Topology of Bi₂Se₃ nanosheets

Recently, the quantum spin-Hall edge channels of two-dimensional colloidal nanocrystals of the topological insulator Bi₂Se₃ were observed directly. Motivated by this development, we reconsider the four-band effective model which has been traditionally employed in the past to describe thin nanosheets of this material. Derived from a three-dimensional boldsymbol{k} boldsymbol{cdot} boldsymbol{p} model, it physically describes the top and bottom electronic surface states at the Γ point that become gapped due to the material's small thickness. However, we find that the four-band model for the surface states alone, as derived directly from the three-dimensional theory, is inadequate for the description of thin films of a few quintuple layers and even yields an incorrect topological invariant within a significant range of thicknesses. To address this limitation we propose an eight-band model which, in addition to the surface states, also incorporates the set of bulk states closest to the Fermi level. We find that the eight-band model not only captures most of the experimental observations, but also agrees with previous first-principles calculations of the mathbb{Z}₂ invariant in thin films of varying thickness. The band inversion around the Γ point, which endows the surface-like bands with topology, is shown to be enabled by the presence of the additional bulk-like states without requiring any reparametrization of the resulting effective Hamiltonian.