Topological semimetal in a Bi-Bi2Se3 infinitely adaptive superlattice phase

We report spin-and angle-resolved photoemission studies of a topological semimetal from the infinitely adaptive series between elemental Bi and Bi2Se3. The compound, based on Bi4Se3, is a 1:1 natural superlattice of alternating Bi-2 layers and Bi2Se3 layers; the inclusion of S allows the growth of large crystals, with the formula Bi4Se2.6S0.4. The crystals cleave along the interfaces between the Bi-2 and Bi2Se3 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 semimetal.