Dirac fermions in monolayer TiB$_{2}$

Monolayer TiB$_{2}$ sheet ($m$-TiB$_{2}$), a two-dimensional metal-diboride, is investigated by first-principles calculations. We demonstrate that $m$-TiB$_{2}$ maintains isotropic Dirac cones near the Fermi level, having a Fermi velocity about one-half of the Fermi velocity of graphene. Different form graphene, these Dirac cones are located between K and $\Gamma $ point in the Brillouin zone, and have primarily the transition metal \textit{d-orbit} characters. Further analysis illustrates that the $d$-band Dirac cones arise from the hybridization of B $p $and Ti $d$ orbitals. Calculations of adsorption of the m-TiB$_{2}$ on hexagonal BN ($h$-BN) substrate reveal a negligible influence of the $h$-BN substrate to the electronic properties of $m$-TiB$_{2}$. Our findings extend the Dirac-band materials to metal-diborides.