Emerging topological states in quasi‐two‐dimensional materials

Huang, Huaqing; Xu, Yong; Wang, Jianfeng; Duan, Wenhui

doi:10.1002/wcms.1296

Cited by 30 publications

(21 citation statements)

References 163 publications

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“…Besides the above quasi‐1D structures, 2D materials, especially topological materials, are of great interest . However, for 2D devices with sizes exceeding the phonon mean free path, electron‐phonon scattering should be taken into consideration.…”

Section: Caloritronic Devicesmentioning

confidence: 97%

Thermal Engineering in Low‐Dimensional Quantum Devices: A Tutorial Review of Nonequilibrium Green's Function Methods

2018

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Thermal engineering of quantum devices has attracted much attention since the discovery of quantized thermal conductance of phonons. Although easily submerged in numerous excitations in macrosystems, quantum behaviors of phonons manifest in nanoscale low‐dimensional systems even at room temperature. Especially in nanotransport devices, phonons move quasiballistically when the transport length is smaller than their bulk mean free paths. It has been shown that the phonon nonequilibrium Green's function method (NEGF) is effective for the investigation of nanoscale quantum transport of phonons. Here, two aspects of thermal engineering of quantum devices are discussed using NEGF methods. One covers transport properties of pure phonons; the other concerns caloritronic effects, which manipulate other degrees of freedom, such as charge, spin, and valley, via the temperature gradient. For each part, the basic theoretical formalisms are outlined first, then a survey of related investigations on models or realistic materials is presented. Particular attention is given to phonon topologies and the generalized phonon NEGF method. Finally, conclusions are offered and the study is summarized with an outlook.

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Section: Caloritronic Devicesmentioning

confidence: 97%

Thermal Engineering in Low‐Dimensional Quantum Devices: A Tutorial Review of Nonequilibrium Green's Function Methods

2018

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“…However, the presence of SOC will open a global bulk gap at the Dirac points and introduce the topologically protected gapless edge states. [13][14][15] Therefore, 2D DMs are formally a quantum spin Hall insulator 13 under time-reversal symmetry [ Fig. 1(a)] or quantum anomalous Hall insulator 12 with time-reversal breaking [ Fig.…”

Section: Introductionmentioning

confidence: 99%

Two-dimensional spin–valley-coupled Dirac semimetals in functionalized SbAs monolayers

et al. 2019

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In the presence of spin-orbit coupling (SOC), achieving both spin and valley polarized Dirac state is significant to promote the fantastic integration of Dirac physics, spintronics and valleytronics. Based on ab initio calculations, here we demonstrate that a class of spin-valley-coupled Dirac semimetals (svc-DSMs) in the functionalized SbAs monolayers (MLs) can host such desired state. Distinguished from the graphene-like 2D Dirac materials, the Dirac cones in svc-DSMs hold giant spin-splitting induced by strong SOC under inversion symmetry breaking. In the 2.3% strained SbAsH 2 ML, the Dirac fermions in inequivalent valleys have opposite Berry curvature and spin moment, giving rise to Dirac spin-valley Hall effect with constant spin Hall conductivity as well as massless and dissipationless transport. Topological analysis reveals that the svc-DSM emerges at the boundary between trivial and 2D topological insulators, which provides a promising platform for realizing the flexible and controllable tuning among different quantum states.The rise of graphene 1-3 has inspired significant efforts in searching for other 2D Dirac materials (DMs) 4-10 with linear energy dispersion. As the host of massless Dirac fermions, 2D DMs have become the playground for investigating many quantum relativistic phenomena [11][12][13] in the emerging field of Dirac physics. Like graphene, without considering spin-orbit coupling (SOC), the Dirac points in these 2D DMs are protected by symmetry. However, the presence of SOC will open a global bulk gap at the Dirac points and introduce the topologically protected gapless edge states. [13][14][15] Therefore, 2D DMs are formally a quantum spin Hall insulator 13 under time-reversal symmetry [ Fig. 1(a)] or quantum anomalous Hall insulator 12 with time-reversal breaking [ Fig. 1(b)]. In a pioneering theoretical study, Yong and Kane 16 employed symmetry analysis and a two-site tight-binding model to examine the possibility that the Dirac points can not be gapped by SOC [ Fig. 1(c)]. They concluded that nonsymmorphic space group symmetry plays an essential role in protecting the 2D Dirac points against SOC, named as spin-orbit Dirac points (SDPs). Although the first realistic 2D material hosting SDPs was predicted recently 17 , its SDPs are not located at the Fermi level and its Dirac dispersion is contaminated by some extraneous non-Dirac bands. Hence, how to achieve a 2D Dirac semimetal hosting SDPs with clean Dirac bands at the Fermi level remains a great challenge.On the other hand, the discovery of valley-dependent effects in MoS 2 monolayer 18,19 without inversion symmetry aroused an upsurge in the field of 2D valleytronics. 20 For hexagonal 2D materials such as graphene and monolayer group-VI transition metal dichalcogenides, the conically shaped valleys at +K and -K 4 FIG. 1. Schematic electronic band structures without SOC (left) and with SOC (right) for different 2D Dirac materials: (a) quantum spin Hall insulator, (b) quantum anomalous Hall insulator, (c) symmetry-protected Dirac semim...

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“…One of the essential conditions to achieve the QSH state is the band inversion between conduction and valence states, which enables the TPT between a normal insulator (NI) and a QSH insulator. Typically, one can realize the band inversion by tuning the on-site energy difference = s − p , the SOC strength λ, and the bonding strength γ [14,15]. According to the universal linear scaling of TPT we derived recently [18], the critical transition point is roughly determined by the condition…”

Section: Topological Phase Diagrammentioning

confidence: 99%

“…On the other hand, quantum spin Hall (QSH) states have been studied in various theoretical models and realistic materials in recent years [12][13][14][15]. The QSH state is manifested by an insulating bulk and topologically protected metallic edges with quantized conductance.…”

Section: Introductionmentioning

confidence: 99%

Comparison of quantum spin Hall states in quasicrystals and crystals

Huang

Liu

2019

Phys. Rev. B

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We theoretically study the quantum spin Hall states in an Ammann-Beenker-type octagonal quasicrystal and a periodic snub-square crystal, both sharing the same basic building blocks. Although the bulk states show significant differences in localization and transport properties, the topological phases manifest similarly in the two systems. This indicates the robustness of the topological properties regardless of symmetry and periodicity. We characterize the topological nature of the two systems with a nonzero topological invariant (spin Bott index B s and Z 2 invariant), robust metallic edge states, and quantized conductance. In spite of some quantitative differences, the topological phase diagram of the two systems also exhibits similar behaviors, indicating that the topological phase transition is mainly determined by similar interactions in the two systems regardless of their structural difference. This is also reflected by the observation that the transition point between the normal insulator and the quantum spin Hall state in both systems follows a universal linear scaling relation for topological phase transitions.

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Emerging topological states in quasi‐two‐dimensional materials

Cited by 30 publications

References 163 publications

Thermal Engineering in Low‐Dimensional Quantum Devices: A Tutorial Review of Nonequilibrium Green's Function Methods

Thermal Engineering in Low‐Dimensional Quantum Devices: A Tutorial Review of Nonequilibrium Green's Function Methods

Two-dimensional spin–valley-coupled Dirac semimetals in functionalized SbAs monolayers

Comparison of quantum spin Hall states in quasicrystals and crystals

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