Evidence for a Structurally-driven Insulator-to-metal Transition in VO2: a View from the Ultrafast Timescale

TL;DR

This study uses ultrafast spectroscopy to reveal that in VO2, the insulator-to-metal transition is primarily driven by structural changes, with electronic effects following on a phonon timescale, suggesting a band-like nature of the insulator.

Contribution

It provides direct time-resolved evidence linking structural dynamics to the insulator-metal transition in VO2, highlighting the importance of lattice effects in the process.

Findings

The insulator-to-metal transition is delayed after hole doping.

Structural changes occur on a timescale matching phonon oscillations.

VO2 exhibits band-like characteristics in its insulating phase.

Abstract

We apply ultrafast spectroscopy to establish a time-domain hierarchy between structural and electronic effects in a strongly-correlated electron system. We discuss the case of the model system VO2, a prototypical non-magnetic compound that exhibits cell doubling, charge localization and a metal-insulator transition below 340 K. We initiate the formation of the metallic phase by prompt hole photo-doping into the valence band of the low-T insulator. The I-M transition is however delayed with respect to hole injection, exhibiting a bottleneck timescale that corresponds to half period of the phonon connecting the two crystallographic phases. This experiment indicates that this controversial insulator may have important band-like character.