Instantaneous band gap collapse in photoexcited monoclinic VO$_2$ due to photocarrier doping

Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO$_2$ quasi-instantaneously into a metal. Thereby, we exclude an 80 femtosecond structural bottleneck for the photoinduced electronic phase transition of VO$_2$. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO$_2$ bandgap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructral insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening emph{before} significant hot-carrier relaxation or ionic motion has occurred.

Direct time domain sampling of sub-THz coherent acoustic phonon spectra in SrTiO$_3$ using ultrafast X-ray diffraction

We synthesize sub-THz longitudinal quasi-monochromatic acoustic phonons in a SrTiO$_3$ single crystal using a SrRuO$_3$/SrTiO$_3$ superlattice as an optical-acoustic transducer. The generated acoustic phonon spectrum is determined using ultrafast X-ray diffraction. The analysis of the generated phonon spectrum in the time domain reveals a k-vector dependent phonon lifetime. It is observed that even at sub-THz frequencies the phonon lifetime agrees with the 1/$omega^2$ power law known from Akhiezers model for hyper sound attenuation. The observed shift of the synthesized spectrum to the higher $q$ is discussed in the framework of non-linear effects appearing due to the high amplitude of the synthesized phonons.