Dominant Role of Electron-Electron Scattering in the Photoinduced Terahertz Conductivity Spectra of Few-Layer WS_{2(1-<i>x</i>)}Se_2<i>x</i> Flakes.

Abstract

Understanding the photoinduced terahertz (THz) conductivity in transition metal dichalcogenides (TMDs) is essential for their envisioned applications in optoelectronics. Recent studies of photoinduced terahertz conductivity in TMDs have revealed strong non-Drude behavior, empirically fitted by the Drude-Lorentz or Drude-Smith models, without adequate physical understanding of the parameters employed. Here, we report the photoexcited spectral response of the nonequilibrium carriers in WS_{2(1-<i>x</i>)}Se_2<i>x</i> (<i>x</i> = 0, 0.5, and 1) laminates consisting of few-layer flakes, using optical pump terahertz probe (OPTP) spectroscopy, and model the data using the Boltzmann transport equation (BTE) with energy-dependent scattering mechanisms. We demonstrate that by incorporating electron-electron (e-e) scattering mechanism into the Boltzmann transport framework, alongside impurity and phonon scattering, the terahertz photoconductivity can be quantitatively understood. Our analysis reveals that e-e scattering is a critical mechanism to understand the fluence-dependent blueshift of the zero-crossing frequency of the imaginary part of the photoinduced terahertz conductivity Δσ_im(ω), without any empirical fitting parameters. Drawing parallels with the hydrodynamic transport regime observed in graphene's Dirac fluid, we show that e-e interactions play a significant role in the carrier dynamics, leading to non-Drude terahertz photoconductivity response. The insight from this study highlights the indispensable role of e-e scattering in describing the nonequilibrium carrier dynamics in TMDs and offers insights for future optoelectronic device design.