Insulator-metal transition in the rutile structure of ultrathin VO₂ films on (001) TiO₂

The origin of the insulator-metal transition (IMT) in VO₂ is not well understood due to the concurrent monoclinic-rutile transition (MRT). The long-standing dilemma is whether the IMT occurs due to the breakdown of Mott electron-electron correlations or due to the melting of Peierls charge density wave under the influence of controllable parameters. To solve the dilemma, we investigate ultrathin VO₂ films on rutile (001) TiO₂ substrates by using synchrotron XRD, electrical transport, and optical techniques. We reveal that below the critical thickness of 7.5 nm, the VO₂ films persist in the rutile structure before and after the IMT avoiding Peierls ordering and, thus, the MRT. The observed pure electronic Mott IMT holds the key to understanding VO₂ and other phase-change materials and to boosting their application potential.