Figure 1: (Top) Idealized surface dispersion for doped topological insulator ${\text{Cu}}_x{\text{Bi}}_2{\text{Se}}_3$. Shaded area corresponds to occupied bulk modes. Linearly dispersing surface modes meet at the time-reversal invariant momentum $k=0$. (Bottom) Crystal structure of ${\text{Cu}}_x{\text{Bi}}_2{\text{Se}}_3$, showing intercalated copper layers.