On the Quantum Spin Hall Gap of Monolayer 1T′‐WTe<sub>2</sub>

Zheng, Feipeng; Cai, Changsi; Ge, Shaofeng; Zhang, Xuefeng; Liu, Xin; Lu, Hong; Zhang, Yudao; Qiu, Jun; Taniguchi, Takashi; Watanabe, Kenji; Jia, Shuang; Qi, Jingshan; Chen, Jianhao; Sun, Dong; Feng, Ji

doi:10.1002/adma.201600100

Cited by 166 publications

(167 citation statements)

References 40 publications

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“…1c-e, which is generally consistent with the literature 10,20,23 . The key bands for the band inversion with opposite parities are marked to track their evolution.…”

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confidence: 91%

“…We note here that different calculation methods give different estimates on the size of the bandgap for strain-free 1T -WTe 2 monolayers. The generalizedgradient approximation (Perdew-Burke-Ernzerhof, PBE) usually underestimates the bandgap and gives a negative bandgap value 10 , while PBE with hybrid function (HSE06) gives a positive value 23 . Figure 2c is an atomically resolved scanning tunnelling microscopy (STM) image of 1T -WTe 2 , from which a ∼7.5…”

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Quantum spin Hall state in monolayer 1T'-WTe2

et al. 2017

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A quantum spin Hall (QSH) insulator is a novel twodimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin-orbit coupling 1,2 . By investigating the electronic structure of epitaxially grown monolayer 1T'-WTe 2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulk bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe 2 as a new class of QSH insulator with large bandgap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).A two-dimensional (2D) topological insulator (TI), or a quantum spin Hall insulator, is characterized by an insulating bulk and a conductive helical edge state, in which carriers with different spins counter-propagate to realize a geometry-independent edge conductance 2e 2 /h (refs 1,2). The only scattering channel for such helical edge current is back scattering, which is prohibited by time reversal symmetry, making QSH insulators a promising material candidate for spintronic and other applications.The prediction of the QSH effect in HgTe quantum wells sparked intense research efforts to realize the QSH state [3][4][5][6][7][8][9][10][11] . So far only a handful of QSH systems have been fabricated, mostly limited to quantum well structures of three-dimensional (3D) semiconductors such as HgTe/CdTe (ref.3) and InAs/GaSb (ref. 6). Edge conduction consistent with a QSH state has been observed 3,6,12 . However, the behaviour under a magnetic field, where time reversal symmetry is broken, cannot be explained within our current understanding of the QSH effect 13,14 . There have been continued efforts to predict and investigate other material systems to further advance the understanding of this novel quantum phenomenon 5,[7][8][9]15 . So far, it has been difficult to make a robust 2D material with a QSH state, a platform needed for widespread study and application. The small bandgaps exhibited by many candidate systems, as well as their vulnerability to strain, chemical adsorption, and element substitution, make them impractical for advanced spectroscopic studies or applications. For example, a QSH insulator candidate stanene, a monolayer analogue of graphene for tin, grown on Bi 2 Se 3 becomes topologically trivial due to the modification of its band structure by the underlying substrate 11,16 . Free-standing Bi film with 2D bonding on a cleaved surface has shown edge conduction 9 , but its topological nature is still debated 17 . It takes 3D out-of-plane bonding with the substrate and large stra...

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“…1c-e, which is generally consistent with the literature 10,20,23 . The key bands for the band inversion with opposite parities are marked to track their evolution.…”

supporting

confidence: 91%

mentioning

confidence: 99%

Quantum spin Hall state in monolayer 1T'-WTe2

et al. 2017

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“…Recent transport measurements indicate that the helical conduction is indeed present on the edge of monolayer WTe 2 , but surprisingly, its interior is insulating without applying an external tensile strain [24], which is consistent with a 2D topological insulator. However, even before the monolayer limit is reached, little is known about the evolution of the electronic structure of WTe 2 as the samples are made thinner and cross over to the 2D regime [24,25].…”

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Thickness-dependent electronic structure in WTe2 thin films

Xiang

Srinivasan

et al. 2018

Phys. Rev. B

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X. (2018). Thickness-dependent electronic structure in WTe2 thin films. Physical Review B: Covering condensed matter and materials physics, 98 (3), 035115-1-035115-10. Thickness-dependent electronic structure in WTe2 thin films AbstractWe study the electronic structure of WTe2 thin films with different thicknesses. High-quality thin-film samples are obtained with carrier mobility up to 5000 cm2 V−1 s−1, which enables us to resolve the four main Fermi pockets from Shubnikov-de Haas (SdH) oscillations. Angle-resolved SdH oscillations show that the WTe2 thin films cross from three-dimensional to two-dimensional electronic systems at a thickness of ∼ 20 nm. Using the field effect, the nature of the Fermi pockets in thin-film WTe2 is identified, and the evolution of SdH oscillation frequencies is traced over different sample thicknesses. It is found that the frequencies dramatically decrease at a thickness of approximately 12 nm, which indicates the onset of finite-size effects on the band structure. Our work pins down two critical length scales of the thickness-dependent electronic structure in WTe2 thin films. Disciplines Engineering | Physical Sciences and Mathematics Publication DetailsXiang, F., Srinivasan, A., Du, Z. Z., Klochan, O., Dou, S., Hamilton, A. We study the electronic structure of WTe 2 thin films with different thicknesses. High-quality thin-film samples are obtained with carrier mobility up to 5000 cm 2 V −1 s −1 , which enables us to resolve the four main Fermi pockets from Shubnikov-de Haas (SdH) oscillations. Angle-resolved SdH oscillations show that the WTe 2 thin films cross from three-dimensional to two-dimensional electronic systems at a thickness of ∼ 20 nm. Using the field effect, the nature of the Fermi pockets in thin-film WTe 2 is identified, and the evolution of SdH oscillation frequencies is traced over different sample thicknesses. It is found that the frequencies dramatically decrease at a thickness of approximately 12 nm, which indicates the onset of finite-size effects on the band structure. Our work pins down two critical length scales of the thickness-dependent electronic structure in WTe 2 thin films.

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“…Recently, WTe 2 , a typical kind of transition metal dichalcogenides (TMDs), has attracted considerable attention and initiated intensive study, since it manifests many novel and intriguing physical properties including extremely large magnetoresistance (XMR) [1], type II Weyl Fermion [2][3][4][5][6], pressure-induced superconductivity [7,8], two-dimensional topological insulator in monolayer [9], and so on [10][11][12][13][14][15]. In WTe 2 , the magnetoresistance is reported to reach an order of at least 10 5 %∼10 6 % at low temperature and remains quadratic up to a field of 60 Tesla with no indication of saturation.…”

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Evidence of Electron-Hole Imbalance in WTe ₂ from High-Resolution Angle-Resolved Photoemission Spectroscopy

Wang¹,

Zhang²,

Huang³

et al. 2017

Chinese Phys. Lett.

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WTe 2 has attracted a great deal of attention because it exhibits extremely large and nonsaturating magnetoresistance. The underlying origin of such a giant magnetoresistance is still under debate. Utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we reveal the complete electronic structure of WTe 2 . This makes it possible to determine accurately the electron and hole concentrations and their temperature dependence. We find that, with increasing the temperature, the overall electron concentration increases while the total hole concentration decreases. It indicates that the electron-hole compensation, if it exists, can only occur in a narrow temperature range, and in most of the temperature range there is an electron-hole imbalance. Our results are not consistent with the perfect electron-hole compensation picture that is commonly considered to be the cause of the unusual magnetoresistance in WTe 2 . We identified a flat band near the Brillouin zone center that is close to the Fermi level and exhibits a pronounced temperature dependence. Such a flat band can play an important role in dictating the transport properties of WTe 2 . Our results provide new insight on understanding the origin of the unusual magnetoresistance in WTe 2 .

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On the Quantum Spin Hall Gap of Monolayer 1T′‐WTe₂

Cited by 166 publications

References 40 publications

Quantum spin Hall state in monolayer 1T'-WTe2

Quantum spin Hall state in monolayer 1T'-WTe2

Thickness-dependent electronic structure in WTe2 thin films

Evidence of Electron-Hole Imbalance in WTe ₂ from High-Resolution Angle-Resolved Photoemission Spectroscopy

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On the Quantum Spin Hall Gap of Monolayer 1T′‐WTe2

Cited by 166 publications

References 40 publications

Quantum spin Hall state in monolayer 1T'-WTe2

Quantum spin Hall state in monolayer 1T'-WTe2

Thickness-dependent electronic structure in WTe2 thin films

Evidence of Electron-Hole Imbalance in WTe 2 from High-Resolution Angle-Resolved Photoemission Spectroscopy

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On the Quantum Spin Hall Gap of Monolayer 1T′‐WTe₂

Evidence of Electron-Hole Imbalance in WTe ₂ from High-Resolution Angle-Resolved Photoemission Spectroscopy