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 (T_N) of VOBr to be 290 K, slightly below room temperature. For VOF and VOI, the Curie temperatures (T_C) 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.