  Here, we report by first-principles calculations one new stable 2D Dirac
material, Ta2Se3 monolayer. For this system, stable layered bulk phase exists,
and exfoliation should be possible. Ta2Se3 monolayer is demonstrated to support
two Dirac points close to the Fermi level, achieving the exotic 2D double Dirac
semimetal. And like 2D single Dirac and 2D node-line semimetals, spin-orbit
coupling could introduce an insulating state in this new class of 2D Dirac
semimetals. Moreover, the Dirac feature in this system is layer-dependent and a
metal-to-insulator transition is identified in Ta2Se3 when reducing the
layer-thickness from bilayer to monolayer. These findings are of fundamental
interests and of great importance for nanoscale device applications.
