconductors

Abstract

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.

Fluctuations and pairing in Fe-based superconductors: light scattering experiments

“…In FeSe, having a structural transition at T S ≈ 90 K and short-ranged but no long-ranged magnetic order above 4 K [157,168], the uctuations survive down to temperatures right above T c ≈ 9 K [53,55,57]. The experimental results, of the three groups of authors agree with each other in the energy and temperature range where they can be compared.…”

“…For FeS the excitation is completely gone. In contrast to Massat et al [53] and Zhang et al [57], Baum and coworkers [55] assign this peak to a two-magnon excitation of nearly frustrated spins.…”

“…If we disregard spurious contributions to the cross section such as luminescence (being a good approximation at low energies; see, however, reference [57]) only (dynamic) interactions can produce the broad continua observed [81,130]. Impurities are present but irrelevant in high-quality single crystals as can be seen directly below T c since impurities reduce the effect of pair breaking in the Raman spectra.…”

“…Recent experiments in FeSe support scattering from localized spins. The B 1g spectra at high (300 K) and low (20 K) temperature are dominated by a broad peak centered at approximately 500 cm −1 , whereas near T S an additional sharp peak appears in the range 100-200 cm −1 [53,55,57] which we disregard here but discuss in detail below in the context of uctuations. The main peak in B 1g symmetry at 500 cm −1 depends continuously on temperature [55] and survives low doping with sulfur [57].…”

Nematic state of the FeSe superconductor

“…Even though the primary order parameter of the phase transition at 90 K is considered as nematic and of electronic origin, the exact nature of the order parameter remains unknown 13 . Several possible order parameters have been suggested, which include spin 14 , charge 15 , or orbital 16 degrees of freedom, antiferroquadrupolar order 17 , stripe quadrupolar order 18 , and collective modes such as a Pomeranchuk instability 15,19 of the Fermi surface. Indeed, in a quantum material such as FeSe, the order parameters cannot be considered as independent and competing.…”