Electronic Structures of Ultrathin Bi and Sb Films on 3D Topological Insulators

Abstract

While two-dimensional (2D) topological insulators (TI’s) initiated the field of topological materials, only very few materials were discovered to date and the direct access to their quantum spin Hall edge states has been challenging due to material issues. Here, we introduce a new 2D TI materials, a single Bi(111) bilayer and Sb few layer films. The electronic states of these films are investigated by scanning tunneling spectroscopy (STS) and ab intio calculations. Well-ordered Bu and Sb bilayer films with zigzag edges are grown epitaxially on a cleaved Bi2Te2Se crystal. The calculations show that the topological nature of their quantum spin Hall states preserved even through the strong hybridization exists between the films and the substrate. The edge-enhanced local density of states are identified and visualized unambiguously by scanning tunneling spectroscopy (STS) in good agreement with the calculation. The maps of local density of states clearly identify robust edge electronic states of Bi and Sb films. In the case of Sb few layer films, the topological edge states emerge through a 2D topological phase transition predicted between three and four bilayer films in recent theory.

On the other hand, by covering the surface of the 3D topological insulators, Bi2Te2Se and Bi2Se3, the original topological surface states (TSS's) are completely removed and new spin helical surface states, originating from a strong hybridization between the Bi/Sb film and the substrates, emerge with different dispersion and spin polarization. These new states play the role of TSS keeping the bulk topological nature intact. This mechanism provides a way to create various different types of topologically protected electron channels on top of a single topological insulator, possibly with tailored properties for various applications. Also, we propose a innovated approach to realize a topological p-n junction. When ultrathin Sb film was partially grown on 3D topological insulators, its surface consists of 2D Sb islands and the bare substrate naturally. On Sb islands new TSS develop due to the strong hybridization. While Bi-chalcogenides 3D TI's have p-type Dirac cones, the hybridized TSS's show strong n-type character. A p-n junction of TSS's is formed on the boundaries of Sb islands. Our experimental and theoretical results clearly substantiate that a topological p-n junction is indeed formed in partial covered 3D TI with Sb(111) film. These results will help vigorously promote studies of topological p-n junctions.