investigated by optical spectroscopy*

Transition metal oxides exhibit diverse emergent phenomena such as strongly correlated Mott insulating states, high-temperature superconductivity, magnetic and structural phase transitions, and metal-insulator transitions. Here, we focus on the thermally-driven metal-insulator transition (MIT) in vanadium dioxide (VO₂). It is controversial whether the MIT in a thick VO₂ film is accompanied by a simultaneous structural phase transition (SPT) or whether the MIT and SPT occur at different temperatures. The SPT is a Peierls ordering of all the vanadium atoms into dimerized pairs. Some theoretical proposals have been advanced to explain the MIT as a Peierls transition in which the SPT is assigned a crucial role. Here we report experiments on ultrathin VO₂ film on rutile (001) TiO₂. We demonstrate an insulating phase in ultrathin, rutile VO₂ without Peierls ordering. The energy gap in rutile, insulating VO₂ arises from electron-electron interactions, not from the pairing of vanadium atoms. Hence, the MIT studied in our work is a manifestation of a Mott transition.