Direct observation of mono-layer, bi-layer, and tri-layer charge density waves in 1T-TaS2 by transmission electron microscopy without a substrate

Sakabe, D.; Liu, Zheng; Suenaga, Kazu; Nakatsugawa, Keiji; Tanda, Satoshi

doi:10.1038/s41535-017-0025-8

Cited by 35 publications

(30 citation statements)

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“…25 for optically excited hidden CDW states), while the second one (type-2) uses McMillan's original free energy 9 analyzed with the method of Nakanishi-Shiba (see the Method section for more detail). The type-2 free energy is important because ring-like diffuse scattering were not observed for ultra-thin sheet of 1T-TaS 2 including monolayer 22 . Figure 2(b-d) show the free energy in the Q (1) space.…”

Section: Resultsmentioning

confidence: 99%

“…We note that this induced anisotropy model has experimental support. The low temperature CDW phase of a free-standing monolayer 1T-TaS 2 with multiple T domain walls has been observed with scanning transmission electron microscopy 22 . In addition, 1T-TaS 2 with a single domain wall has been observed using scanning tunneling microscopy 36 .…”

Section: Discussionmentioning

confidence: 99%

“…Recent progress in STM technique has deepened the understanding of the CDW states 22,25,29,30 . These materials exhibit various CDW phases which are characterized by domain walls (topological defects).…”

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confidence: 99%

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Multivalley Free Energy Landscape and the Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX2 Compounds

Nakatsugawa

Tanda

Ikeda

2020

Sci Rep

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Ultrathin sheets of transition metal dichalcogenides (MX 2 ) with charge density waves (cDWs) is increasingly gaining interest as a promising candidate for graphene-like devices. Although experimental data including stripe/quasi-stripe structure and hidden states have been reported, the ground state of ultrathin MX 2 compounds and, in particular, the origin of anisotropic (stripe and quasi-stripe) cDW phases is a long-standing problem. Anisotropic cDW phases have been explained by coulomb interaction between domain walls and inter-layer interaction. However, these models assume that anisotropic domain walls can exist in the first place. Here, we report that anisotropic CDW domain walls can appear naturally without assuming anisotropic interactions: We explain the origin of these phases by topological defect theory (line defects in a two-dimensional plane) and interference between harmonics of macroscopic cDW wave functions. We revisit the McMillan-nakanishi-Shiba model for monolayer 1T-taS 2 and 2H-taSe 2 and show that CDWs with wave vectors that are separated by 120° (i.e. the three-fold rotation symmetry of the underlying lattice) contain a free-energy landscape with many local minima. Then, we remove this 120° constraint and show that free energy local minima corresponding to the stripe and quasi-stripe phases appear. our results imply that coulomb interaction between domain walls and inter-layer interaction may be secondary factors for the appearance of stripe and quasi-stripe CDW phases. Furthermore, this model explains our recent experimental result (appearance of the quasi-stripe structure in monolayer 1T-taS 2 ) and can predict new CDW phases, hence it may become the basis to study cDW further. We anticipate our results to be a starting point for further study in two-dimensional physics, such as explanation of "Hidden CDW states", study the interplay between supersolid symmetry and lattice symmetry, and application to other van der Waals structures.Usually, anisotropic structures such as stripe phases can be explained by anisotropic interaction between the constituent atoms, electrons, or liquid crystal polymers 1 . Anisotropic structures in charge density waves (CDWs) have been explained likewise. CDWs are periodic modulations of electric charge density in low-dimensional conductors 2-6 . The stability of CDWs containing stripe domain walls 7,8 was first studied by free energy theories 9,10 . But these theories assumed that stripe domain walls can exist in the first place, probably due to weak computational facilities at that time. The appearance of stripe domain walls and quasi-stripe (triclinic) domain walls 8,11,12 has been explained by Coulomb interaction between domain walls 13 and three-dimensional stacking 14 . However, are these interactions indispensable? It would be ideal to have rich structures with the least amount of interactions.In this article we report that stripe and quasi-stripe CDW domain walls can appear without anisotropic interactions and explain the origin of these phases by topolo...

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Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Multivalley Free Energy Landscape and the Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX2 Compounds

Nakatsugawa

Tanda

Ikeda

2020

Sci Rep

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“…Due to confinement effects, interesting properties are already predicted for the CDW/PLD state, e.g., for single-layer 1T-TaS 2 which obtains a triclinic stripe order or for single-layer 2H-NbSe 2 which shows an enhanced transition temperature. 17,18 So far, there are different qualitative models for the CDW phase transition, e.g., the Fermi surface nesting, 5,19,20 Peierls distortion, 7 and giant Kohn anomaly, 4 but a "coherent and realistic microscopy theory has not yet emerged." 5 For 1T-TaSe 2 , thickness-and temperature-dependent properties have already been reported in the commensurate charge density wave CCDW/PLD state: It is metallic in the bulk structure but insulating in a single-layer, 21,22 and the transition temperature to the commensurate (C)CDW/PLD phase in the bulk structure is 437 K, which is reduced with the decreasing thickness.…”

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Observation of charge density waves in free-standing 1T-TaSe2 monolayers by transmission electron microscopy

Börner

Kinyanjui

Björkman

et al. 2018

Applied Physics Letters

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While bulk 1T-TaSe2 is characterized by a commensurate charge density wave (CCDW) state below 473 K, the stability of the CCDW state in a 1T-TaSe2 monolayer, although theoretically predicted, has not been experimentally confirmed so far. As charge density waves and periodic lattice distortions (PLDs) always come together, we evaluate the PLD in a 1T-TaSe2 monolayer from low-voltage aberration-corrected high-resolution transmission electron microscopy experiments. To prevent fast degradation of 1T-TaSe2 during exposure to the electron-beam, a 1T-TaSe2/graphene heterostructure was prepared. We also perform the image simulations based on atom coordinates obtained using density functional theory calculations. From the agreement between the experimental and simulated images, we confirm the stability of the CCDW/PLD in a monolayer 1T-TaSe2/graphene heterostructure at room temperature in the form of a 13×13 superstructure. At the same time, we find that in comparison to multi-layer structures, the superstructure is less pronounced.

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“…To this end, it will be interesting to look at ultrathin crystals in addition to bulk samples. For example, the √ 13 × √ 13 structure in free-standing trilayer crystals has been reported to be more robust than in the bulk crystal, being stable even at room temperature (22). It will also be interesting to grow atomically thin samples such as monolayers by molecular beam epitaxy (MBE).…”

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confidence: 99%

1T-TaS ₂ as a quantum spin liquid

Law

Lee

2017

Proc. Natl. Acad. Sci. U.S.A.

258

154

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1T-TaS 2 is unique among transition metal dichalcogenides in that it is understood to be a correlation-driven insulator, where the unpaired electron in a 13-site cluster experiences enough correlation to form a Mott insulator. We argue, based on existing data, that this well-known material should be considered as a quantum spin liquid, either a fully gapped Z2 spin liquid or a Dirac spin liquid. We discuss the exotic states that emerge upon doping and propose further experimental probes.spin liquid | Mott insulator | transition metal dichalcogenide T he transition metal dichalcogenide (TMD) is an old subject that has enjoyed a revival recently due to the interests in its topological properties and unusual superconductivity. The layer structure is easy to cleave or intercalate and can exist in singlelayer form (1, 2). This material was studied intensively in the 1970s and 1980s and was considered the prototypical example of a charge density wave (CDW) system (3). Due to imperfect nesting in two dimensions, in most of these materials, the onset of CDW gaps out only part of the Fermi surface, leaving behind a metallic state that often becomes superconducting. Conventional band theory and electron-phonon coupling appear to account for the qualitative behavior (3). There is, however, one notable exception, namely 1T-TaS2. The Ta forms a triangular lattice, sandwiched between two triangular layers of S, forming an ABCtype stacking. As a result, the Ta is surrounded by S, forming an approximate octahedron. In contrast, the 2H-TaS2 forms an ABA-type stacking, and the Ta is surrounded by S, forming a trigonal prism. In a single layer, inversion symmetry is broken in 2H structure. The system is a good metal below the CDW onset around 90 K, and, eventually, the spin-orbit coupling gives rise to a special kind of superconductivity called Ising superconductivity (4-6). In 1T-TaS2, inversion symmetry is preserved. The system undergoes a CDW transition at about 350 K with a jump in the resistivity. It is known that this transition is driven by an incommensurate triple-Q CDW (ICDW). A similar transition is seen in 1T-TaSe2 at 470 K. However, whereas TaSe2 stays metallic below this transition, 1T-TaS2 exhibits a further resistivity jump around 200 K that is hysteretic, indicative of the firstorder nature of this transition. These transitions are also visible in the spin susceptibility data shown in Fig. 2. In early samples, the resistivity rises only by about a factor of 10 as the temperature is lowered from 200 K to 2 K and, below that, obeys Mott hopping law (log resistivity goes as T −1/3 ) (7). More recent samples show better insulating behavior, and it is generally agreed that the ground state is insulating. The 200 K transition is accompanied by a lock-in to a commensurate CDW (CCDW), forming a √ 13 × √ 13 structure. As shown in Fig. 1, this structure is described as clusters of stars of David where the sites of the stars move inward toward the site in the middle. The stars of David are packed in such a way that they form ...

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Direct observation of mono-layer, bi-layer, and tri-layer charge density waves in 1T-TaS2 by transmission electron microscopy without a substrate

Cited by 35 publications

References 39 publications

Multivalley Free Energy Landscape and the Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX2 Compounds

Multivalley Free Energy Landscape and the Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX2 Compounds

Observation of charge density waves in free-standing 1T-TaSe2 monolayers by transmission electron microscopy

1T-TaS ₂ as a quantum spin liquid

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Direct observation of mono-layer, bi-layer, and tri-layer charge density waves in 1T-TaS2 by transmission electron microscopy without a substrate

Cited by 35 publications

References 39 publications

Multivalley Free Energy Landscape and the Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX2 Compounds

Multivalley Free Energy Landscape and the Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX2 Compounds

Observation of charge density waves in free-standing 1T-TaSe2 monolayers by transmission electron microscopy

1T-TaS 2 as a quantum spin liquid

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1T-TaS ₂ as a quantum spin liquid