Bi$_{2}$$_{−}$$_{δ}$Ca$_{δ}$Se$_{3}$.NO$_{2}$, tuned to the topological transport regime. (f) Angle-integrated intensity near (red) shows a linear trend. The inset shows the expected density of states (DOS) of a helical Dirac cone, which is 1/2 that of a graphene Dirac cone due to its single spin degeneracy. (g) Schematic of the chiral Dirac fermion ground state of graphene, which exhibits spin-degenerate Dirac cones that intersect away from the Kramers’ points. (h) Schematic of the helical Dirac fermion ground state of Bi$_{2}$$_{−}$$_{δ}$Ca$_{δ}$Se$_{3}$.NO$_{2}$, which exhibits a spin-polarized Dirac cone that intersects at a Kramers’ point and guarantees a ν$_{0}$ = 1 topological quantum number for the ground state. 15