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...
We report on the observation of photogalvanic effects in epitaxially grown Sb2Te3 and Bi2Te3 three-dimensional (3D) topological insulators (TI). We show that asymmetric scattering of Dirac fermions driven back and forth by the terahertz electric field results in a dc electric current. Because of the "symmetry filtration" the dc current is generated by the surface electrons only and provides an optoelectronic access to probe the electron transport in TI, surface domains orientation, and details of electron scattering in 3D TI even at room temperature.
ReS is considered as a promising candidate for novel electronic and sensor applications. The low crystal symmetry of this van der Waals compound leads to a highly anisotropic optical, vibrational, and transport behavior. However, the details of the electronic band structure of this fascinating material are still largely unexplored. We present a momentum-resolved study of the electronic structure of monolayer, bilayer, and bulk ReS using k-space photoemission microscopy in combination with first-principles calculations. We demonstrate that the valence electrons in bulk ReS are-contrary to assumptions in recent literature-significantly delocalized across the van der Waals gap. Furthermore, we directly observe the evolution of the valence band dispersion as a function of the number of layers, revealing the transition from an indirect band gap in bulk ReS to a direct gap in the bilayer and the monolayer. We also find a significantly increased effective hole mass in single-layer crystals. Our results establish bilayer ReS as an advantageous building block for two-dimensional devices and van der Waals heterostructures.
Three-dimensional (3D) topological insulators are a new state of quantum matter, which exhibits both a bulk band structure with an insulating energy gap as well as metallic spin-polarized Dirac fermion states when interfaced with a topologically trivial material. There have been various attempts to tune the Dirac point to a desired energetic position for exploring its unusual quantum properties. Here we show a direct experimental proof by angle-resolved photoemission of the realization of a vertical topological p–n junction made of a heterostructure of two different binary 3D TI materials Bi2Te3 and Sb2Te3 epitaxially grown on Si(111). We demonstrate that the chemical potential is tunable by about 200 meV when decreasing the upper Sb2Te3 layer thickness from 25 to 6 quintuple layers without applying any external bias. These results make it realistic to observe the topological exciton condensate and pave the way for exploring other exotic quantum phenomena in the near future.
New three-dimensional (3D) topological phases can emerge in superlattices containing constituents of known two-dimensional topologies. Here we demonstrate that stoichiometric Bi1Te1, which is a natural superlattice of alternating two Bi2Te3 quintuple layers and one Bi bilayer, is a dual 3D topological insulator where a weak topological insulator phase and topological crystalline insulator phase appear simultaneously. By density functional theory, we find indices (0;001) and a non-zero mirror Chern number. We have synthesized Bi1Te1 by molecular beam epitaxy and found evidence for its topological crystalline and weak topological character by spin- and angle-resolved photoemission spectroscopy. The dual topology opens the possibility to gap the differently protected metallic surface states on different surfaces independently by breaking the respective symmetries, for example, by magnetic field on one surface and by strain on another surface.
Ternary (Bi 1−x Sb x ) 2 Te 3 films with an Sb content between 0 and 100% were deposited on a Si(111) substrate by means of molecular beam epitaxy. X-ray diffraction measurements confirm single crystal growth in all cases. The Sb content is determined by X-ray photoelectron spectroscopy. Consistent values of the Sb content are obtained from Raman spectroscopy. Scanning Raman spectroscopy reveals that the (Bi 1−x Sb x ) 2 Te 3 layers with an intermediate Sb content show spatial composition inhomogeneities. The observed spectra broadening in angular-resolved photoemission spectroscopy (ARPES) is also attributed to this phenomena. Upon increasing the Sb content from x = 0 to 1 the ARPES measurements show a shift of the Fermi level from the conduction band to the valence band. This shift is also confirmed by corresponding magnetotransport measurements where the conductance changes from n-to p-type. In this transition region, an increase of the resistivity is found, indicating a location of the Fermi level within the band gap region. More detailed measurements in the transition region reveals that the transport takes place in two independent channels. By means of a gate electrode the transport can be changed from n-to p-type, thus allowing a tuning of the Fermi level within the topologically protected surface states.
We report on the observation of a terahertz radiation-induced photon drag effect in epitaxially grown nand p-type (Bi 1−x Sb x ) 2 Te 3 three-dimensional topological insulators with different antimony concentrations x varying from 0 to 1. We demonstrate that the excitation with polarized terahertz radiation results in a dc electric photocurrent. While at normal incidence a current arises due to the photogalvanic effect in the surface states, at oblique incidence it is outweighed by the trigonal photon drag effect. The developed microscopic model and theory show that the photon drag photocurrent can be generated in surface states. It arises due to the dynamical momentum alignment by time-and space-dependent radiation electric field and implies the radiation-induced asymmetric scattering in the electron momentum space. We show that the photon drag current may also be generated in the bulk. Both surface states and bulk photon drag currents behave identically upon variation of such macroscopic parameters as radiation polarization and photocurrent direction with respect to the radiation propagation. This fact complicates the assignment of the trigonal photon drag effect to a specific electronic system.
In order to stabilize Majorana excitations within vortices of proximity induced topological superconductors, it is mandatory that the Dirac point matches the Fermi level rather exactly, such that the conventionally confined states within the vortex are well separated from the Majorana-type excitation. Here, we show by angle resolved photoelectron spectroscopy that (Bi 1−x Sb x ) 2 Te 3 thin films with x = 0.94 prepared by molecular beam epitaxy and transferred in ultrahigh vacuum from the molecular beam epitaxy system to the photoemission setup matches this condition. The Dirac point is within 10 meV around the Fermi level and we do not observe any bulk bands intersecting the Fermi level.A topological insulator (TI) is characterized by a bulk energy gap which hosts conducting helical surface states. 1,2 These surface states are protected by time reversal symmetry according to a topological Z 2 invariant. 3,4 A non-trivial Z 2 number, implying helical surface states, is induced, e.g., by spin-orbit coupling which inverts the band order around the gap at some high symmetry points of the Brillouin zone. 5-7
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.