A Quantum Monte Carlo study of the structural, energetic, and magnetic properties of two-dimensional (2D) H and T phase VSe$_2$

TL;DR

This study employs advanced quantum Monte Carlo methods combined with structural optimization to resolve discrepancies in the structural and magnetic properties of 2D VSe₂ phases, providing accurate phase stability and geometry insights.

Contribution

The paper introduces a combined DMC and surrogate Hessian optimization approach to accurately determine the structure and phase stability of 2D VSe₂, clarifying previous conflicting results.

Findings

DMC accurately predicts the geometry of 2D VSe₂ phases

Constructed a phase diagram for 2D VSe₂

Resolved discrepancies in magnetic property reports

Abstract

Previous works have controversially claimed near-room temperature ferromagnetism in two-dimensional (2D) VSe$_2$, with conflicting results throughout the literature. These discrepancies in magnetic properties between both phases (T and H phase) of 2D VSe$_2$ are most likely due to the structural parameters being coupled to the magnetic properties. Specifically, both phases 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. Our findings demonstrate the successes of the DMC method coupled with the surrogate Hessian structural optimization technique when applied to a 2D magnetic system.