Franckeite: A new naturally occurring topological insulator*

Franckeite is a layered material formed by alternating tin disulfide-based Sn(Sb)S₂ - pseudohexagonal (H) - and lead sulfide-based Pb(Sb)S - pseudotetragonal (Q) - layers. These layers exhibit different symmetry and periodicity, leading to an incommensurate crystal structure bound by not-so-weak van der Waals interactions between interleaved layers. The electronic properties at large concentration of Sb on the H layer are those of an indirect gap heterostructure (holes and electrons belonging to different layers) [1]. Here we present DFT results on the opposite limit of concentration. For 100% Sn on the H layer, franckeite becomes a topological insulator. This is corroborated by Wilson loop and Z2 number evaluations (the latter when forced into a centro-symmetric structure). We have also evaluated the structural composition of the energetically favorable thinnest possible layer, corresponding this to 0.5Q/H/0.5Q. This layer behaves as a 2D topological insulator, very much like InAs/GaSb quantum wells. [1] Molina-Mendoza et al., Nature Communications 8, 14409 (2017).