Evolution of electronic structure upon Cu doping in the topological insulator Bi₂Se₃

We have performed angle-resolved photoemission spectroscopy of Cu _xBi₂Se₃ as a function of Cu doping (x=0.0-0.25) to investigate the doping-induced evolution of the electronic structure. We found that the topological surface state is preserved even in the heavy-doping region (x = 0.25), indicative of the robustness of the surface state against doping and impurities. The estimated carrier concentration is far smaller than that expected from a simple intercalation picture, and saturates at x∼0.1 where superconductivity emerges. This indicates that the carrier concentration responsible for superconductivity is dominated by a subtle balance between two competing processes of electron and hole doping.