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

TL;DR

This study uses advanced computational methods to analyze how epitaxial strain, film thickness, and surface effects influence the metal-insulator transition in CaVO$_3$ thin films, revealing that strain and surface phenomena can induce insulating states.

Contribution

The paper demonstrates that both tensile epitaxial strain and ultra-thin film thickness can independently induce a Mott-insulating phase in CaVO$_3$, highlighting the role of surface-induced crystal-field splitting.

Findings

Tensile strain of 3-4% induces a Mott insulator phase.

Ultra-thin films (<4 perovskite units) become insulating due to surface effects.

Surface layers show reduced quasiparticle spectral weight, indicating enhanced correlations.

Abstract

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 from substrate-induced epitaxial strain from those related to the reduced thickness of the film. We show that tensile epitaxial strain of around 3-4% is sufficient to induce a transition to a paramagnetic Mott-insulating phase. This corresponds to the level of strain that could be achieved on a SrTiO$_3$ substrate. Using free-standing slab models, we then demonstrate that reduced film thickness can also cause a MIT in CaVO$_3$, however, only for thicknesses of less than 4 perovskite units. Our calculations indicate that the MIT in such ultra-thin films results mainly from a surface-induced crystal-field splitting between the $t_{2g}$-orbitals, favoring the formation of an orbitally-polarized Mott insulator. This surface-induced crystal-field splitting is of the same type as the one resulting from tensile epitaxial strain, and thus the two effects can also cooperate. Furthermore, our calculations confirm an enhancement of correlation effects at the film surface, resulting in a reduced quasiparticle spectral weight in the outermost layer, whereas bulk-like properties are recovered within only a few layers away from the surface.