A DFT+U and many-body Quantum Monte Carlo study of monolayer MnO₂ and VSe₂

In the past decade, the search for two-dimensional (2D) ferromagnets has been at the forefront of the materials science community. Two promising 2D ferromagnets that have been synthesized and predicted to exceed the Curie temperature of CrI₃ are monolayer MnO₂ [1] and VSe₂ [2]. The theoretical methods used to predict the properties of these materials, such as density functional theory (DFT), have a demonstrated dependence on the approximations which can result in flawed predictions for the magnetic properties. For the case of VSe₂, DFT yields the semiconducting (H) or metallic (T) phases more favorable depending on which functional is used. For the case of MnO₂, the preferred magnetic coupling (ferromagnetic or antiferromagnetic) is also functional dependent. Due to these reasons, we employed many-body Diffusion Monte Carlo (DMC) to calculate the properties of monolayer MnO₂ and VSe₂. DMC has a much weaker dependence on the trial density functional and can accurately provide energetic and magnetic properties in 2D and bulk materials. Our results have achieved the statistical uncertainty required to resolve the energy differences between the magnetic and structural phases of 2D MnO₂ and VSe₂. Our work intends to serve as a terminal theoretical benchmark for MnO₂ and VSe₂​​​​​​​ monolayers, guide experimentalists, and aid in the future computational studies of other 2D magnetic materials using many-body methods.

[1] J. Phys. Chem. Lett., 4, 20, 3382–3386 (2013)

[2] Nature Nanotechnology, 13, 289–293 (2018)