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We use density functional theory plus dynamical mean-field theory (DFT+DMFT)to study multiple control parameters for tuning the metal-insulator transition(MIT) in CaVO$_3$ thin films. We focus on separating the effects resulting fromsubstrate-induced epitaxial strain from those related to the reduced thicknessof the film. We show that tensile epitaxial strain of around 3-4% is sufficientto induce a transition to a paramagnetic Mott-insulating phase. Thiscorresponds to the level of strain that could be achieved on a SrTiO$_3$substrate. Using free-standing slab models, we then demonstrate that reducedfilm thickness can also cause a MIT in CaVO$_3$, however, only for thicknessesof less than 4 perovskite units. Our calculations indicate that the MIT in suchultra-thin films results mainly from a surface-induced crystal-field splittingbetween the $t_{2g}$-orbitals, favoring the formation of an orbitally-polarizedMott insulator. This surface-induced crystal-field splitting is of the sametype as the one resulting from tensile epitaxial strain, and thus the twoeffects can also cooperate. Furthermore, our calculations confirm anenhancement of correlation effects at the film surface, resulting in a reducedquasiparticle spectral weight in the outermost layer, whereas bulk-likeproperties are recovered within only a few layers away from the surface.

Metal-insulator transition in CaVO$_3$ thin films: interplay between epitaxial strain, dimensional confinement, and surface effects