3D Topological Insulator Research Articles

Low Power In-Memory Computation with Reciprocal Ferromagnet/Topological Insulator Heterostructures.

The surface state of a 3D topological insulator (3DTI) is a spin-momentum locked conductive state, whose large spin hall angle can be used for the energy-efficient spin-orbit torque based switching of an overlying ferromagnet (FM). Conversely, the gated switching of the magnetization of a separate FM in or out of the TI surface plane can turn on and off the TI surface current. By exploiting this reciprocal behavior, we can use two FM/3DTI heterostructures to design an integrated 1-transistor 1-magnetic tunnel junction random access memory unit (1T1MTJ RAM) for an ultra low power Processing-in-Memory (PiM) architecture. Our calculation involves combining the Fokker-Planck equation with the Nonequilibrium Green Function (NEGF) based flow of conduction electrons and Landau-Lifshitz-Gilbert (LLG) based dynamics of magnetization. Our combined approach allows us to connect device performance metrics with underlying material parameters, which can guide proposed experimental and fabrication efforts.

A generic designing rule for realizing quantum anomalous Hall phase in a transition-metal trichalcogenide family

Different from introducing ferromagnetism in three-dimensional (3D) topological insulators, empowering nontrivial band topology with 2D ferromagnetic (2DFM) materials is another way to realize quantum anomalous Hall effect (QAHE). The recently discovered 2DFM insulators provide unprecedented opportunities for realizing such an intriguing quantum phenomenon. Here, by using first-principles approaches, we design a generic rule for realizing QAHE in the transition-metal trichalcogenide family materials of Cr2A2X6 (where A = Si, Ge, Sn; X = S, Se, Te), whose design principle is based on that a 2D topologically trivial magnetic insulator can be converted into a QAH system via proper surface modification. Taking silicene-passivated Cr2Sn2Se6 as an illustration, we show that the Cuire temperature of Cr2Sn2Se6 monolayer can be enhanced up to ∼92 K after silicene passivation. Most strikingly, such a heterostructure harbors QAHE with a band gap of ∼30 meV. Further low-effective model analyses reveal that the topologically nontrivial states are attributed to the honeycomb lattice of Cr atoms. All those results demonstrate that our proposal is general, which opens a new avenue to explore high-temperature QAHE.

Fredholm Homotopies for Strongly-Disordered 2D Insulators

We study topological indices of Fermionic time-reversal invariant topological insulators in two dimensions, in the regime of strong Anderson localization. We devise a method to interpolate between certain Fredholm operators arising in the context of these systems. We use this technique to prove the bulk-edge correspondence for mobility-gapped 2D topological insulators possessing a (Fermionic) time-reversal symmetry (class AII) and provide an alternative route to a theorem by Elgart-Graf-Schenker (Commun Math Phys 259(1):185–221, 2005) about the bulk-edge correspondence for strongly-disordered integer quantum Hall systems. We furthermore provide a proof of the stability of the $$\mathbb {Z}_2$$ index in the mobility gap regime. These two-dimensional results serve as a model for the study of higher dimensional $$\mathbb {Z}_2$$ indices.

Steep-slope transistors enabled with 2D quantum coupling stacks

As down scaling of transistors continues, there is a growing interest in developing steep-slope transistors with reduced subthreshold slope (SS) below the Boltzmann limit. In this work, we successfully fabricated steep-slope MoS2 transistors by incorporating a graphene layer, inserted in the gate stack. For our comprehensive study, we have applied density functional theory to simulate and calculate the change of SS effected by different 2D quantum materials, including graphene, germanene and 2D topological insulators, inserted within the gate dielectric. This theoretical study showed that graphene/MoS2 devices had steep SS (27.2 mV/decade), validating our experimental approach (49.2 mV/decade). Furthermore, the simulations demonstrated very steep SS (8.6 mV/decade) in WTe2/MoS2 devices. We conclude that appropriate combination of various 2D quantum materials for the gate-channel stacks, leads to steep SS and is an effective method to extend the scaling of transistors with exceptional performance.

TaIrTe4 Monolayer with Topological Insulator Characteristic: A New and Highly Efficient Electrocatalyst toward Oxygen Reduction Reaction

Topological materials with robust topological surface states can be a class of very promising catalysts in electrocatalysis. In this study, a freestanding 2D topological insulator (TI) nanomaterial, namely, TaIrTe4 monolayer with low Ir-loading, which can be exfoliated from the bulk phase with the layered structure, is demonstrated to be dynamically, mechanically, and thermodynamically stable under the first-principles calculations. More intriguingly, coupled with good conductivity, a 2D TaIrTe4 monolayer can be considered as an alternative to noble metal Pt-based electrocatalyst for the oxygen reduction reaction (ORR). Our calculations revealed that on the surface of the TaIrTe4 monolayer, the adsorbed O2 molecules can be effectively activated and dissociated into two separated O* species through a very small energy barrier (0.28 eV, even lower than that of Pt(111)). When the four-electron (4e) dissociation path of ORR occurs on the TaIrTe4 monolayer, a very small overpotential of 0.47 V can be observed, which can be even comparable to the Pt(111) surface. In addition, the 2D TaIrTe4 monolayer can also exhibit high selectivity, effectively inhibiting the formation of byproduct H2O2. It can be revealed that the TI topological states can have an important effect on the excellent ORR catalytic activity of the 2D TaIrTe4 monolayer. Undoubtedly, the design of ORR electrocatalysts based on the fascinating topological materials can be a new and effective way to replace the precious metal Pt-based ORR electrocatalysts.

Prediction of ferromagnetic Weyl semimetal and antiferromagnetic topological insulator phases in MnHg2Te3

Based on first-principles band-structure calculations, we predict that FM MnHg2Te3 is a Weyl semimetal candidate. When the direction of spin polarization is toward the c-axis, there are six Weyl points in the whole Brillouin zone. With spin orientation along the a-axis, there exist eight Weyl points. For AFM MnHg2Te3, when the spin direction is along the c-axis, the band structure is fully gapped. The calculation of the Z2 number confirms that AFM-c MnHg2Te3 is a 3D AFM topological insulator. Adjusting the spin direction from the c-axis to the a-axis only changes the bandgap without affecting the topological properties of this system. The gapless surface-state on the (100) surface is also obtained, the results of which correspond with the properties of the AFM topological insulator.

Homotopic analysis of quantum states in two-dimensional polymorphs by a herringbone lattice model

We report a homotopic analysis of quantum states in two-dimensional (2D) polymorphs. Two typical models, namely the honeycomb-type and Shastry-Sutherland-type models, are continuously connected through a deformable herringbone lattice model having a nonsymmorphic group symmetry. The evolution of Dirac fermions is characterized within the phase space spanned by the deformation parameters. Our tight-binding models, based on single orbitals for each site with and without spin-orbit coupling (SOC), reveal how the SOC convolutes, the manner being important in facilitating the search and design of 2D topological insulators focused on the homotopic feature of symmetry-protected properties independent of specific materials.

Selective Fabrication of Bismuthene and α-Bi on Hydrogen-Terminated SiC(0001).

Nanomaterials based on monoatomic bismuth (Bi) are attracting particular attention because they are candidates of two-dimensional (2D) topological insulators and Rashba metals useful for spintronic applications. We report convenient selective fabrication of two different types of ultrathin Bi films, bismuthene and α-Bi on hydrogen-terminated SiC(0001), by combining the molecular-beam-epitaxy (MBE) method and the low-temperature and low-pressure hydrogen chemical etching of SiC. We have succeeded in selectively fabricating these two different Bi phases by simply tuning the substrate temperature during the MBE process. We observed that while bismuthene and α-Bi showed a similar low-energy electron diffraction pattern of the (√3 × √3)R30° periodicity, angle-resolved photoemission spectroscopy revealed a sizable difference in the band structure; bismuthene shows a massive Dirac cone, a signature of 2D topological insulators, whereas α-Bi exhibits an insulating behavior with a large band gap of more than 1.8 eV. We discuss the underlying mechanism of selective fabrication in terms of hydrogen desorption from the hydrogen-terminated SiC substrate.

Emergent helical edge states in a hybridized three-dimensional topological insulator

As the thickness of a three-dimensional (3D) topological insulator (TI) becomes comparable to the penetration depth of surface states, quantum tunneling between surfaces turns their gapless Dirac electronic structure into a gapped spectrum. Whether the surface hybridization gap can host topological edge states is still an open question. Herein, we provide transport evidence of 2D topological states in the quantum tunneling regime of a bulk insulating 3D TI BiSbTeSe2. Different from its trivial insulating phase, this 2D topological state exhibits a finite longitudinal conductance at ~2e2/h when the Fermi level is aligned within the surface gap, indicating an emergent quantum spin Hall (QSH) state. The transition from the QSH to quantum Hall (QH) state in a transverse magnetic field further supports the existence of this distinguished 2D topological phase. In addition, we demonstrate a second route to realize the 2D topological state via surface gap-closing and topological phase transition mechanism mediated by a transverse electric field. The experimental realization of the 2D topological phase in a 3D TI enriches its phase diagram and marks an important step toward functionalized topological quantum devices.

Evaluation of the influence of thermal annealing on the performance of vertical topological insulator p-n heterojunction broadband photodetector

The rapid progress and continuum search for new broadband photodetectors generation with ultrafast response synchronizes with the conductive surface electronic structure and the strong broadband absorption properties of 3D topological insulators. Here, we introduce the fabrication of a vertical p-n heterojunction based on thermally evaporated thin films of Sb2Te3/Bi2Te3 on Si substrate for broadband photodetection. The realization of these characteristics dictated us to study the structural, optical absorption, and morphological properties of each layer in the fabricated device as a function of annealing temperature using XRD, spectrophotometric measurements, and FE-SEM techniques, respectively. The annealing process up to T = 423 K enhanced the film’s crystallinity, and grain growth process besides improving the film quality and surface roughness. Furthermore, the optoelectronic characterizations showed that all fabricated photodetectors exhibit a good response to halogen light illumination in the wavelength range (310–1100) nm with different intensities (10–50) mW/cm2. Whilst the 423 K annealed photodetector showed the highest photosensitivity with a conspicuous stable and fast response. The main device parameters, including spectral responsivity, specific detectivity, and response times were determined in addition to explaining the forward and reverse bias conduction mechanisms. The estimated spectral responsivity of the fabricated devices was about 0.247, 0.397, and 0.148 A/W for the as fabricated and 423 K and 573 K, respectively. This significant improvement of the device’s performance rather than other topological insulator devices may be due to the strong built-in field at the topological insulators’ interfaces. Moreover, the fabricated photodetectors showed improved and repeatable ON/OFF transient switching behavior with 85.23 ms and 84.66 ms as a rise and fall time, respectively. According to the prominent results in addition to the simple architecture of the fabricated photodetectors, this topological insulator photosensor can be considered a potential candidate in prospective high-performance optoelectronics.

Near‐Field Plasmons Imaging and Energy Analysis on Bi2Te3 Film

AbstractTopological materials have great potential in lossless electronic transportation. Bi2Te3 as three‐dimensional (3D) topological insulator is predicted to exhibit many amazing electromagnetic characteristics, which are widely studied all over the world. However, current research available on Bi2Te3 materials is not adequate, and a comprehensive understanding of its properties is necessary. In this work, near‐field plasmon imaging based on Bi2Te3 nanostructures and the kinetic energy spectra of emitted electrons from Bi2Te3 film by using photoemission electron microscope instruments is studied. Under UV laser excitation of 273 nm, the electrons can be excited from the trap states in bandgap, which provides a feasible idea for observing the trap energy levels of Bi2Te3 materials. Moreover, the imaging applications of nanostructures in Bi2Te3 film based on their polarization‐dependence and wavelength‐dependence properties are also demonstrated, providing a more degrees of freedom for imaging, and can realize multidimensional and multi‐multiplexing imaging technology of topological materials. This work provides a novel method to enrich the Bi2Te3 film properties from the view of near‐field plasmon imaging and kinetic energy distribution, paving the way for realizing practical applications of topological materials.

DFT-1/2 method applied to 3D topological insulators

In this paper, we present results and describe the methodology of application of DFT-1/2 method for five three-dimensional topological insulators materials that have been extensively studied in last years: Bi2Se3, Bi2Te3, Sb2Te3, CuTlSe2 and CuTlS2. There are many differences between the results of simple DFT calculations and quasiparticle energy correction methods for these materials, especially for band dispersion in the character band inversion region. The DFT-1/2 leads to quite accurate results not only for band gaps, but also for the shape and atomic character of the bands in the neighborhood of the inversion region as well as the topological invariants, essential quantities to describe the topological properties of materials. The methodology is efficient and ease to apply for the different approaches used to obtain the topological invariant Z 2, with the benefit of not increasing the computational cost in comparison with standard DFT, possibilitating its application for materials with a high number of atoms and complex systems.

Photonic topological insulator induced by a dislocation in three dimensions.

The hallmark of topological insulators (TIs) is the scatter-free propagation of waves in topologically protected edge channels1. This transport is strictly chiral on the outer edge of the medium and therefore capable of bypassing sharp corners and imperfections, even in the presence of substantial disorder. In photonics, two-dimensional (2D) topological edge states have been demonstrated on several different platforms2-4 and are emerging as a promising tool for robust lasers5, quantum devices6-8 and other applications. More recently, 3D TIs were demonstrated in microwaves9 andacoustic waves10-13, where the topological protectionin the latter isinduced by dislocations. However, at optical frequencies, 3D photonic TIs have so far remained out of experimental reach. Here we demonstrate a photonic TI with protected topological surface states in three dimensions. The topological protection is enabled by a screw dislocation. For this purpose, we use the concept of synthetic dimensions14-17 in a 2D photonic waveguide array18 by introducing a further modal dimension to transform the system into a 3D topological system. The lattice dislocation endows the system with edge states propagating along 3D trajectories, with topological protection akin to strong photonic TIs19,20. Our work paves the way for utilizing 3D topology in photonic science and technology.

Nontrivial gapless electronic states at the stacking faults of weak topological insulators

Lattice defects such as stacking faults may obscure electronic topological features of real materials. In fact, defects are a source of disorder that can enhance the density of states and conductivity of the bulk of the system and they break crystal symmetries that can protect the topological states. On the other hand, in recent years it has been shown that lattice defects can act as a source of nontrivial topology. Motivated by recent experiments on three-dimensional (3D) topological systems such as Bi$_2$TeI and Bi$_{14}$Rh$_3$I$_9$, we examine the effect of stacking faults on the electronic properties of weak topological insulators (WTIs). Working with a simple model consisting of a 3D WTI formed by weakly-coupled two-dimensional (2D) topological layers separated by trivial spacers, we find that 2D stacking faults can carry their own, topologically nontrivial gapless states. Depending on the WTI properties, as well as the way in which the stacking fault is realized, the latter can form a topologically protected 2D semimetal, but also a 2D topological insulator which is embedded in the higher-dimensional WTI bulk. This suggests the possibility of using stacking faults in real materials as a source of topologically nontrivial, symmetry-protected conducting states.

Two dimensional LiMgAs: A topological quantum catalyst for hydrogen evolution reaction

Quantum materials, such as topological insulators (TIs), are promising due to diverse applications of their robust surface/edge states in the bulk three-dimensional (3D) and two-dimensional (2D) regimes. Such conducting surface states in 3D systems host “electron baths,” which are known to facilitate catalysis. However, the analogous effects in 2D scenarios wherein conducting helical edge states leading to Fermionic accumulation have been scarcely addressed. Using first-principles calculations, we demonstrate that the conducting edge states in 2D TIs, such as LiMgAs, can be exploited to facilitate excellent catalytic response toward hydrogen evolution reactions. The Gibbs free energy in such cases was found to be as low as −0.02 eV, which is quite superior compared to other materials reported in the literature. The concept presented herein can be extended to other well-known 2D TIs and used to realize unconventional topological quantum catalysts for ultra-high performance and efficient catalytic applications.

Observation of Dirac Hierarchy in Three-Dimensional Acoustic Topological Insulators.

Dirac cones (DCs) play a pivotal role in various unique phenomena ranging from massless electrons in graphene to robust surface states in topological insulators (TIs). Recent studies have theoretically revealed a full Dirac hierarchy comprising an eightfold bulk DC, a fourfold surface DC, and a twofold hinge DC, associated with a hierarchy of topological phases including first-order to third-order three-dimensional (3D) topological insulators, using the same 3D base lattice. Here, we report the first experimental observation of the Dirac hierarchy in 3D acoustic TIs. Using acoustic measurements, we unambiguously reveal that lifting of multifold DCs in each hierarchy can induce two-dimensional topological surface states with a fourfold DC in a first-order 3D TI, one-dimensional topological hinge states with a twofold DC in a second-order 3D TI, and zero-dimensional topological corner states in a third-order 3D TI. Our Letter not only expands the fundamental research scope of Dirac physics, but also opens up a new route for multidimensional robust wave manipulation.

Fast-Response Photodetector Based on Hybrid Bi2Te3/PbS Colloidal Quantum Dots.

Colloidal quantum dots (CQDs) as photodetector materials have attracted much attention in recent years due to their tunable energy bands, low cost, and solution processability. However, their intrinsically low carrier mobility and three-dimensional (3D) confinement of charges are unsuitable for use in fast-response and highly sensitive photodetectors, hence greatly restricting their application in many fields. Currently, 3D topological insulators, such as bismuth telluride (Bi2Te3), have been employed in high-speed broadband photodetectors due to their narrow bulk bandgap, high carrier mobility, and strong light absorption. In this work, the advantages of topological insulators and CQDs were realized by developing a hybrid Bi2Te3/PbS CQDs photodetector that exhibited a maximum responsivity and detectivity of 18 A/W and 2.1 × 1011 Jones, respectively, with a rise time of 128 μs at 660 nm light illumination. The results indicate that such a photodetector has potential application in the field of fast-response and large-scale integrated optoelectronic devices.

Nanometric Moiré Stripes on the Surface of Bi2Se3 Topological Insulator.

Mismatch between adjacent atomic layers in low-dimensional materials, generating moiré patterns, has recently emerged as a suitable method to tune electronic properties by inducing strong electron correlations and generating novel phenomena. Beyond graphene, van der Waals structures such as three-dimensional (3D) topological insulators (TIs) appear as ideal candidates for the study of these phenomena due to the weak coupling between layers. Here we discover and investigate the origin of 1D moiré stripes on the surface of Bi2Se3 TI thin films and nanobelts. Scanning tunneling microscopy and high-resolution transmission electron microscopy reveal a unidirectional strained top layer, in the range 14–25%, with respect to the relaxed bulk structure, which cannot be ascribed to the mismatch with the substrate lattice but rather to strain induced by a specific growth mechanism. The 1D stripes are characterized by a spatial modulation of the local density of states, which is strongly enhanced compared to the bulk system. Density functional theory calculations confirm the experimental findings, showing that the TI surface Dirac cone is preserved in the 1D moiré stripes, as expected from the topology, though with a heavily renormalized Fermi velocity that also changes between the top and valley of the stripes. The strongly enhanced density of surface states in the TI 1D moiré superstructure can be instrumental in promoting strong correlations in the topological surface states, which can be responsible for surface magnetism and topological superconductivity.

Integration and Applications of Nanomaterials for Ultrafast Photonics

AbstractNanomaterials with remarkable optical, mechanical, and electrical properties have shed new light on various fields including optoelectronics, sensors, biomedicine, and ultrafast photonics. Particularly, owing to their nonlinear optical properties, fast recovery time, and broadband operation, nanomaterials are well qualified as saturable absorbers in ultrafast pulsed lasers. Over the development of the past decades, various nanomaterials have been developed as saturable absorbers, which contribute to a diversity of lasers with excellent performance. Therefore, it is important for researchers to provide a landmark of the applications of nanomaterials in ultrafast photonics concerning cutting‐edge development. Herein, the integration and applications of nanomaterials in ultrafast photonics are reviewed. First, end‐face, lateral, as well as photonic crystal fibers padding integration methods are introduced along with their process and characteristics. Then the ultrafast applications of nanomaterials including carbon‐based (carbon nanotubes and graphene), typical 2D (topological insulators, transition metal dichalcogenides, black phosphorus, and MXenes), and metal‐based nanomaterials (gold, silver, copper, and metal oxides) are summarized. Major parameters of ultrafast lasers and features of each nanomaterial are presented simultaneously. Finally, the perspectives of nanomaterials for further development in ultrafast photonics are discussed.

Strong Anharmonicity‐Induced Low Thermal Conductivity and High n‐type Mobility in the Topological Insulator Bi1.1Sb0.9Te2S

AbstractIntrinsically low lattice thermal conductivity (κlat) while maintaining the high carrier mobility (μ) is of the utmost importance for thermoelectrics. Topological insulators (TI) can possess high μ due to the metallic surface states. TIs with heavy constituents and layered structure can give rise to high anharmonicity and are expected to show low κlat. Here, we demonstrate that Bi1.1Sb0.9Te2S (BSTS), which is a 3D bulk TI, exhibits ultra‐low κlat of 0.46 Wm−1 K−1 along with high μ of ≈401 cm2 V−1 s−1. Sound velocity measurements and theoretical calculations suggest that chemical bonding hierarchy and high anharmonicity play a crucial role behind such ultra‐low κlat. BSTS possesses low energy optical phonons which strongly couple with the heat carrying acoustic phonons leading to ultra‐low κlat. Further, Cl has been doped at the S site of BSTS which increases the electron concentration and reduces the κlat resulting in a promising n‐type thermoelectric figure of merit (zT) of ≈0.6 at 573 K.