Effect of Bi bilayers on the topological states ofBi2Se3: A first-principles study

Abstract

${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is a three-dimensional topological insulator which has been extensively studied because it has a single Dirac cone on the surface, inside a relatively large bulk band gap. However, the effect of two-dimensional topological insulator Bi bilayers on the properties of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and vice versa, has not been explored much. Bi bilayers are often present between the quintuple layers of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, since ${({\mathrm{Bi}}_{2})}_{n}{({\mathrm{Bi}}_{2}{\mathrm{Se}}_{3})}_{m}$ form stable ground-state structures. Moreover, ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is a good substrate for growing ultrathin Bi bilayers. By first-principles techniques, we first show that there is no preferable surface termination by either Bi or Se. Next, we investigate the electronic structure of Bi bilayers on top of, or inside a ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ slab. If the Bi bilayers are on top, we observe a charge transfer to the quintuple layers that increases the binding energy of the surface Dirac cones. The extra states, originating from the Bi bilayers, were declared to form a topological Dirac cone, but here we show that these are ordinary Rashba-split states. This result, together with the appearance of a new Dirac cone that is localized slightly deeper, might necessitate the reinterpretation of several experimental results. When the Bi bilayers are located inside the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ slab, they tend to split the slab into two topological insulators with clear surface states. Interface states can also be observed, but an energy gap persists because of strong coupling between the neighboring quintuple layers and the Bi bilayers.