  Vanadium sesquioxide (V2O3) is an archetypal Mott insulator in which the
atomic positions and electron correlations change as temperature, pressure or
doping are varied giving rise to different structural, magnetic or electronic
phase transitions. Remarkably, the isostructural Mott transition in Cr-doped
V2O3 between paramagnetic metallic and insulating phase observed in bulk has
been elusive in thin film compounds so far. Here, via continuous lattice
deformations induced by heteroepitaxy we demonstrate a room temperature Mott
metal-insulator transition in 1.5% Cr-doped and pure V2O3 thin films. By means
of a controlled epitaxial strain, not only the structure but also the intrinsic
electronic and optical properties of the thin films are stabilized at different
intermediate states between the metallic and insulating phases, inaccessible in
bulk materials. This leads to films with unique features such as a colossal
change in room temperature resistivity (DR/R up to 100,000 %) and a broad range
of optical constant values, as consequence of a strain-modulated bandgap. We
propose a new phase diagram for pure and Cr-doped V2O3 thin films with the
engineered in-plane lattice constant as a tuneable parameter. Our results
demonstrate that controlling phase transitions in correlated systems by
epitaxial strain offers a radical new approach to create the next generation of
Mott devices.
