Monolayer Mg2C : Negative Poisson's ratio and unconventional two-dimensional emergent fermions

Two-dimensional (2D) emergent fermions and negative Poisson's ratio in 2D materials are fascinating subjects of research. Here, based on first-principles calculations and theoretical analysis, we predict that the hexacoordinated Mg₂C monolayer hosts both exotic properties. We analyze its phonon spectrum, reveal the Raman-active modes, and show that it has small in-plane stiffness constants. Particularly, under the tensile strain in the zigzag direction, the Mg₂C monolayer shows an intrinsic negative Poisson's ratio ∼-0.023 , stemming from its unique puckered hinge structure. The material is metallic at its equilibrium state. A moderate biaxial strain can induce a metal-semimetal-semiconductor phase transition, during which several types of 2D unconventional fermions emerge, including the anisotropic Dirac fermions around 12 tilted Dirac points in the metallic phase, the 2D double Weyl fermions in the semimetal phase where the conduction and valence bands touch quadratically at a single Fermi point, and the 2D pseudospin-1 fermions at the critical point of the semimetal-semiconductor phase transition where three bands cross at a single point on the Fermi level. In addition, uniaxial strains along the high-symmetry directions break the threefold rotational symmetry and reduce the number of Dirac points. Interestingly, it also generates 2D type-II Dirac points. We construct effective models to characterize the properties of these fermions. Our result reveals Mg₂C monolayer as an intriguing platform for the study of 2D unconventional fermions, and also suggests its great potential for nanoscale device applications.