Many metallic transition-metal oxides turn insulating when grown as films that are only a few unit cells thick. The microscopic origins of these thickness-induced metal-to-insulator transitions, however, remain in dispute. Here, we simulate the extreme case of a monolayer of an inconspicuous correlated metal—the strontium vanadate $\mathrm{Sr}{\mathrm{VO}}_3$—deposited on a ${\mathrm{SrTiO}}_3$ substrate. Crucially, our system can have a termination to vacuum consisting of either a SrO or a ${\mathrm{VO}}_2$ top layer. While we find that both lead to Mott insulating behavior at nominal stoichiometry, the phase diagram emerging upon doping—chemically or through an applied gate voltage—is qualitatively different. Indeed, our many-body calculations reveal a cornucopia of nonlocal fluctuations associated with (in)commensurate antiferromagnetic, ferromagnetic, and stripe and checkerboard orbital ordering instabilities. Identifying that the two geometries yield crystal-field splittings of opposite signs, we elucidate the ensuing phases through the lens of the orbital degrees of freedom. Quite generally, our work highlights that interface and surface reconstruction and the deformation or severing of coordination polyhedra in ultrathin films drive rich properties that are radically different from the material's bulk physics.

• Received 27 August 2020
• Revised 31 May 2021
• Accepted 12 July 2021

DOI:https://doi.org/10.1103/PhysRevB.104.024307