Topology of Bi2Se3 nanosheets

Abstract

The arrival of advanced materials and their unique electronic properties has opened up new investigations in the field of condensed-matter physics, offering exciting opportunities for next-generation electronic devices and applications. One class of these materials was discovered a little more than a decade ago, which exhibits a fascinating new phase of matter. These materials are called topological insulators. These solid-state materials behave as “normal” insulators in their bulk, but have conducting states that live on the boundary of the material. The hope is that it is eventually possible to manipulate such states for practical applications, such as high-performance electronics, spintronics and quantum computing.

Specifically in our work, we investigated the band structure and underlying topology of nanosheets of the topological insulator bismuth selenide (Bi2Se3). These nanosheets live on the crossover from 3D to 2D samples of Bi2Se3, and are described in the literature by an effective 2D model, or “4-band” model. We found that this 4-band model is able to describe thinner nanosheets, but breaks down for thicker nanosheets. Hence, we extended the model to an “8-band model” that can accurately describe the band structure and topological properties of thicker nanosheets, in agreement with state-of-the-art ab initio calculations and experiments. The topological properties are symmetry-protected by the (nontrivial) topological invariants assigned to the system. We calculated these invariants and observed that they arise due to a twist in the occupied valence band eigenstates. This twist makes itself known through the nontrivial winding of the parity eigenvalue and spin-z expectation value of the eigenstates over the considered Brillouin zone. Experimentalists are currently investigating Bi2Se3 nanosheets, which inspired us to build the 8-band model that may help explain their observations.