Direct observation of the M2 phase with its Mott transition in a VO$_2$ film

TL;DR

This study provides direct evidence of the M2 phase in VO₂ and demonstrates its Mott insulating nature through microscopic observations of phase coexistence and conductivity patterns during the insulator-metal transition.

Contribution

It offers the first direct observation of the M2 phase in VO₂ and clarifies its Mott insulator characteristics using advanced microscopy and spectroscopy techniques.

Findings

Identification of the M2 phase during the transition

Conductivity enhancement at grain boundaries

Evidence of intra-grain percolation process

Abstract

In VO$_2$, the explicit origin of the insulator-to-metal transition is still disputable between Peierls and Mott insulators. Along with the controversy, its second monoclinic (M2) phase has received considerable attention due to the presence of electron correlation in undimerized vanadium ions. However, the origin of the M2 phase is still obscure. Here, we study a granular VO$_2$ film using conductive atomic force microscopy and Raman scattering. Upon the structural transition from monoclinic to rutile, we observe directly an intermediate state showing the coexistence of monoclinic M1 and M2 phases. The conductivity near the grain boundary in this regime is six times larger than that of the grain core, producing a donut-like landscape. Our results reveal an intra-grain percolation process, indicating that VO$_2$ with the M2 phase is a Mott insulator.