Strain engineering a 4a×√3a charge-density-wave phase in transition-metal dichalcogenide 1T−VSe2

Abstract

We report a rectangular charge density wave (CDW) phase in strained 1T-VSe2 thin films synthesized by molecular beam epitaxy on c-sapphire substrates. The observed CDW structure exhibits an unconventional rectangular 4a×√3a periodicity, as opposed to the previously reported hexagonal 4a×4a structure in bulk crystals and exfoliated thin layered samples. Tunneling spectroscopy shows a strong modulation of the local density of states of the same 4a×√3a CDW periodicity and an energy gap of 2ΔCDW = (9.1 ± 0.1) meV. The CDW energy gap evolves into a full gap at temperatures below 500 mK, indicating a transition to an insulating phase at ultra-low temperatures. First-principles calculations confirm the stability of both 4a×4a and 4a×√3a structures arising from soft modes in the phonon dispersion. The unconventional structure becomes preferred in the presence of strain, in agreement with experimental findings.

Electronic Structure and Enhanced Charge-Density Wave Order of Monolayer VSe2

“…We assign these as chargeorder peaks indicative of a CDW instability persisting to the monolayer limit of VSe 2 , as further supported below. The location of the new features that we observe in LEED is consistent with the Γ {11} family of charge order spots for an ordering wavevector q CDW = (1/4, 1/4), retaining the in-plane periodicity of the bulk CDW order, 15,30,41 where the Γ {01} spots would be shadowed by the electron gun in our LEED optics. Although this is certainly plausible, further measurements, for example scanning tunnelling microscopy or x-ray scattering, will be required to unambiguously classify q CDW here.…”

Epitaxially grown monolayer VSe 2 : an air-stable magnetic two-dimensional material with low work function at edges

“…S2 online). This kind of excited state is often reported in the TMDC materials that exists at low temperature [36,37]. We further made a STS linear mapping (Fig.…”

Effect of strain on charge density wave order in the Holstein model

“…The application of strain has additional complicating effects, including changes in the phonon spectrum and of the relative placement of different orbitals (energy bands). In particular, 1T -VSe 2 has a transition from hexagonal to rectangular charge order with strain, which seems to originate in the softening of certain phonon modes 16 . The aforementioned DFT investigations have explored many of these details.…”

“…CDW formation on surfaces and in quasi-2D materials have been motivating theoretical studies of the Holstein model in two dimensions. In addition to the choice of the CDW driving interaction (electron-electron-like or electron-phonon-like), lattice geometry plays an arXiv:1905.05831v2 [cond-mat.str-el] 16 May 2019 important role in the presence of charge ordering. For instance, for the Holstein model in a honeycomb lattice, one may show that a finite critical electron-phonon coupling is required for CDW 22,23 , while in the triangular lattice its ground state exhibits SC 24 .…”