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

TL;DR

This study demonstrates that photoexcitation causes an almost immediate transition of monoclinic VO$_2$ into a metallic state, driven by electronic effects rather than structural changes, as shown by spectroscopy and theoretical calculations.

Contribution

It provides evidence that the insulator-to-metal transition in VO$_2$ is electronically driven and occurs quasi-instantaneously, challenging previous notions of a structural bottleneck.

Findings

Photoexcitation induces an ultrafast metallic state in VO$_2$.

The transition occurs without significant ionic motion or hot-carrier relaxation.

The bandgap sensitivity is linked to changes in Coulomb screening.

Abstract

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.