Mechanisms of electron-phonon coupling unraveled in momentum and time: The case of soft phonons in TiSe ₂

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 1T-TiSe$_2$ both momentum time. are able distinguish effects due the real part of many-body bare electronic susceptibility, $\Re\left[\chi_0(\mathbf{q})\right]$, from those vertex, $g_{\mathbf{q}}$, by following response semi-metallic (normal phase) selective photo-doping into pocket at Fermi level. Quasi-impulsive wavevector-specific renormalization soft zone-boundary phonon frequencies (stiffening) observed, followed wavevector-independent equilibration. These results underlying mechanisms driving softening that associated with density wave transition lower temperatures.