Low-frequency Raman response near the Ising-nematic quantum critical point: A memory-matrix approach

Recent Raman scattering experiments have revealed a quasielastic peak in FeSe1-xSx near an Ising-nematic quantum critical point (QCP) [Zhang et al., PNAS 118, 20 (2021)]. Notably, the peak occurs at subtemperature frequencies, and softens as T-alpha when temperature is decreased toward the QCP, with alpha > 1. This temperature dependence is inconsistent with an impurity scattering scenario, and suggests that quantum critical fluctuations play an important role. In this work,we incorporate these effects in the framework of a memory matrix approach. The quasielastic peak is associated with the relaxation of an Ising-nematic deformation of the Fermi surface. We identify the dynamical scattering rate tau(-1) of this deformation as the product of the quasielastic peak frequency Gamma and the Ising-nematic thermodynamic susceptibility chi. Over a broad temperature regime, we find that tau(-1)(T) exhibits a quasilinear dependence on temperature, in qualitative agreement with experiments. This behavior reflects a crossover from quantum critical scaling to a regime where the lifetime is governed by scattering from quasielastic thermal fluctuations. At frequencies larger than the temperature, we find that the Raman response is proportional to omega(1/3), consistently with earlier theoretical predictions.

Automated summary

Recent Raman scattering experiments have discovered a quasielastic peak in FeSe1-xSx near an Ising-nematic quantum critical point. The peak appears at subtemperature frequencies and softens as temperature decreases towards the quantum critical point. This behavior is inconsistent with impurity scattering and suggests the involvement of quantum critical fluctuations. In this study, the researchers incorporate these effects using a memory matrix approach and find that the quasielastic peak is associated with the relaxation of an Ising-nematic deformation of the Fermi surface. The dynamical scattering rate of this deformation is found to be proportional to the product of the quasielastic peak frequency and the Ising-nematic thermodynamic susceptibility. They also observe a quasilinear dependence of the scattering rate on temperature over a broad temperature range, in agreement with experimental results.