Topological insulator in a Bi-Bi$_2$Se$_3$ infinitely adaptive   superlattice phase
  We report spin- and angle-resolved photoemission studies of a topological insulator from the infinitely adaptive series between elemental Bi and Bi$_2$Se$_3$. The compound, based on Bi$_4$Se$_3$, is a 1:1 natural superlattice of alternating Bi$_2$ layers and Bi$_2$Se$_3$ layers; the inclusion of S allows the growth of large crystals, with the formula Bi$_4$Se$_{2.6}$S$_{0.4}$. The crystals cleave along the interfaces between the Bi$_2$ and Bi$_2$Se$_3$ layers, with the surfaces obtained having alternating Bi or Se termination. The resulting terraces, observed by photoemission electron microscopy, create avenues suitable for the study of one-dimensional topological physics. The electronic structure, determined by spin- and angle- resolved photoemission spectroscopy, shows the existence of a surface state that forms a large, hexagonally shaped Fermi surface around the $\Gamma$ point of the surface Brillouin zone, with the spin structure indicating that this material is a topological insulator. 
T. Valla, Huiwen Ji, L. M. Schoop, A. P. Weber, Z.-H. Pan, J. T.   Sadowski, E. Vescovo, A. V. Fedorov, A. N. Caruso, Q. D. ...
cond-mat.mtrl-sci cond-mat.str-el