Peierls transition, ferroelectricity, and spin-singlet formation in monolayer VOI2

Using ab initio density functional theory and single-orbital Hubbard model calculations via the density matrix renormalization group method, we systematically studied the monolayer VOI₂ with a 3d¹ electronic configuration. Our phonon calculations indicate that the orthorhombic Pmm2 FE-II phase is the most likely ground state, involving a ferroelectric (FE) 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_3z²-r²) and O 2p states is proposed as the mechanism that stabilizes the FE 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. Additionally, we found very short-range antiferromagnetic coupling along the V-V chain due to the formation of nearly decoupled spin singlets in the ground state.