Electronic Structure of the Dark Surface of the Weak Topological Insulator Bi14Rh3I9

TL;DR

This study investigates the electronic structure of the dark surface of the weak topological insulator Bi14Rh3I9, revealing intrinsic doping effects, the position of edge states relative to the Fermi level, and potential pathways for accessing topological transport properties.

Contribution

The paper combines experimental and theoretical methods to show that the surface doping is intrinsic and well-screened, and identifies conditions under which topological edge states can be accessed for transport.

Findings

Edge states are 0.25 eV below the Fermi level, hindering transport.

Surface doping is caused by the polar surface and is well screened.

A buried edge state likely exists at the Fermi level, accessible via multilayer steps.

Abstract

The compound Bi14Rh3I9 consists of ionic stacks of intermetallic [(Bi4Rh)3I]2+ and insulating [Bi2I8]2- layers and has been identified to be a weak topological insulator. Scanning tunneling microscopy revealed the robust edge states at all step edges of the cationic layer as a topological fingerprint. However, these edge states are found 0.25 eV below the Fermi level which is an obstacle for transport experiments. Here, we address this obstacle by comparing results of density functional slab calculations with scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. We show that the n-type doping of the intermetallic layer is intrinsically caused by the polar surface and is well screened towards the bulk. In contrast, the anionic "spacer" layer shows a gap at the Fermi level, both, on the surface and in the bulk, i.e. it is not surface-doped due to iodine desorption. The well screened surface dipole implies that a buried edge state, probably already below a single spacer layer, is located at the Fermi level. Consequently, a multilayer step covered by a spacer layer could provide access to the transport properties of the topological edge states. In addition, we find a lateral electronic modulation of the topologically non-trivial surface layer which is traced back to the coupling with the underlying zigzag chain structure of the spacer layer.