The nature of the pressure-induced metallization in VO₂

We utilize ultrafast optical pump - THz probe spectroscopy in order to investigate the pressure-driven insulator-to-metal transition (IMT) in vanadium dioxide (VO₂). The probe pulses with central frequency of 30 THz enable a sensitive detection of the photoinduced metallization. The threshold pump fluence necessary for generation of a metastable metallic phase has been systematically measured for pressures up to 19 GPa. Initial pressure application leads to a notable increase of the threshold fluence. This contrasts the thermally-driven IMT in VO₂ where it decreases on approaching the transition temperature. Above the IMT, that occurs at approximately 6-8 GPa, we observe a sharp drop of the threshold fluence. However, the clear threshold behavior characteristic for systems with cooperative electronic localization still could be observed also in the metallic state up to the highest applied pressure. Our results support a view of the pressure-induced IMT in VO₂ as a purely electronic bandwidth-driven Mott-Hubbard transition, that does not involve any change in the crystal structure.