Decoupled few-femtosecond phase transitions in vanadium dioxide

TL;DR

This study uses ultra-broadband few-femtosecond spectroscopy to resolve the ultrafast electronic and structural phase transitions in VO2, revealing a two-stage process involving a rapid electronic change followed by a structural rearrangement.

Contribution

It demonstrates the capability of few-femtosecond spectroscopy to simultaneously resolve electronic and structural dynamics in quantum materials, providing new insights into VO2 phase transitions.

Findings

VO2 transforms into a bad-metallic phase within 10 fs

Structural transition involves vanadium dimer separation and untwisting

Electronic oscillations and partial bandgap re-opening observed during transition

Abstract

The nature of the insulator-to-metal phase transition in vanadium dioxide (VO2) is one of the longest-standing problems in condensed-matter physics. Ultrafast spectroscopy has long promised to determine whether the transition is primarily driven by the electronic or structural degree of freedom, but measurements to date have been stymied by their sensitivity to only one of these components and/or their limited temporal resolution. Here we use ultra-broadband few-femtosecond pump-probe spectroscopy to resolve the electronic and structural phase transitions in VO2 at their fundamental time scales. We find that the system transforms into a bad-metallic phase within 10 fs after photoexcitation, but requires another 100 fs to complete the transition, during which we observe electronic oscillations and a partial re-opening of the bandgap, signalling a transient semi-metallic state. Comparisons with tensor-network simulations and density-functional theory calculations show these features originate from oscillations around the equilibrium high-symmetry atomic positions during an unprecedentedly fast structural transition, in which the vanadium dimers separate and untwist with two different timescales. Our results resolve the complete structural and electronic nature of the light-induced phase transition in VO2 and establish ultra-broadband few-femtosecond spectroscopy as a powerful new tool for studying quantum materials out of equilibrium.