Manipulation of Topological States and Bulk Band Gap Using Natural   Heterostructures of a Topological Insulator
  We have performed angle-resolved photoemission spectroscopy on (PbSe)5(Bi2Se3)3m, which forms a natural multilayer heterostructure consisting of a topological insulator (TI) and an ordinary insulator. For m = 2, we observed a gapped Dirac-cone state within the bulk-band gap, suggesting that the topological interface states are effectively encapsulated by block layers; furthermore, it was found that the quantum confinement effect of the band dispersions of Bi2Se3 layers enhances the effective bulk-band gap to 0.5 eV, the largest ever observed in TIs. In addition, we found that the system is no longer in the topological phase at m = 1, pointing to a topological phase transition between m = 1 and 2. These results demonstrate that utilization of naturally-occurring heterostructures is a new promising strategy for realizing exotic quantum phenomena and device applications of TIs. 
K. Nakayama, K. Eto, Y. Tanaka, T. Sato, S. Souma, T. Takahashi, Kouji   Segawa, and Yoichi Ando
cond-mat.mes-hall cond-mat.mtrl-sci