Spectroscopic evidence of a topological quantum phase transition in ultrathin Bi 2 Se 3 films

We have measured the band dispersion of ultrathin ${\text{Bi}}_{2}{\text{Se}}_{3}$ films formed on a silicon substrate using angle-resolved photoemission spectroscopy. An energy gap opening in the topological surface-state bands of the three-dimensional topological insulator ${\text{Bi}}_{2}{\text{Se}}_{3}$ is observed due to the hybridization of the top and bottom surfaces. The drastic change in the shape of the band structure and the reversal of the size of the energy gap between 2 and 3 quintuple layer (QL) films indicate that the parity of the conduction and valence bands are exchanged by finite-size effects. The analyses based on an effective four-band model also support the occurrence of a quantum topological phase transition, thus, implying that 2 QL films are trivial while the 3 QL films are in the quantum spin Hall state.