Topology of Bi2Se3 nanosheets

Recently, the quantum spin-Hall edge channels of two-dimensional colloidal nanocrystals of the topological insulator ${\mathrm{Bi}}₂{\mathrm{Se}}₃$ 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 $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ model, it physically describes the top and bottom electronic surface states at the $\mathrm{\ensuremath{\Gamma}}$ 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 $\mathrm{\ensuremath{\Gamma}}$ point, which endows the surfacelike bands with topology, is shown to be enabled by the presence of the additional bulklike states without requiring any reparametrization of the resulting effective Hamiltonian.