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 Bi ₂Se₃. The compound, based on Bi₄Se₃, is a 1:1 natural superlattice of alternating Bi₂ layers and Bi ₂Se₃ layers; the inclusion of S allows the growth of large crystals, with the formula Bi₄Se_2.6S_0.4. The crystals cleave along the interfaces between the Bi₂ and Bi ₂Se₃ 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 Γ point of the surface Brillouin zone, with the spin structure indicating that this material is a topological semimetal.