X-ray induced insulator-metal transition in a thin film of electron-doped VO₂

We have observed a persistent x-ray induced insulator-metal transition at low temperature for an epitaxial film of electron-doped VO₂, a material as characterized by strong electron-electron and electron-lattice interactions. The volume fraction of the photo-generated metallic patches, ranging from 0 to 100% of the whole film, can be scaled well with the total dose of x-ray irradiation, irrespective of the x-ray intensity. This indicates the monomolecular process of the insulator-metal phase conversion that corresponds to the instantaneous creation of the metallic patch extending over as many as 105 V sites per one x-ray absorbed photon. The typical percolation behavior is observed in the conductivity change with the finely photo-controlled volume ratio of the metallic phase.