Persistent insulator-to-metal transition of a VO₂ thin film induced by soft X-ray irradiation

We present a persistent soft-X-ray-induced insulator-to-metal transition at room temperature for VO₂ thin films. Continuous soft X-ray irradiation renders the insulating electronic state metallic in the VO ₂ thin films, which has been found by photoemission spectroscopy measurement. The observed transition is irreversible and the metallic state persists when soft X-ray irradiation is stopped. The analysis of valence band spectra reveals that the density of states (DOS) of the V 3d band increases with irradiation time, while the DOS of the O 2p band decreases. We propose a simple model where the persistent insulator-to-metal transition is driven by oxygen desorption from VO₂ thin films under soft X-ray irradiation. The present results will help in developing a new route for controlling the phase transition of VO₂ by light.