Large negative Poisson's ratios and multiple single-spin Dirac cones in 2D VOX (X = F, Cl, Br, I) monolayers.

Half-metallicity and negative Poisson's ratio (NPR) are critical physical properties of two-dimensional (2D) materials. In this work, using first-principles calculations, we predicted a new class of stable 2D non-planar materials VOX (X = F, Cl, Br, I) that exhibit both half-metallicity and NPR characteristics. Among these, the VOF monolayer achieves an impressive NPR of -1.47, followed by VOBr (-0.318) and VOI (-0.232), respectively. Our calculations reveal that 2D VOX (X = F, Cl, Br, I) are promising half-metals with significant exchange splitting and robust ferromagnetic or antiferromagnetic ordering. Notably, multiple single-spin Dirac cones are observed in the electronic band structures of these 2D materials. Using Monte Carlo simulations based on the Heisenberg model, we determine the Néel temperature () of VOBr to be 290 K, slightly below room temperature. For VOF and VOI, the Curie temperatures () are identified as 232 K and 118 K, respectively. This work positions VOX (X = F, Cl, Br, I) as a promising material platform for auxetic flexible electronics and spintronic devices.