Session E08: Two-dimensional Topological Insulators: Transport (I)

Observing Topological State Residing at Step Edge of bulk WTe2

Topological states emerge at the boundary of solids as a consequence of the nontrivial topology of the bulk. In two dimensional topological insulators, the topological edge states support quantum spin Hall effect, where the electrons at the edge of the system possess different spins when propagating along opposite directions. Spectroscopic imaging scanning tunneling microscopy (SISTM) can access the electronic states of the system with high energy and spatial resolution, and thus offers an idea probe to the topological edge states. Recently, theory predicts a topological edge state on single layer transition metal dichalcogenides with 1T’ structure. However, its existence still lacks experimental proof. Here, we report the direct observations of the topological states at the step edge of WTe₂ by SISTM. A one-dimensional electronic state residing at the step edge of WTe₂ is observed, which exhibits remarkable robustness against edge imperfections. First principles calculations rigorously verify the edge state has a topological origin, and its topological nature is unaffected by the presence of the substrate. Our study supports the existence of topological edge states in 1T’-WTe₂, which may envision in-depth study of its topological physics and device applications.   

Transport Properties of Thin Film α-Sn

The Army Research Lab is growing α-Sn via molecular beam epitaxy and investigating its topological nature for use in mesoscopic devices. The reported observation of nontrivial topology in α-Sn indicates its candidacy for transport studies in which the protected edge state plays a vital role. By measuring the magnetoresistance and temperature-dependent conductance of fabricated transport devices like hall bars, we characterize the electronic topology and band structure properties of α-Sn. Such transport experiments are necessary to evaluate the potential for highly efficient quantum and spintronic devices using α-Sn.   

Anomalous Edge Plasmon in magnetically doped topological insulator

Magnetically doped topological insulators (MTIs) with ferromagnetic dopants such as chromium support a Quantum Anomalous Hall Effect state. We show that the quantum anomalous effect necessarily leads to a chiral one dimensional edge plasmon collective mode with approximately linear dispersion, which we refer to as anomalous edge plasmons (AEPs). AEPs are closely related to the so-called Edge Magneto Plasmon (EMP) excitations of two-dimensional electron gases on quantum Hall plateaus in a strong magnetic field, and typically have frequencies in Microwave or Radio frequency ranges, depending on samples size. Recently, researchers in Stanford University used Microwave Impedance Microscopy (MIM) to image quantum anomalous Hall states, and identified response that is localized near Hall bar edges. We interpret these observations in terms of the properties of AEPs, addressing in particular the dependence of the degree of edge localization on the MIM imaging frequency. We also address the influence of magnetic domains on the MIM signal, and discuss the drastic difference between the MIM signal of the quantum anomalous Hall state and the axion insulator state which forms when the magnetization has opposite signs near the top and bottom surfaces of MTI thin films.   

Topological states on grain boundary of 1T’-MoTe2

We show that on a transition metal dichalcogenides 1T’ MoTe₂ two types of grain boundaries can exist and that electronic states associated with themselves strongly depend on their crystal symmetries as well as topological index. Using first-principles computational methods and model Hamiltonian analysis, we investigate their structural stabilities and electronic structures. We clearly demonstrate that one-dimensional metallic state along one of the grain boundaries is a topologically protected state. The spatial symmetries created by grain boundaries is also shown to play an important role in characterizing the protected metallic states.   

High-temperature quantum anomalous Hall effect on post-transition-metal-decorated graphene.

Quantum anomalous Hall (QAH) insulators are a highly promising class of materials for spintronic devices and quantum computations because of their precise quantization nature, robust properties against defects, and relatively low energy consumption for operation. To realize the QAH effect quantum spin Hall (QSH) insulators must be utilized, which requires transition metal doping or surface functionality control. Here, we propose a new way to introduce ferromagnetism to large-gap QSH insulators: we release the onsite magnetic momentum by increasing the lattice constants of stanene and germanene. If the lattice constant is increased to 9.5 Å, ab initio band structure calculations show that their spin–orbit coupling gaps are about 0.25 and 0.05 eV, respectively. Furthermore, the Curie temperatures, calculated by the Monte Carlo method, are 780 and 420 K. Both results indicate that the room-temperature QAH effect can be realized on these systems. We also provide a possible experimental realization of this system on the 2√3×2√3 graphene substrate. Our calculations predict the first room-temperature QAH insulator in the realistic materials system.   

Counterpropagating Topological Interface States in Nano Punctured Graphene

It has been known that superstructures such as a regular array of holes (antidot lattice) on a graphene sheet can induce a gapped state. We analyze the band structure of graphene with a hole array in more detail, and reveal that different arrangements of holes, triangular or honeycomb, can lead to topologically distinct states. The topological nontriviality is shown by explicitly calculating interface states between two regions with triangular and honeycomb hole arrangements, where the interface state appears as cross-shape bands mimicking helical edge states in a quantum spin Hall state. The topological character is also discussed in terms of the parity of the wave function at the high symmetry point in the Brillouin zone, in relation to the state-of-art argument on topological crystalline insulators.   

Topologically Protected Metallic States Induced by a One-Dimensional Extended Defect in the Bulk of a 2D Topological Insulator

We report ab initio calculations [1] showing that a one-dimensional extended defect generates topologically protected metallic states immersed in the bulk of two-dimensional topological insulators. We find that a narrow extended defect, composed of periodic units consisting of one octagonal and two pentagonal rings (a 558 extended defect), embedded in the hexagonal bulk of a bismuth bilayer, introduces two pairs of one-dimensional counterpropagating helical-Fermion electronic bands with the opposite spin-momentum locking characteristic of the topological metallic states that appear at the edges in two-dimensional topological insulators. Each one of these pairs of helical-Fermion bands is localized, respectively, along each one of the zigzag chains of bismuth atoms at the core of the 558 extended defect, and their hybridization leads to the opening of very small gaps (6 meV or less) in the helical-Fermion dispersions of these defect-related modes. We discuss the connection between the defect-induced metallic modes and the helical-Fermion edge states that occur on bismuth bilayer ribbons.
[1] Nano Lett., 2016, 16 (7), pp 4025–4031   

One-dimensional physics in the edge states of the high-temperature quantum spin Hall system bismuthene on SiC(0001)

Bismuthene (a mono-atomic honeycomb lattice of Bi-atoms) chemisorbed on a SiC(0001) substrate has recently been synthesized and shown to be a promising candidate for the realization of a room-temperature Quantum Spin Hall (QSH) effect which is based on a novel QSH mechanism [1]. Experiments with angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling microscopy (STM) found excellent agreement with the calculated topological band structure. In particular, while the bismuthene film displays a large bulk band gap of ~0.8 eV, conducting edge states are observed at the boundaries of the honeycomb layer, e.g. at terrace steps of the substrate, as expected for a two-dimensional topological insulator. Here we demonstrate, by a detailed analysis of tunneling spectra, that these edge states are indeed one-dimensional (1D) and correlated in nature. The spectra display power law behavior with energy and temperature as well as universal scaling, consistent with the expectations for tunneling into a Tomonaga-Luttinger liquid and in excellent agreement with experimental observations in other 1D systems.

[1] F. Reis et al., Science 357, 287 (2017)
  

MBE growth and electronic properties of 2D topological insulators on Bi2Te3

Two-dimensional topological insulators (TIs) are distinguished by their Dirac edge states, and a topological phase transition between trivial and nontrivial states that can be tuned by composition, strain, or film thickness. In this work, we demonstrate the molecular beam epitaxial growth of Bi and Sb films on 3D TI Bi₂Te₃, and investigate their electronic properties using scanning tunneling microscopy/spectroscopy and density functional theory. We find that while Bi grows layer-by-layer, the growth mode for Sb is thickness dependent: below three bilayers (BLs) is step flow, and above layer-by-layer. This facilities a systematic study of their thickness-dependent electronic properties by scanning tunneling spectroscopy. We show that while few BL Bi films are 2D TIs, Sb film is a trivial insulator at below three BLs, which evolves into a 2D TI above three BLs, confirming earlier theoretical predictions.   

A topological quantum optics interface

Topological photonics have opened up a multitude of new avenues in designing photonic devices. While exotic physics has been explored with topological photonic states in classical domain, strong light matter interaction with topological photonic states in the quantum regime remains largely unexplored. Towards this goal, we fabricate a topological photonic crystal to mediate coupling between single quantum emitter and topological photonic states. Developed on a thin slab of Gallium Arsenide membrane with electron beam lithography, such a device supports two robust counter-propagating edge states at the boundary of two distinct topological photonic crystals at near-IR wavelength. We show chiral coupling of circularly polarized lights emitted from a single Indium Arsenide quantum dot under strong magnetic field into these topological edge modes and demonstrate their robustness against sharp bends. Our technique could open up opportunities to explore many-body interaction, fault-tolerant photonic circuits and unconventional quantum states of light.   

Thickness Dependence of the Energy Band Structure and Topological Property of Topological Crystalline Insulator SnTe Films

A topological crystalline insulator (TCI) is characterized by gapless surface states that are protected by the crystalline symmetry. For an enough thin film of three-dimensional TCI, the top and bottom surface states hybridize, opening up an energy gap at the Dirac surface states. According to theoretical prediction, the quantum spin Hall (QSH) phase may occur in such a film of certain thickness, and the topological phase diagram is dependent on the electrical field perpendicular to the film plane. Here we present a systematic angle-resolved photoemission spectroscopy (ARPES) study on molecular beam epitaxy-grown TCI SnTe (111) films with different thicknesses and on different substrates. A thickness-dependent oscillation of the surface state gap was observed in the films and was found to strongly depend on the substrate used. The origin of the oscillation and its relationship with topological phase transitions were discussed.   

Solution to the hole-doping problem and tunable quantum Hall effect in Bi2Se3 thin films

Bi₂Se₃, one of the most widely studied topological insulators (TIs), is naturally electron-doped due to n-type native defects. However, many years of efforts to achieve p-type Bi₂Se₃ thin films have failed so far. In this presentation, we provide a solution to this long-standing problem, showing that the main culprit has been the high density of interfacial defects. By suppressing these defects through an interfacial engineering scheme, we have successfully implemented p-type Bi₂Se₃ thin films down to the thinnest topological regime. On this platform, we present the first tunable quantum Hall effect (QHE) study in Bi₂Se₃ thin films, and reveal not only significantly asymmetric QHE signatures across the Dirac point but also the presence of competing anomalous states near the zeroth Landau level.   

Quantum Anomalous Hall Effect in Two-dimensional Organic Mn2L3 Lattice

Using first-principles calculations, we predict the existence of nontrivial topological states in a monolayer metal-organic framework Mn₂L₃ (L = C₆O₄Cl₂), which has been experimentally synthesized. A band gap of 7.8 meV at the Dirac point near the Fermi level is opened by spin-orbital coupling, with the attributes of C and O p-orbitals mediated by Mn d-orbitals. We further construct a tight-binding model to characterize the nonzero Chern number and edge states within the Dirac gap, confirming its nontrivial topological properties. Our results suggest that Mn₂L₃ could provide an organic platform for developing low-energy-consumption spintronics devices based on the quantum anomalous Hall effect.   

STM study of Stanene

The tin analog to graphene, stanene, is predicted to be a Quantum Spin Hall Insulator whereas, stanane is anticipated to behave as a trivial insulator. This suggests that tuning the physical properties of stanene could be achieved by direct modification of the surface. However, experimentally realizing stanene has been challenging as the parent crystal cannot be exfoliated and growing the material directly requires molecular beam epitaxy. Here we present Scanning Tunneling Microscopy study of stanene and its surface alloy on Cu(111). By thermal evaporation we deposited sub-monolayer coverage of tin on the surface. We observe a triangular lattice with 4.6Å spacing, consistent with the upward sites of the buckled honeycomb structure of stanene as calculated by DFT. Co-adsorbed on the surface a 5.2Å lattice, consistent with a 2x2 reconstruction of the Cu(111) surface, indicates a CuSn alloy. Tunneling spectroscopy on stanene do not clearly show the anticipated spin-orbit gap, and we see contributions of the underlying Cu(111) states, similar to what we previously reported for graphene. Consequently, we show progress towards isolating stanene with an in-situ growth of a hexagonal boron nitride buffer layer.