Twisted charge-density-wave patterns in bilayer 2D crystals and modulated electronic states

Abstract The twistronics of the atomic-thick two-dimensional (2D) van der Waals materials has opened a new venue to investigate the interlayer coupling. Till now, most studies focus on the twist of atomic lattices and the resulted moiré superstructures, while the reports about the twist of charge density waves (CDW), the superstructures of which are from individual layers are limited. Here, using molecular beam epitaxy, we construct bilayer (BL) 1T-NbSe2 vertical structures. With high resolution scanning tunneling microscopy observations, we identify two cases of CDW twisted stacking with atomic precision. The typical twist angles are 0o and 60o between the 1st and the 2nd layer, although the top Se atomic lattices of these two layers are parallel. Compared to the single layer case, the dI/dV at BL shows an insulator-to-metal transition, with the Hubbard bands shrinking towards the Fermi level (EF ). More intriguingly, interlayer coupling states rise near EF , which are dependent on the CDW twist angles. These findings give fresh insight into the engineering of 2D materials by CDW twisting and are potentially applicable for future nanoelectronic devices.

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Direct identification of Mott Hubbard band pattern beyond charge density wave superlattice in monolayer 1T-NbSe2

Nature Communications ◽

10.1038/s41467-021-22233-w ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Liwei Liu ◽

Han Yang ◽

Yuting Huang ◽

Xuan Song ◽

Quanzhen Zhang ◽

...

Keyword(s):

Charge Density ◽

Density Wave ◽

Charge Density Waves ◽

Scanning Tunneling ◽

Tunneling Microscopy ◽

Hubbard Band ◽

Spin Polarized ◽

Spatially Distributed ◽

Device Applications ◽

The Relationship

AbstractUnderstanding Mott insulators and charge density waves (CDW) is critical for both fundamental physics and future device applications. However, the relationship between these two phenomena remains unclear, particularly in systems close to two-dimensional (2D) limit. In this study, we utilize scanning tunneling microscopy/spectroscopy to investigate monolayer 1T-NbSe2 to elucidate the energy of the Mott upper Hubbard band (UHB), and reveal that the spin-polarized UHB is spatially distributed away from the dz2 orbital at the center of the CDW unit. Moreover, the UHB shows a √3 × √3 R30° periodicity in addition to the typically observed CDW pattern. Furthermore, a pattern similar to the CDW order is visible deep in the Mott gap, exhibiting CDW without contribution of the Mott Hubbard band. Based on these findings in monolayer 1T-NbSe2, we provide novel insights into the relation between the correlated and collective electronic structures in monolayer 2D systems.

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Scanning Tunneling Microscopy on Charge Density Waves in Layered Compounds

MRS Proceedings ◽

10.1557/proc-295-15 ◽

1992 ◽

Vol 295 ◽

Cited By ~ 1

Author(s):

J. Th. M. De Hosson ◽

G. P. E. M. Van Bakel

Keyword(s):

Scanning Tunneling Microscopy ◽

Charge Density ◽

Density Wave ◽

Charge Density Waves ◽

Layered Compounds ◽

Scanning Tunneling ◽

Periodic Lattice ◽

Electronic Charge ◽

Density Waves ◽

Tunneling Microscopy

AbstractDifferent layered transition metal dichalcogenides were subjected to scanning tunneling microscopy to reveal the electronic charge distribution associated with the charge density wave (CDW) part of the superstructure, in addition to the atomic corrugation. The observations presented display three regimes ranging from localized CDW centred around defects/impurities in the case of lT-TiS2, via an intermediate regime governed by overlapping envelope functions in 2H-NbSe2, to a fully developed CDW system in 1T-TaSe2 (as well in a large number of other compounds). The fact that these observations have been made in solids ranging from (dirty) semiconductor (1T-TiS2) to semimetal (1T-TaSe2) to metallic (2H-NbSe2) points at the general applicability of the phenomenological Ginzburg-Landau theory, employed to describe the various regimes in which the formation of charge density waves and the accompanying periodic lattice distortions appear to act.

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Atomic origin of the scanning tunneling microscopy images of charge-density-waves on 1T-TaSe2

Physica B Condensed Matter ◽

10.1016/j.physb.2007.11.026 ◽

2008 ◽

Vol 403 (13-16) ◽

pp. 2207-2210 ◽

Cited By ~ 1

Author(s):

D. Stoltz ◽

M. Bielmann ◽

L. Schlapbach ◽

M. Bovet ◽

H. Berger ◽

...

Keyword(s):

Scanning Tunneling Microscopy ◽

Charge Density ◽

Charge Density Waves ◽

Scanning Tunneling ◽

Density Waves ◽

Tunneling Microscopy ◽

Microscopy Images

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Nanostructured and Modulated Low-Dimensional Systems

Solid State Phenomena ◽

10.4028/www.scientific.net/ssp.203-204.42 ◽

2013 ◽

Vol 203-204 ◽

pp. 42-47

Author(s):

Albert Prodan ◽

Herman J.P. van Midden ◽

Erik Zupanič ◽

Rok Žitko

Keyword(s):

Charge Density ◽

Structural Information ◽

Density Wave ◽

Scanning Tunneling ◽

Tunneling Microscopy ◽

Additional Information ◽

Related Pair ◽

Good Accord ◽

Long Range Ordering ◽

Low Dimensional

Charge density wave (CDW) ordering in NbSe3 and the structurally related quasi one-dimensional compounds is reconsidered. Since the modulated ground state is characterized by unstable nano-domains, the structural information obtained from diffraction experiments is to be supplemented by some additional information from a method, able to reveal details on a unit cell level. Low-temperature (LT) scanning tunneling microscopy (STM) can resolve both, the local atomic structure and the superimposed charge density modulation. It is shown that the established model for NbSe3 with two incommensurate (IC) modes, q1 = (0,0.241,0) and q2 = (0.5,0.260,0.5), locked in at T1=144K and T2=59K and separately confined to two of the three available types of bi-capped trigonal prismatic (BCTP) columns, must be modified. The alternative explanation is based on the existence of modulated layered nano-domains and is in good accord with the available LT STM results. These confirm i.a. the presence of both IC modes above the lower CDW transition temperature. Two BCTP columns, belonging to a symmetry-related pair, are as a rule alternatively modulated by the two modes. Such pairs of columns are ordered into unstable layered nano-domains, whose q1 and q2 sub-layers are easily interchanged. The mutually interchangeable sections of the two unstable IC modes keep a temperature dependent long-range ordering. Both modes can formally be replaced by a single highly inharmonic long-period commensurate CDW.

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