Mechanisms of electron-phonon coupling unraveled in momentum and time: The case of soft-phonons in TiSe$_2$

TL;DR

This study uses ultrafast electron diffuse scattering to dissect momentum- and time-resolved electron-phonon interactions in TiSe₂, revealing how soft phonons relate to charge density wave phenomena.

Contribution

It demonstrates a method to distinguish between electronic susceptibility effects and electron-phonon coupling in momentum and time domains in TiSe₂.

Findings

Wavevector-specific phonon stiffening observed

Electron-phonon equilibration occurs independently of wavevector

Results clarify mechanisms behind phonon softening and charge density wave transition

Abstract

The complex coupling between charge carriers and phonons is responsible for diverse phenomena in condensed matter. We apply ultrafast electron diffuse scattering to unravel electron-phonon coupling phenomena in 1T-TiSe$_2$ in both momentum and time. We are able to distinguish effects due to the real part of the many-body bare electronic susceptibility, $\Re\left[\chi_0(\mathbf{q})\right]$, from those due to the electron-phonon coupling vertex, $g_{\mathbf{q}}$, by following the response of semi-metallic (normal phase) 1T-TiSe$_2$ to the selective photo-doping of carriers into the electron pocket at the Fermi level. Quasi-impulsive and wavevector-specific renormalization of soft zone-boundary phonon frequencies (stiffening) is observed, followed by wavevector-independent electron-phonon equilibration. These results unravel the underlying mechanisms driving the phonon softening that is associated with the charge density wave transition at lower temperatures.