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Quantum spin Hall (QSH) insulators represent a quintessential example of a topological phase of matter, characterized by a conducting edge mode existing within a bulk energy gap. The pursuit of a tunable QSH state stands as a pivotal objective in the development of QSH-based topological devices. In this study, we employ first-principles calculations to identify three strain-tunable quantum spin Hall (QSH) insulators based on monolayer MAlGaTe4 (where M represents Mg, Ca, or Sr). These monolayers exhibit dynamic stability, with no imaginary modes detected in their phonon dispersion. While MgAlGaTe4 is a normal insulator under zero strain, it transitions into the QSH phase when subjected to external strains. Conversely, CaAlGaTe4 and SrAlGaTe4 already exhibit the QSH phase at zero strain. Intriguingly, upon the application of biaxial strain, these two compounds undergo phase transitions, encompassing metallic (M), normal/trivial insulator (NI), and topological insulator (TI) phases, thereby illustrating their strain-tunable electronic and topological properties. (Ca, Sr)-AlGaTe4, in particular, undergo M-TI/TI-M transitions under applied strain, while MgAlGaTe4 additionally experiences an M-NI/NI-M transition, signifying it as a material featuring a metal–insulator transition (MIT). Remarkably, the observation of metal-trivial insulator-topological insulator transitions in MgAlGaTe4 introduces it as a unique material platform in which both MIT and topological phase transitions can be controlled through the same physical parameter. Moreover, the piezoelectric properties of these materials enable strain-induced tuning, allowing them to be strain-biased for application in novel devices such as reconfigurable circuit components for compact electronic devices and topological field-effect transistors. Our study thus introduces a novel material platform distinguished by highly strain-tunable electronic and topological properties, offering promising prospects for the development of next-generation, low-power topological devices. [ABSTRACT FROM AUTHOR]

The precession of a ferromagnet leads to the injection of spin current and heat into an adjacent non-magnetic material. Besides, spin-orbit entanglement causes an additional charge current injection. Such a device has been recently proposed where a quantum-spin hall insulator (QSHI) in proximity to a ferromagnetic insulator (FI) and superconductor (SC) leads to the pumping of charge, spin, and heat. Here we build a circuit-compatible Verilog-A-based compact model for the QSHI-FI-SC device capable of generating two topologically robust modes enabling the device operation. Our model also captures the dependence on the ferromagnetic precision, drain voltage, and temperature with an excellent (> 99%) accuracy.

Two-dimensional Bi grown on semiconductor substrate, a large-gap quantum spin Hall insulator characterized by a ( p _x , p _y )-orbital hexagonal lattice, has been theoretically proposed and experimentally confirmed. Here, by combining tight-binding modeling with first-principles calculations, we investigate the electronic structures and quantum transport properties of Bi nanoribbons (NRs), focusing on the topological edge states for nanoelectronics. We reveal that band gap emerges due to the quantum confinement, and the gaps size depends crucially on the width and edge shape: for zigzag NRs, the gap decreases monotonically with the increase of width; while for armchair NRs, it can be categorized into three subgroups with band-gap hierarchies of $E_ {\mathrm{g}(3p-1)} \gt E_ {\mathrm{g}(3p)} \gt E_ {\mathrm{g}(3p+1)}$ , so that the overall relation is an oscillating dependence dumped by 1/width decay. Quantum transport calculations demonstrate that the conductance is quantized to 2 $\mathrm{e}^{2}/h$ , and an applied gate voltage can efficiently regulate the conductance plateau, originating from the interplay between gate voltage and topological gaps. Furthermore, the quantized conductance remains robust against strong disorder, suggesting the unique advantage of topological states for electronic transport. This work not only provides fundamental insights into the electronic properties of topological insulator nanostructures, but also sheds light on the potential applications of exotic states for quantum devices compatible with semiconductor technology.

Topological insulator (TIs), a novel quantum state of materials, has a lot of significance in the development of low-power electronic equipments as the conducting edge states display exceptional endurance against back-scattering. The absence of suitable materials with high fabrication feasibility and significant nontrivial bandgap, is now the biggest hurdle in their potential applications in devices. Here, we illustrate using first principles density functional calculations that the quintuplet layers of EuMg 2 Bi 2 and YbMg 2 Bi 2 crystals are potential two-dimensional TIs with a sizeable nontrivial gaps of 72 meV and 147 meV respectively. Dynamical stability of these quintuplet layers of EuMg 2 Bi 2 and YbMg 2 Bi 2 is confirmed by our phonon calculations. The weakly coupled layered structure of parent compounds makes it possible for simple exfoliation from a three-dimensional structure. We observed gapless edge states inside the bulk band gap in both the systems which indicate their TI nature. Further, we observed the anomalous and spin Hall conductivities to be quantized in two dimensional EuMg 2 Bi 2 and YbMg 2 Bi 2 respectively. Our findings predict two viable candidate materials as two dimensional quantum TIs which can be explored by future experimental investigations and possible applications of quantized spin and anomalous Hall conductance in spintronics., (© 2024 IOP Publishing Ltd.)

The remarkable development of colloidal nanocrystals with controlled dimensions and surface chemistry has resulted in vast optoelectronic applications. But can they also form a platform for quantum materials, in which electronic coherence is key? Here, we use colloidal, two-dimensional Bi 2 Se 3 crystals, with precise and uniform thickness and finite lateral dimensions in the 100 nm range, to study the evolution of a topological insulator from three to two dimensions. For a thickness of 4-6 quintuple layers, scanning tunneling spectroscopy shows an 8 nm wide, nonscattering state encircling the platelet. We discuss the nature of this edge state with a low-energy continuum model and ab initio GW-Tight Binding theory. Our results also provide an indication of the maximum density of such states on a device.

Monolayer transition metal dichalcogenides (TMDCs) with the 1T′ structure are a new class of large-gap two-dimensional (2D) topological insulators, hosting topologically protected conduction channels on the edges. However, the 1T′ phase is metastable compared to the 2H phase for most of 2D TMDCs, among which the 1T′ phase is least favored in monolayer MoS 2. Here we report a clean and controllable technique to locally induce nanometer-sized 1T′ phase in monolayer 2H-MoS 2 via a weak Argon-plasma treatment, resulting in topological phase boundaries of high density. We found that the stabilization of 1T′ phase arises from the concerted effects of S vacancies and the tensile strain. Scanning tunneling spectroscopy (STS) clearly reveals a spin-orbit band gap (~60 meV) and topologically protected in-gap states residing at the 1T′-2H phase boundary, which are corroborated by density-functional theory (DFT) calculations. The strategy developed in this work can be generalized to a large variety of TMDCs materials, with potentials to realize scalable electronics and spintronics with low dissipation. [ABSTRACT FROM AUTHOR]

Based on first-principles calculations, we predict that the monolayer AuTe2Cl is a quantum spin Hall (QSH) insulator with a topological band gap about 10 meV. The three-dimensional (3D) AuTe2Cl is a topological semimetal that can be viewed as the monolayer stacking along b axis. By studying the energy-level distribution of p orbitals of Te atoms for the bulk and the monolayer, we find that the confinement effect driven p y − − p z + band inversion is responsible for the topological nontrivial nature of monolayer. Since 3D bulk AuTe2Cl has already been experimentally synthesized, we expect that monolayer AuTe2Cl can be exfoliated from a bulk sample and the predicted QSH effect can be observed. [ABSTRACT FROM AUTHOR]

Large-gap quantum spin Hall (QSH) insulators were previously predicted in stanene and its derivatives. Beyond stanene that is the thinnest α -Sn(111) film, we propose to explore QSH insulators in α -Sn films with different crystallographic orientations. Our first-principles calculations reveal that the thickness-dependent band gap of α -Sn(100) and α -Sn(110) films does not show a monotonic decrease as typically expected by quantum confinement, but displays an oscillating change behavior, an indicative of topological quantum phase transition. While these films are normal insulators in the ultrathin limit, the QSH phase emerges above a critical film thickness of around 10 layers. Remarkably, the QSH insulators are obtainable within a wide thickness range and their energy gaps are sizable (even > 0.1 eV), which facilitates experimental realization of the high-temperature QSH effect. [ABSTRACT FROM AUTHOR]

Abstract The quantum spin Hall (QSH) insulators possess potential applications in dissipationless spintronics. With the rapid development of flexible electronics, it is highly desirable to search or design high-performance QSH insulators with excellent flexibility and large bandgap, which is still a huge challenge. Here, using first-principles calculations, we report that surface oxygen functionalization can transform two-dimensional (2D) GaSe monolayer from a trivial insulator to a nontrivial topological insulator with superior stretchability. The fully O-functionalized GaSe monolayer, GaSeO, exhibits a sizable QSH gap of 178 meV, in which the topological nature is characterized by the nontrivial Z 2 topological invariant and the topological edge states. The nontrivial band topology originates from the s-p band inversion in the crystal field. Remarkably, the GaSeO monolayer shows isotropic mechanical flexibility and can sustain large strains of up to 26%, beyond graphene and 2D flexible Ti 2 C. Furthermore, the QSH state of GaSeO is robust against small elastic strains. The discovery of this 2D flexible and large-gap QSH insulator opens up new opportunities for developing stretchable and low-power electronic devices. Graphical abstract Image 1 Highlights • The fully O-functionalized GaSe monolayer, GaSeO, is a quantum spin Hall insulator with a sizable bandgap of 178 meV. • The nontrivial band topology originates from the s-p band inversion in the crystal field. • The GaSeO monolayer exhibits excellent mechanical flexibility sustaining critical strains of up to 26%. • Promising candidates for applications in flexible, portable and ultra-low-power spintronic devices. [ABSTRACT FROM AUTHOR]

Coulomb interactions among electrons and holes in 2D semimetals with overlapping valence and conduction bands can give rise to a correlated insulating ground state via exciton formation and condensation. One candidate material in which such excitonic state uniquely combines with non-trivial band topology are atomic monolayers of tungsten ditelluride (WTe 2 ), in which a 2D topological excitonic insulator (2D TEI) forms. However, the detailed mechanism of the 2D bulk gap formation in WTe 2 , in particular with regard to the role of Coulomb interactions, has remained a subject of ongoing debate. Here, it shows that WTe 2 is susceptible to a gate-tunable quantum phase transition, evident from an abrupt collapse of its 2D bulk energy gap upon ambipolar field-effect doping. Such gate tunability of a 2D TEI, into either n- and p-type semimetals, promises novel handles of control over non-trivial 2D superconductivity with excitonic pairing., (© 2023 Wiley-VCH GmbH.)

Thin films of rare-earth monopnictide (RE-V) semimetals are expected to turn into semiconductors due to quantum confinement effects (QCE), lifting the overlap between electron pockets at Brillouin zone edges (X) and hole pockets at the zone center (Γ). Instead, using LaSb as an example, we find the emergence of the quantum spin Hall (QSH) insulator phase in (001)-oriented films as the thickness is reduced to 7, 5, or 3 monolayers (MLs). This is attributed to a strong QCE on the in-plane electron pockets and the lack of quantum confinement on the out-of-plane pocket projected onto the zone center, resulting in a band inversion. Spin-orbit coupling (SOC) opens a sizable nontrivial gap in the band structure of ultrathin films. Such effect is anticipated to be general in rare-earth monopnictides and may lead to interesting phenomena when coupled with the 4 f magnetic moments present in other members of this family of materials.

Topological insulators (TIs) are promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, currently realized two-dimensional (2D) TIs, quantum spin Hall (QSH) insulators, suffer from ultrahigh vacuum and extremely low temperature. Thus, seeking for desirable QSH insulators with high feasibility of experimental preparation and large nontrivial gap is of great importance for wide applications in spintronics. On the basis of the first-principles calculations, we predict a novel family of 2D QSH insulators in transition-metal halide MX (M = Zr, Hf; X = Cl, Br, and I) monolayers, especially, which is the first case based on transition-metal halide-based QSH insulators. MX family has the large nontrivial gaps of 0.12-0.4 eV, comparable with bismuth (111) bilayer (0.2 eV), stanene (0.3 eV), and larger than ZrTe5 (0.1 eV) monolayers and graphene-based sandwiched heterstructures (30-70 meV). Their corresponding 3D bulk materials are weak topological insulators from stacking QSH layers, and some of bulk compounds have already been synthesized in experiment. The mechanism for 2D QSH effect in this system originates from a novel d-d band inversion, significantly different from conventional band inversion between s-p, p-p, or d-p orbitals. The realization of pure layered MX monolayers may be prepared by exfoliation from their 3D bulk phases, thus holding great promise for nanoscale device applications and stimulating further efforts on transition metal-based QSH materials. [ABSTRACT FROM AUTHOR]

Based on first-principles calculations, we propose one new category of two-dimensional topological insulators (2D TIs) in chemically functionalized (-CH _3 and -OH) arsenene films. The results show that the surface decorated arsenene (AsCH _3 and AsOH) films are intrinsic 2D TIs with sizeable bulk gap. The bulk energy gaps are 0.184 eV, and 0.304 eV in AsCH _3 and AsOH films, respectively. Such large bulk gaps make them suitable to realize quantum spin Hall effect in an experimentally accessible temperature regime. Topologically helical edge states in these systems are desirable for dissipationless transport. Moreover, we find that the topological properties in these systems are robust against mechanical deformation by exerting biaxial strain. These novel 2D TIs with large bulk gaps are potential candidate in future electronic devices with ultralow dissipation.

Topological edge states at the boundary of quantum spin Hall (QSH) insulators hold great promise for dissipationless electron transport. The device application of topological edge states has several critical requirements for QSH insulator materials, e.g. a large band gap, appropriate insulating substrates, and multiple conducting channels. In this paper, based on first-principles calculations, we show that Bi _4 Br _4 is a suitable candidate. Single-layer Bi _4 Br _4 was recently demonstrated to be a QSH insulator with sizable gap. Here we find that, in multilayer systems, both the band gaps and low-energy electronic structures are only slightly affected by the interlayer coupling. On the intrinsic insulating substrate of bulk Bi _4 Br _4 , the single-layer Bi _4 Br _4 preserves its topological edge states well. Moreover, at the boundary of multilayer Bi _4 Br _4 , the topological edge states stemming from different single-layers are weakly coupled, and can be fully decoupled by constructing a stair-stepped edge. The decoupled topological edge states are very suitable for multi-channel dissipationless transport. Our work indicates that an ideal QSH insulator can be prepared by nano-fabricaton on the cleaved surface of layered Bi _4 Br _4 single crystal.

Although Pb harbors a strong spin-orbit coupling effect, pristine plumbene (the last group-IV cousin of graphene) hosts topologically trivial states. Based on first-principles calculations, we demonstrate that epitaxial growth of plumbene on the BaTe(111) surface converts the trivial Pb lattice into a quantum spin Hall (QSH) phase with a large gap of ∼0.3 eV via a selective substrate-orbital-filtering effect. Tight-binding model analyses show the p z orbital in half of the Pb overlayer is selectively removed by the BaTe substrate, leaving behind a p z - p x , y band inversion. Based on the same working principle, the gap can be further increased to ∼0.5-0.6 eV by surface adsorption of H or halogen atoms that filters out the other half of the Pb p z orbitals. The mechanism of selective substrate-orbital-filtering is general, opening an avenue to explore large-gap QSH insulators in heavy-metal-based materials. It is worth noting that plumbene has already been widely grown on various substrates experimentally.

Although the 1T ' phase is rare in the transition metal dichalcogenides (TMDCs) family, it has attracted rapid growing research interest due to the coexistence of superconductivity, unsaturated magneto-resistance, topological phases etc. Among them, the quantum spin Hall (QSH) state in monolayer 1T ' -TMDCs is especially interesting because of its unique van der Waals crystal structure, bringing advantages in the fundamental research and application. For example, the van der Waals two-dimensional (2D) layer is vital in building novel functional vertical heterostructure. The monolayer 1T ' -TMDCs has become one of the widely studied QSH insulator. In this review, we review the recent progress in fabrications of monolayer 1T ' -TMDCs and evidence that establishes it as QSH insulator., (© 2020 IOP Publishing Ltd.)

Both spin and orbital degrees of freedom contribute to the magnetic moment of isolated atoms. However, when inserted in crystals, atomic orbital moments are quenched because of the lack of rotational symmetry that protects them when isolated. Thus, the dominant contribution to the magnetization of magnetic materials comes from electronic spin. Here we show that nanoislands of quantum spin Hall insulators can host robust orbital edge magnetism whenever their highest occupied Kramers doublet is singly occupied, upgrading the spin edge current into a charge current. The resulting orbital magnetization scales linearly with size, outweighing the spin contribution for islands of a few nm in size. This linear scaling is specific of the Dirac edge states and very different from Schrodinger electrons in quantum rings. By modeling Bi(111) flakes, whose edge states have been recently observed, we show that orbital magnetization is robust with respect to disorder, thermal agitation, shape of the island, and crystallographic direction of the edges, reflecting its topological protection.

Quantum spin Hall (QSH) effect is promising for achieving dissipationless transport devices but presently is achieved only at extremely low temperature. Searching for the large-gap QSH insulators with strong spin-orbit coupling (SOC) is the key to increase the operating temperature. We demonstrate theoretically that this can be solved in the chloridized gallium bismuthide (GaBiCl2) monolayer, which has nontrivial gaps of 0.95 eV at the Γ point, and 0.65 eV for bulk, as well as gapless edge states in the nanoribbon structures. The nontrivial gaps due to the band inversion and SOC are robust against external strain. The realization of the GaBiCl2 monolayer will be beneficial for achieving QSH effect and related applications at high temperatures.