Understanding topological phase transition in monolayer transition metal dichalcogenides

Choe, Duk‐Hyun; Sung, Ha-Jun; Chang, Kee Joo

doi:10.1103/physrevb.93.125109

Cited by 99 publications

(99 citation statements)

References 48 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%

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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%

“…The lattice distortion from the 1T phase to the 1T phase induces band inversion and causes 1T -WTe 2 to become topologically nontrivial 10,19,20 . Figure 1b schematically summarizes this topological phase transition in 1T -WTe 2 .…”

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confidence: 99%

Quantum spin Hall state in monolayer 1T'-WTe2

et al. 2017

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“…2c). A similar theoretical analysis for WTe 2 shows that the relevant reverse-ordered states near the zone center are both dominated by the W 5 d states 30 . The Te states play a relatively minor role.…”

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confidence: 56%

Large quantum-spin-Hall gap in single-layer 1T′ WSe2

et al. 2018

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Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum-spin-Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of a quasi-freestanding WSe2 single layer with the 1T′ structure that does not exist in the bulk form of WSe2. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we observe a gap of 129 meV in the 1T′ layer and an in-gap edge state located near the layer boundary. The system′s 2D TI characters are confirmed by first-principles calculations. The observed gap diminishes with doping by Rb adsorption, ultimately leading to an insulator–semimetal transition. The discovery of this large-gap 2D TI with a tunable band gap opens up opportunities for developing advanced nanoscale systems and quantum devices.

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“…WTe 2 , however, does not show any transition and stays at the γ-phase 21,22 . Since the structural parameters of a single layer of 1T ′ -MoTe 2 and 1T ′ -WTe 2 are almost the same 15,17,23 and Mo and W belong to the same group in the periodic table, the different phase transition behaviors are intriguing and origins of the contrasting features are yet to be clarified. To understand the phase transition, the proper treatment of long and short range interlayer interaction in TMDs is essential.…”

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confidence: 99%

Origins of the structural phase transitions in MoTe2 and WTe2

et al. 2017

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Layered transition metal dichalcogenides MoTe2 and WTe2 share almost similar lattice constants as well as topological electronic properties except their structural phase transitions. While the former shows a first-order phase transition between monoclinic and orthorhombic structures, the latter does not. Using a recently proposed van der Waals density functional method, we investigate structural stability of the two materials and uncover that the disparate phase transitions originate from delicate differences between their interlayer bonding states near the Fermi energy. By exploiting the relation between the structural phase transitions and the low energy electronic properties, we show that a charge doping can control the transition substantially, thereby suggesting a way to stabilize or to eliminate their topological electronic energy bands. Because of the layered structures of TMDs, several polymorphs can exist and show characteristic physical properties depending on their structures 8 . A typical TMD shows the trigonal prismatic (2H) or the octahedral (1T ) structures 9-12 . For MoTe 2 and WTe 2 , the 2H structure (α-phase, P 6 3 /mmc) is a stable semiconductor while the 1T form is unstable 7,13 . The unstable 1T structure turns into the distorted octahedral one (1T ′ ) 7,14 . The stacked 1T ′ single layer forms a threedimensional bulk with the monoclinic structure (β-phase, P 2 1 /m) or the orthorhombic one (γ-phase, P mn2 1 ) (see Fig. 1) [15][16][17] . Interestingly, the β phase with a few layers is a potential candidate of QSH insulator 7 and the bulk γ phase shows type-II Weyl semimetalic energy bands 4,18,19 , respectively. Since the structural differences between β and γ phases are minute (∼4• tilting of axis along out-of-plane direction in β phase with respect to one in γ phase), the sensitive change in their topological low energy electronic properties is remarkable and the transition between different structures can lead to alternation of topological properties of the system.A phase transition between the β-and γ-phase in the layered TMDs has been known for a long time 16,20 . MoTe 2 shows a first-order transition from the β-to γ-structure at around 250 K 20 when temperature decreases. WTe 2 , however, does not show any transition and stays at the γ-phase 21,22 . Since the structural parameters of a single layer of 1T ′ -MoTe 2 and 1T ′ -WTe 2 are almost the same 15,17,23 and Mo and W belong to the same group in the periodic table, the different phase transition behaviors are intriguing and origins of the contrasting features are yet to be clarified. To understand the phase transition, the proper treatment of long and short range interlayer interaction in TMDs is essential. Most of the theoretical studies, however, fail to reproduce the experimental crystal structures of the two phases of MoTe 2 and WTe 2 so do their topological electronic structures using crystal structures obtained from ab initio calculations [24][25][26][27][28][29][30] . Instead, the atomic structures from experiment data are r...

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Understanding topological phase transition in monolayer transition metal dichalcogenides

Cited by 99 publications

References 48 publications

Quantum spin Hall state in monolayer 1T'-WTe2

Quantum spin Hall state in monolayer 1T'-WTe2

Large quantum-spin-Hall gap in single-layer 1T′ WSe2

Origins of the structural phase transitions in MoTe2 and WTe2

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