Prediction of a Dirac state in monolayer TiB2

We predict the existence of a Dirac state in a monolayer TiB2 sheet (m-TiB2), a two-dimensionalmetal diboride, based on first-principles calculations. The band structure of m-TiB2 is found to be characterized with anisotropic Dirac cones with the largest Fermi velocity of 0.57 × 106 m/s, which is about one-half of that of graphene. The Dirac point is located at the Fermi level between the K and points, with the Dirac states arising primarily from the d orbitals of Ti. Freestanding m-TiB2 exhibits a bending instability, so that a planar m-TiB2 needs to be stabilized on a substrate. The calculation of m-TiB2 on a h-BN substrate reveals a negligible influence of the h-BN substrate on the electronic properties of m-TiB2. Our findings extend the Dirac materials to metal diborides.