Subnanometre-wide electron channels protected by topology

Abstract

Helical locking of spin and momentum and prohibited backscattering are the key properties of topologically protected states 1,2 . They are expected to enable novel types of information processing by providing pure spin currents 3,4 , or fault tolerant quantum computation by using the Majorana fermions at interfaces of topological states with superconductors 5 . So far, the required helical conduction channels used to realize Majorana fermions are generated through the application of an axial magnetic field to conventional semiconductor nanowires 6 . Avoiding the magnetic field enhances the possibilities for circuit design significantly 7 . Here, we show that subnanometre-wide electron channels with natural helicity are present at surface step edges of the weak topological insulator Bi 14 Rh 3 I 9 (ref. 8). Scanning tunneling spectroscopy reveals the electron channels to be continuous in both energy and space within a large bandgap of 200 meV, evidencing its non-trivial topology. The absence of these channels in the closely related, but topologically trivial compound Bi 13 Pt 3 I 7 corroborates the channels' topological nature. The backscatter-free electron channels are a direct consequence of Bi 14 Rh 3 I 9 's structure: a stack of two-dimensional topologically insulating, graphene-like planes separated by trivial insulators. We demonstrate that the surface of Bi 14 Rh 3 I 9 can be engraved using an atomic force microscope, allowing networks of protected channels to be patterned with nanometre precision.The compound Bi 14 Rh 3 I 9 consists of two types of layers being alternately stacked. One layer, [(Bi 4 Rh) 3 I] 2+ , exhibits a graphenelike honeycomb lattice formed by rhodium-centred bismuth cubes, as revealed by X-ray diffraction (XRD) (red layer, Fig. 1b) and is a two-dimensional topological insulator (2DTI) according to density functional theory (DFT; ref. 8). Its structure mimics the originally proposed quantum spin Hall insulator in graphene 9 , but with an inverted bandgap being four orders of magnitude larger. The other layer separating the 2DTIs is a [Bi 2 I 8 ] 2− spacer with a trivial bandgap (blue layer, Fig. 1b). Such a stack of layers has been proposed to be a weak three-dimensional topological insulator (3DTI; ref. 10), as the only alternative of time-reversal protected 3DTIs to the meanwhile intensely studied strong 3DTIs, such as, for example, Bi 2 Se 3 (refs 1,2). However, weak 3DTIs remained elusive until DFT results in good correspondence with angle-resolved photoemission spectroscopy (ARPES) data confirmed the synthesized compound Bi 14 Rh 3 I 9 to be one 8 . Theory predicts that weak 3DTIs feature helical edge states at step edges on the surface that is perpendicular to the stacking direction 11 . These edge states are topologically protected and immune to backscattering as long as time-reversal symmetry persists. Thus, perfect conduction of these channels with conductivity e 2 /h is anticipated 11,12 . Moreover, partially interfacing these channels with superconductors is predicted to in...

Spatially resolved Landau level spectroscopy of the topological Dirac cone of bulk-typeSb2Te3(0001): Potential fluctuations and quasiparticle lifetime

“…They have been used to categorize the quantum Hall effect [6,7] as well as to predict a non-magnetic quantized transversal conductance in superfluids [8]. More recently, they led to the experimental discovery of two-dimensional (2D) topological insulators (TIs) [9][10][11], strong and weak three-dimensional (3D) TIs [12][13][14][15][16][17], topological crystalline insulators [18,19] and the anomalous quantum Hall effect [20,21]. The strong 3DTIs are so far the most versatile class in terms of different realizations in materials [3-5, 22, 23].…”

Chemical Gating of a Weak Topological Insulator: Bi14Rh3I9

“…15 Subsequently, this hypothesis was strengthened by scanning tunneling microscopy (STM) experiments. 16 By STM topography, the investigated [001] surface was found to exhibit areas with both types of layers. Clear signatures of one-dimensional (1D) states were observed in the band gap only at step edges of cationic surface layers.…”

“…However, the related surface-layer gap was found 0.25 eV below the Fermi level (E F ). 16 The [(Bi 4 Rh) 3 A confirmation of the weak 3D TI state of Bi 14 Rh 3 I 9 would require to observe the QSH effect on the mentioned 1D edge states. 22 However, related transport experiments make only sense if the observed intrinsic doping is compensated by reasonable means and, thus, the topological gap with the edge states is shifted to the Fermi level.…”

Manipulating quantum channels in weak topological insulator nanoarchitectures

“…16 Yet, in spite of the number of realizations of 3D topological insulators, 17 there have not been many proposals for realizing a WTI in stoichiometric compounds. 18 WTI may be realized in superlattice systems.…”