Topological phase transition and Dirac fermion transfer in Bi ₂Se₃ films

Topological surface states, while protected by time-reversal symmetry in the bulk limit, can be missing in films with thicknesses much greater than the decay lengths of the surface states. This novel effect is demonstrated theoretically in Bi₂Se₃, the best known topological insulator. When the spin-orbit-coupling strength is tuned through the quantum critical point (realizable experimentally by low-Z element substitution), there is a wide dead zone where the film is topological but without topological surface states within the projected bulk gap. This dead zone can be suppressed by interfacial bonding.