Dirac Semimetals in Two Dimensions

Understand

Graphene is famous for being a host of 2D Dirac fermions.

• However, spin-orbit coupling introduces a small gap, so that graphene is formally a quantum spin hall insulator.
• Here we present symmetry-protected 2D Dirac semimetals, which feature Dirac cones at high-symmetry points that are \emph{not} gapped by spin-orbit interactions, and exhibit behavior distinct from both graphene and 3D Dirac semimetals.
• Using a two-site tight-binding model, we construct representatives of three possible distinct Dirac semimetal phases, and show that single symmetry-protected Dirac points are impossible in two dimensions.