Evidence of charge density wave with anisotropic gap in a monolayer 
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 film

Wang, Yuan; Ren, Junhai; Li, Jiaheng; Wang, Yujia; Peng, Huining; Yu, Pu; Duan, Wenhui; Zhou, Shuyun

doi:10.1103/physrevb.100.241404

Cited by 58 publications

(77 citation statements)

References 35 publications

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“…In contrast, the calculations show that the VBM is dominated by iodine states in both materials 43 , 48 . We note that in some other vanadium based vdW systems, such as VSe and VTe , the less-dispersive V 3 d states are also at the VBM, while the strongly dispersive Se 4 p and Te 5 p states are just below it, similar to our results from VI 54 , 55 . Similarly, the iodine-derived bands associated with VI in the VI /CrI heterostructure 33 have their maxima at and then disperse downward in both - M and - K directions, resembling the states just below the flat band in our experiment.…”

Section: Resultssupporting

confidence: 89%

Valence band electronic structure of the van der Waals ferromagnetic insulators: VI$$_3$$ and CrI$$_3$$

Kundu

Liu²,

Petrović³

et al. 2020

Sci Rep

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Ferromagnetic van der Waals (vdW) insulators are of great scientific interest for their promising applications in spintronics. It has been indicated that in the two materials within this class, CrI$$_3$$ 3 and VI$$_3$$ 3 , the magnetic ground state, the band gap, and the Fermi level could be manipulated by varying the layer thickness, strain or doping. To understand how these factors impact the properties, a detailed understanding of the electronic structure would be required. However, the experimental studies of the electronic structure of these materials are still very sparse. Here, we present the detailed electronic structure of CrI$$_3$$ 3 and VI$$_3$$ 3 measured by angle-resolved photoemission spectroscopy (ARPES). Our results show a band-gap of the order of 1 eV, sharply contrasting some theoretical predictions such as Dirac half-metallicity and metallic phases, indicating that the intra-atomic interaction parameter (U) and spin-orbit coupling (SOC) were not properly accounted for in the calculations. We also find significant differences in the electronic properties of these two materials, in spite of similarities in their crystal structure. In CrI$$_3$$ 3 , the valence band maximum is dominated by the I 5p, whereas in VI$$_3$$ 3 it is dominated by the V 3d derived states. Our results represent valuable input for further improvements in the theoretical modeling of these systems.

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Section: Resultssupporting

confidence: 89%

Valence band electronic structure of the van der Waals ferromagnetic insulators: VI$$_3$$ and CrI$$_3$$

Kundu

Liu²,

Petrović³

et al. 2020

Sci Rep

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“…Nevertheless, when the sample was in-situ heated to 140 K, the CDW structures were still observed in the STM experiments. It implies the CDW transition temperature lying in the range from 140 K to 300 K, consistent with the previously reported value 39 .…”

Section: / 18supporting

confidence: 92%

Multimorphism and gap opening of charge-density-wave phases in monolayer VTe2

Liu

et al. 2020

Nano Res.

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Vanadium dichalcogenides have attracted increasing interests for the charge density wave phenomena and possible ferromagnetism. Here, we report on the multiphase behavior and gap opening in monolayer VTe2 grown by molecular beam epitaxy. Scanning tunneling microscopy (STM) and spectroscopy study revealed the (4×4) metallic and gapped (2√3×2√3) charge-density wave (CDW) phases with an energy gap of ~40 meV. Through the in-plane condensation of vanadium atoms, the typical star-of-David clusters and truncated triangle-shaped clusters are formed in the (4×4) and (2√3×2√3) phases respectively, resulting in different surface morphologies and electronic structures as confirmed by density functional theory (DFT) calculations with on-site Coulomb repulsion. The CDW-driven reorganization of the atomic structure weakens the ferromagnetic superexchange coupling and strengthens the antiferromagnetic exchange coupling on the contrary, suppressing the long-range magnetic order in monolayer VTe2.The electron correlation is found to be important to explain the gap opening in the (2√3×2√3) phase. / 18Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are recently attracting great attention for their diverse physical properties such as ferromagnetism 1,2 , Mott-insulator phase 3,4 , quantum Hall effect 5-7 , superconductivity and charge density wave (CDW) [8][9][10][11] . CDW is a periodic symmetry-lowering redistribution of charge in a material, accompanied with periodic modulation of the atomic lattice and electronic densities. The CDW transition in the group VB dichalcogenides, such as VS2, VSe2, NbSe2, TaS2, TaSe2 and TaTe2, has been extensively studied [12][13][14][15][16][17][18][19][20] . The phase transition exhibits a layer-dependent behavior, for instance, VSe2 and NbSe2 have increased CDW transition temperatures as the thicknesses reduce down to a few monolayers. In monolayer VSe2 and NbSe2, the TCDW can be enhanced to 140 K and 145 K compared to 110 K and 33 K of the bulk TCDW respectively [21][22][23][24] . The intricate interplay between CDW and other collective electronic states such as superconductivity has also been explored in NbSe2 and TaS2 25,26 . While the driving mechanisms of CDW in twodimensional TMDs consisting of Fermi surface nesting 27,28 , saddle point singularities 29,30 and electron-phonon coupling [31][32][33][34] have been proposed, a consistent picture on the CDW formation is still to emerge.As for VTe2, CDW phase transition has been observed in VTe2 thin films as the change of resistance at 240 K and 135 K, which can be driven by an electric field due to local Joule heating 35 . When the thickness reduces from multilayer to single layer, the absence of interlayer interaction result in the structural transition from a monoclinic distorted 1Tʹ structure to a hexagonal 1T structure 36 . The 1Tʹ structure has a (3×1) double zigzag chain-like modulation of V atoms, which arise from the in-plane metal coordination through d orbital hybridization. As the temperature is cooled below TCDW, the mo...

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“…The experimental band structure of CrTe 2 is in sharp contrast with the band structure calculated without the inclusion of spin polarization (Supplementary Fig. 15), where hole pockets near E F are degenerate at Г point as in the cases of VTe 2 59 and VSe 2 18,19 . There are no exchange splitting of band dispersion in MBE grown VSe 2 films, indicating the absence of ferromagnetism 18,19 .…”

Section: Resultsmentioning

confidence: 78%

Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films

et al. 2021

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While the discovery of two-dimensional (2D) magnets opens the door for fundamental physics and next-generation spintronics, it is technically challenging to achieve the room-temperature ferromagnetic (FM) order in a way compatible with potential device applications. Here, we report the growth and properties of single- and few-layer CrTe2, a van der Waals (vdW) material, on bilayer graphene by molecular beam epitaxy (MBE). Intrinsic ferromagnetism with a Curie temperature (TC) up to 300 K, an atomic magnetic moment of ~0.21 $${\mu }_{{\rm{B}}}$$ μ B /Cr and perpendicular magnetic anisotropy (PMA) constant (Ku) of 4.89 × 105 erg/cm3 at room temperature in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device and X-ray magnetic circular dichroism. This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with ~0.2 eV at Г point using angle-resolved photoemission spectroscopy. The FM order is preserved with the film thickness down to a monolayer (TC ~ 200 K), benefiting from the strong PMA and weak interlayer coupling. The successful MBE growth of 2D FM CrTe2 films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices.

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Evidence of charge density wave with anisotropic gap in a monolayer VTe2 film

Cited by 58 publications

References 35 publications

Valence band electronic structure of the van der Waals ferromagnetic insulators: VI$$_3$$ and CrI$$_3$$

Valence band electronic structure of the van der Waals ferromagnetic insulators: VI$$_3$$ and CrI$$_3$$

Multimorphism and gap opening of charge-density-wave phases in monolayer VTe2

Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films

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