Tuning a Strain-Induced Orbital Selective Mott Transition in Epitaxial VO$_2$

TL;DR

This study demonstrates how epitaxial strain can induce and control an orbital selective Mott transition in VO$_2$, revealing strain-dependent electron correlation effects and orbital anisotropy through spectroscopic and theoretical analysis.

Contribution

It provides a novel understanding of strain-induced orbital selective Mott transitions in VO$_2$ using spectroscopic evidence and a theoretical model based on the U(1) slave spin formalism.

Findings

Strain modulates electron correlation effects in VO$_2$.

Orbital anisotropy increases with strain.

Strain can induce an orbital selective Mott transition.

Abstract

We present evidence of strain-induced modulation of electron correlation effects and increased orbital anisotropy in the rutile phase of epitaxial VO$_2$/TiO$_2$ films from hard x-ray photoelectron spectroscopy and soft V L-edge x-ray absorption spectroscopy, respectively. By using the U(1) slave spin formalism, we further argue that the observed anisotropic correlation effects can be understood by a model of orbital selective Mott transition at a filling that is non-integer, but close to the half-filling. Because the overlaps of wave functions between $d$ orbitals are modified by the strain, orbitally-dependent renormalizations of the bandwidths and the crystal fields occur with the application of strain. These renormalizations generally result in different occupation numbers in different orbitals. We find that if the system has a non-integer filling number near the half-filling such as for VO$_2$, certain orbitals could reach an occupation number closer to half-filling under the strain, resulting in a strong reduction in the quasiparticle weight $Z_{\alpha}$ of that orbital. Moreover, an orbital selective Mott transition, defined as the case with $Z_{\alpha} = 0$ in some, but not all orbitals, could be accessed by epitaxial strain-engineering of correlated electron systems.