Molecular orbital formation and metastable short-range ordered structure in VO$_2$

TL;DR

This study investigates the crystal structure and orbital states of VO$_2$, revealing molecular orbital formation in the insulating phase and short-range order in the metallic phase, offering new insights into the metal-insulator transition.

Contribution

It provides direct experimental evidence of molecular orbital formation and short-range order in VO$_2$, clarifying their roles in the metal-insulator transition.

Findings

Molecular orbital formation observed in the low-temperature M1 phase.

Diffuse scattering indicates short-range V displacement correlations in the high-temperature phase.

Short-range order in the rutile phase is key to understanding the MIT.

Abstract

The metal-insulator transition (MIT) in vanadium dioxide VO$_2$ due to V-V dimerization has been extensively discussed for decades. While it is widely acknowledged that electron correlations, Peierls instabilities, and molecular orbital formations are crucial for understanding the MIT of VO$_2$, the primary origin of the MIT remains controversial. In this study, we delve into the crystal structure and orbital state of VO$_2$ through synchrotron x-ray diffraction experiments. The molecular orbital formation corresponding to the V-V dimerization is directly observed in the low-temperature insulating monoclinic phase, called the M1 phase, as indicated by the valence electron density distribution. Moreover, diffuse scattering observed in the high-temperature metal phase of rutile structure suggests the presence of short-range correlation of V displacements, which is not directly attributed to the structural fluctuation of the M1 phase. The short-range order in the rutile phase will be the key to understanding the MIT in this system.