Dynamic instabilities in strongly correlated VSe2 monolayers and bilayers

Abstract

With the emergence of graphene and other two-dimensional (2D) materials, transition-metal dichalcogenides have been investigated intensely as potential 2D materials using experimental and theoretical methods. ${\mathrm{VSe}}_{2}$ is an especially interesting material since its bulk modification exhibits a charge-density wave (CDW), the CDW is retained even for few-layer nanosheets, and monolayers of ${\mathrm{VSe}}_{2}$ are predicted to be ferromagnetic. In this work, we show that electron correlation has a profound effect on the magnetic properties and dynamic stability of ${\mathrm{VSe}}_{2}$ monolayers and bilayers. Including a Hubbard-$U$ term in the density-functional-theory calculations strongly affects the magnetocrystalline anisotropy in the $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ structure while leaving the $2H$-polytype virtually unchanged. This demonstrates the importance of electronic correlations for the electrical and magnetic properties of $1T\ensuremath{-}{\mathrm{VSe}}_{2}$. The Hubbard-$U$ term changes the dynamic stability and the presence of imaginary modes of ferromagnetic $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ while affecting only the amplitudes in the nonmagnetic phase. The Fermi surface of nonmagnetic $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ allows for nesting along the CDW vector, but it plays no role in ferromagnetic $1T\ensuremath{-}{\mathrm{VSe}}_{2}$. Following the eigenvectors of the soft modes in nonmagnetic $1T\ensuremath{-}{\mathrm{VSe}}_{2}$ monolayers yields a CDW structure with a $4\ifmmode\times\else\texttimes\fi{}4$ supercell and Peierls-type distortion in the atomic positions and electronic structure. The magnetic order indicates the potential for spin-density-wave structures.