Monolayer Mg$_{2}$C: Negative Poisson's ratio and unconventional 2D emergent fermions

TL;DR

This study predicts that monolayer Mg₂C exhibits negative Poisson's ratio and hosts various emergent 2D fermions, with potential applications in nanoscale devices, based on first-principles calculations and theoretical analysis.

Contribution

The paper introduces a novel 2D material, Mg₂C monolayer, that simultaneously exhibits negative Poisson's ratio and hosts multiple types of emergent 2D fermions, expanding the understanding of 2D material properties.

Findings

Mg₂C monolayer has a negative Poisson's ratio of approximately -0.023.

Strain induces phase transitions with emergence of novel 2D fermions.

Mg₂C monolayer is a promising platform for studying 2D fermions and nanoscale applications.

Abstract

Novel 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 Mg$_{2}$C 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 Mg$_{2}$C monolayer shows an intrinsic negative Poisson's ratio $\sim -0.023$, 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 novel types of 2D fermions emerge, including the anisotropic Dirac fermions around 12 tilted Dirac points in the metallic phase, the $2$D 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 three-fold 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 novel fermions. Our result reveals Mg$_{2}$C monolayer as an intriguing platform for the study of novel 2D fermions, and also suggests its great potential for nanoscale device applications.