Peierls transition, ferroelectricity, and spin-singlet formation in the monolayer VOI$_2$

TL;DR

This study combines ab initio and Hubbard model calculations to reveal the ground state of monolayer VOI$_2$, showing ferroelectric distortion, V-V dimerization, Peierls instability, and spin-singlet formation, advancing understanding of its electronic and structural properties.

Contribution

It provides a comprehensive theoretical analysis of monolayer VOI$_2$, identifying the ferroelectric ground state and mechanisms behind its structural and magnetic features, which were not previously detailed.

Findings

Orthorhombic Pmm2 ferroelectric phase is most stable.

Ferroelectric distortion stabilized by pseudo Jahn-Teller effect.

V-V dimerization driven by Peierls instability and spin-singlet formation.

Abstract

Using {\it ab initio} density functional theory and single-orbital Hubbard model calculations via the density matrix renormalization group method, we systematically studied the monolayer VOI$_2$ with a $3d^1$ electronic configuration. Our phonon calculations indicate that the orthorhombic $Pmm2$ FE-II phase is the most likely ground state, involving a ferroelectric distortion along the $a$-axis and V-V dimerization along the $b$-axis. Specifically, the "pseudo Jahn-Teller" effect caused by the coupling between empty V ($d_{xz/yz}$ and $d_{3z^2-r^2}$) and O $2p$ states is proposed as the mechanism that stabilizes the ferroelectric distortion from the paraelectric phase. Moreover, the half-filled metallic $d_{xy}$ band displays a Peierls instability along the $b$-axis, inducing a V-V dimerization. We also found very short-range antiferromagnetic coupling along the V-V chain due to the formation of nearly-decoupled spin singlets in the ground state.