A Many-Body Diffusion Monte Carlo and DFT+U Study of Monolayer VSe₂

Previous works have controversially claimed near-room temperature ferromagnetism in two-dimensional (2D) VSe₂, with conflicting results throughout the literature. For T-phase VSe₂, competing magnetic and non-magnetic states are predicted to exist, which can drive charge density wave (CDW) formation. Although density functional theory (DFT) calculations can capture the differences in chemical composition and polymorphism, strong coupling between the magnetic and structural properties in this material makes it difficult to trust the results from approximate density functionals. For example, T and H-VSe₂ have a close lattice match and similar total energies, which makes it difficult to determine which phase is being observed experimentally. In this study, we used a combination of density functional theory (DFT), highly accurate diffusion Monte Carlo (DMC) and a surrogate Hessian line-search optimization technique to resolve the previously reported discrepancy in structural parameters and relative phase stability. With DMC accuracy, we determined the freestanding geometry of both phases and constructed a phase diagram. In addition, we studied the energy differences between the ferromagnetic, antiferromagnetic and CDW phases through a many-body lens to resolve these discrepancies for T-VSe₂. Going one step further, we also studied the effect of strain and Se vacancies on the electronic and magnetic properties of these competing magnetic states. Our findings demonstrate the successes of the DMC method when applied to a correlated 2D magnetic system such as VSe₂.<br/>[1] <i>J. Phys. Chem. Lett.</i> 2023, <b>14</b>, 14, 3553–3560 (2023)<br/>[2] <i>Phys. Rev. B</i> <b>106</b>, 085117 (2022)