Nematicity in SnS single crystals probed by ultrafast spectroscopy

SnS is one of the promising materials for the applications of optoelectronics and photovoltaics. This study determines the nematic dynamics of photoexcited electrons and phonons in SnS single crystals using polarization-dependent ultrafast spectroscopy at various temperatures. Besides the fast relaxation of photoexcited electrons, a damped oscillation component with a frequency of 36~41 GHz is also present in transient reflectivity change (ΔR/R) spectra, which is generated by the thermoelastic effect. The results of this study show that electrons and coherent acoustic phonons demonstrate significant anisotropy on the ac-plane in the transition region from 330 K to 430 K, possibly because of strong electron-phonon coupling (e.g., 1.16 along a-axis at 300 K). However, this in-plane anisotropy weakens dramatically in the low-temperature (< 330 K) and high-temperature (> 430 K) phases. These play an important role in anisotropic heat dissipation and charge carrier mobility in polarization-sensitive optical and optoelectronic devices.