Room Temperature Quantum Spin Hall Insulators with a Buckled Square Lattice

Luo, Wenhua; Xiang, Hongjun

doi:10.1021/acs.nanolett.5b00418

Cited by 116 publications

(87 citation statements)

References 40 publications

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“…However, the extremely small band gap of graphene makes it very difficult to verify the DTC in this material experimentally [12]. To date, 2D TIs have been identified experimentally in HgTe/CdTe [13] and InAs/GaSb [14] quantum wells at low temperatures, and many 2D TIs with giant band gaps have been predicted to exist as a result of the substrate interaction effect [15], chemical functionalization [16][17][18][19][20], or global structure optimization [21]. In many cases, the complex structures and the lack of mirror symmetry in such materials prevent the formation of a 2D TCI phase.…”

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confidence: 99%

Robust dual topological character with spin-valley polarization in a monolayer of the Dirac semimetal Na3Bi

et al. 2017

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Topological materials with both insulating and semimetal phases can be protected by crystalline (e.g., mirror) symmetry. The insulating phase, called a topological crystalline insulator (TCI), has been investigated intensively and observed in three-dimensional materials. However, the predicted two-dimensional (2D) materials with TCI phase are explored much less than 3D TCIs and 2D topological insulators, while the 2D TCIs considered thus far possess almost exclusively a square-lattice structure with the mirror Chern number C M = −2. Here, we predict theoretically that a hexagonal monolayer of Dirac semimetal Na 3 Bi is a 2D TCI with a mirror Chern number C M = −1. The large nontrivial gap of 0.31 eV is tunable and can be made much larger via strain engineering, while the topological phases are robust against strain, indicating a high possibility for room-temperature observation of quantized conductance. In addition, a nonzero spin Chern number C S = −1 is obtained, indicating the coexistence of a 2D topological insulator and a 2D TCI, i.e., the dual topological character. Remarkably, a spin-valley polarization is revealed in the Na 3 Bi monolayer due to the breaking of crystal inversion symmetry. The dual topological character is further explicitly confirmed via the unusual behavior of the edge states under the corresponding symmetry breaking. DOI: 10.1103/PhysRevB.95.075404The discovery of topological insulators (TIs) [1,2] has triggered an explosion of novel topologically nontrivial phases, such as the topological crystalline insulator (TCI), for which the role of time-reversal symmetry is replaced by crystal (mirror) symmetry [3][4][5]. The hallmark of a TCI, similar to a TI, is the presence of gapless surface/edge states with Dirac points inside of the insulating bulk energy gap. In the presence of crystal mirror symmetry, the coexistence of TI and TCI phases has been predicted in three dimensions for Bi 1−x Sb x [6] and Bi chalcogenides [7][8][9], and thus they exhibit a dual topological character (DTC). Recently, unusual topological surface states for a three-dimensional (3D) DTC system have been observed experimentally [8,9]. In the 2D case, graphene may be a prototypical example of a DTC [10,11]. However, the extremely small band gap of graphene makes it very difficult to verify the DTC in this material experimentally For both 2D TIs and 2D TCIs, spin-orbit coupling (SOC) is known to play a vital role. In addition, SOC together with inversion symmetry breaking can lead to coupled spin and valley physics, in which the new degree of freedom offers a promising route to the eventual realization of valleytronic devices [28,29]. Spin-valley polarization has been observed experimentally in the MoS 2 monolayer [30], which is a topologically trivial insulator. Therefore, a natural question arises as to whether spin-valley polarization in nontrivial insulators, such as 2D TIs and 2D TCIs, is possible. Recently, thin films of the Dirac semimetal Na 3 Bi [31,32] were fabricated by molecular-beam epitaxy [33], and th...

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confidence: 99%

Robust dual topological character with spin-valley polarization in a monolayer of the Dirac semimetal Na3Bi

et al. 2017

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“…A few large-gap QSH insulators have been theoretically proposed [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], predominantly in ultrathin materials of free-standing form. Among them, stanene, the graphene counterpart of tin (Sn), is of special interest [5].…”

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confidence: 99%

“…The combined effects would lead to distinct electronic structures for ultrathin materials like stanene. The important influence of substrate, however, has been largely ignored in previous works [5][6][7][8][9][10][11][12][13][14][15][16]. Realizing large-gap QSH states in ultrathin materials grown on substrate emerges as a challenging issue.…”

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confidence: 99%

Large-gap quantum spin Hall states in decorated stanene grown on a substrate

2015

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Two-dimensional stanene is a promising candidate material for realizing room-temperature quantum spin Hall (QSH) effect. Monolayer stanene has recently been fabricated by molecular beam epitaxy, but shows metallic features on Bi 2 Te 3 (111) substrate, which motivates us to study the important influence of substrate. Based on first-principles calculations, we find that varying substrate conditions considerably tunes electronic properties of stanene. The supported stanene gives either trivial or QSH states, with significant Rashba splitting induced by inversion asymmetry.More importantly, large-gap (up to 0.3 eV) QSH states are realizable when growing stanene on various substrates, like the anion-terminated (111) surfaces of SrTe, PbTe, BaSe and BaTe. These findings provide significant guidance for future research of stanene and large-gap QSH states.

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“…These large-gap 2D TIs include silicene, 9 Bi(1 1 1) bilayers, 10 III-Bi bilayers, 11 BiF 2D crystals, 12 Bi 4 Br 4 single layers, 13 chemically modified Ge/Sn [14][15][16] and Bi/Sb 17,18 honeycomb lattices, ZrTe 5 /HfTe 5 19 and 2D transition metal dichalcogenides. [20][21][22][23] In terms of geometrical motifs, square, hexagonal and pentagonal rings are considered to be the three basic building blocks of 2D materials.…”

Section: Introductionmentioning

confidence: 99%

“…[20][21][22][23] In terms of geometrical motifs, square, hexagonal and pentagonal rings are considered to be the three basic building blocks of 2D materials. However, among all predicted or synthesized 2D TIs, [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] square and hexagonal rings are almost the only building blocks that have been found. Such a limitation mostly arises from the fact that the presence of a gapless band structure in these 2D materials makes them promising for harboring QSH states.…”

Section: Introductionmentioning

confidence: 99%

Room temperature quantum spin Hall states in two-dimensional crystals composed of pentagonal rings and their quantum wells

Kou

et al. 2016

NPG Asia Mater

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Quantum spin Hall (QSH) insulators are a peculiar phase of matter exhibiting excellent quantum transport properties with potential applications in lower-power-consuming electronic devices. Currently, among all predicted or synthesized QSH insulators, square and hexagonal atomic rings are the dominant structural motifs, and QSH insulators composed of pentagonal rings have not yet been reported. Here, based on first-principles calculations, we predict a family of large-gap QSH insulators in SnX 2 (X = S, Se, or Te) two-dimensional (2D) crystals by the direct calculation of Z 2 topological invariants and edge states. Remarkably, in contrast to all known QSH insulators, the QSH insulators predicted here are composed entirely of pentagonal rings. Moreover, these systems can produce sizeable nontrivial gaps ranging from 121 to 224 meV, which is sufficiently large for practical applications at room temperature. Additionally, we propose a quantum well by sandwiching an SnTe 2 2D crystal between two BiOBiS 2 sheets and reveal that the considered 2D crystal remains topologically nontrivial with a sizeable gap. This finding demonstrates the robustness of its band topology against the effect of the substrate and provides a viable method for further experimental studies. The resultant Dirac surface states in three-dimensional (3D) TIs as well as the helical edge states in two-dimensional (2D) TIs are spin locked because of protection by time-reversal symmetry; thus, they are robust against perturbations. Particularly in 2D TIs, also known as quantum spin Hall (QSH) insulators, 3,4 all the low-energy scatterings of the edge states caused by the nonmagnetic defects are completely forbidden because the edge electrons can only propagate along two directions with opposite spins, which makes 2D TIs more suitable for lowpower-consuming applications than 3D TIs. Unfortunately, the material realization of 2D TIs is challenging: while 3D TIs have been discovered in many materials, such as the Bi 2 Se 3 family, 5,6 the experimental realization of 2D TIs is thus far limited to the HgTe/ CdTe 7 and InAs/GaSb 8 quantum wells. Moreover, the QSH effect in these two quantum wells can occur only at ultralow temperature (o10 K) because of their extremely small bulk band gaps, and this limitation greatly obstructs their possible applications. The search for new 2D TIs with large band gaps that could support room temperature applications has thus become critically important.

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Room Temperature Quantum Spin Hall Insulators with a Buckled Square Lattice

Cited by 116 publications

References 40 publications

Robust dual topological character with spin-valley polarization in a monolayer of the Dirac semimetal Na3Bi

Robust dual topological character with spin-valley polarization in a monolayer of the Dirac semimetal Na3Bi

Large-gap quantum spin Hall states in decorated stanene grown on a substrate

Room temperature quantum spin Hall states in two-dimensional crystals composed of pentagonal rings and their quantum wells

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