van der Waals Heterostructures with High Accuracy Rotational Alignment

Kim, Kyounghwan; Yankowitz, Matthew; Fallahazad, Babak; Kang, Sangwoo; Movva, Hema C. P.; Huang, Sunxiang; Larentis, Stefano; Corbet, Chris M.; Taniguchi, Takashi; Watanabe, Kenji; LeRoy, Brian J.; Tutuc, Emanuel

doi:10.1021/acs.nanolett.5b05263

Cited by 591 publications

(527 citation statements)

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“…Our STA bilayer graphene samples are fabricated by sequential graphene and hexagonal boron-nitride (hBN) flake pick-up steps using a hemispherical handle substrate (16) that allows an individual flake to be detached from a substrate while leaving flakes in its immediate proximity intact. To realize STA bilayer graphene, we start with a single graphene flake and split it into two separate sections (Fig.…”

Section: Methodsmentioning

confidence: 99%

Tunable moiré bands and strong correlations in small-twist-angle bilayer graphene

Kim

DaSilva

Huang

et al. 2017

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

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541

408

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According to electronic structure theory, bilayer graphene is expected to have anomalous electronic properties when it has long-period moiré patterns produced by small misalignments between its individual layer honeycomb lattices. We have realized bilayer graphene moiré crystals with accurately controlled twist angles smaller than 1°and studied their properties using scanning probe microscopy and electron transport. We observe conductivity minima at charge neutrality, satellite gaps that appear at anomalous carrier densities for twist angles smaller than 1°, and tunneling densities-of-states that are strongly dependent on carrier density. These features are robust up to large transverse electric fields. In perpendicular magnetic fields, we observe the emergence of a Hofstadter butterfly in the energy spectrum, with fourfold degenerate Landau levels, and broken symmetry quantum Hall states at filling factors ±1, 2, 3. These observations demonstrate that at small twist angles, the electronic properties of bilayer graphene moiré crystals are strongly altered by electron-electron interactions.moiré crystal | graphene | twisted bilayer | moiré band | Hofstadter butterfly M oiré patterns form when nearly identical two-dimensional (2D) crystals are overlaid with a small relative twist angle (1-4). The electronic properties of moiré crystals depend sensitively on the ratio of the interlayer hybridization strength, which is independent of twist angle, to the band energy shifts produced by momentum space rotation (5-12). In bilayer graphene, this ratio is small when twist angles exceed about 2°(10, 13), allowing moiré crystal electronic structure to be easily understood using perturbation theory (5). At smaller twist angles, electronic properties become increasingly complex. Theory (14, 15) has predicted that extremely flat bands appear at a series of magic angles, the largest of which is close to 1°. Flat bands in 2D electron systems, for example the Landau level bands that appear in the presence of external magnetic fields, allow for physical properties that are dominated by electron-electron interactions, and have been friendly territory for the discovery of fundamentally new states of matter. Here we report transport and scanning probe microscopy (SPM) studies of bilayer graphene moiré crystals with carefully controlled small-twist angles (STA), below 1°. We find that conductivity minima emerge in transport at neutrality, and at anomalous satellite densities that correspond to ±8 additional electrons in the moiré crystal unit cell, and that the conductivity minimum at neutrality is not weakened by a transverse electric field applied between the layers. Our observations can be explained only by strong electronic correlations. MethodsOur STA bilayer graphene samples are fabricated by sequential graphene and hexagonal boron-nitride (hBN) flake pick-up steps using a hemispherical handle substrate (16) that allows an individual flake to be detached from a substrate while leaving flakes in its immediate proximity intact. To...

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Section: Methodsmentioning

confidence: 99%

Tunable moiré bands and strong correlations in small-twist-angle bilayer graphene

Kim

DaSilva

Huang

et al. 2017

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

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541

408

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“…A local metallic bottom gate is used to screen the charge impurities present in the silicon oxide substrate, and onedimensional edge contacts are used to contact the TBLG [33]. A "tear-and-stack" technique was also developed to enable subdegree control of the twist angle [25,34]. We used an ab initio tight-binding model for the calculation of band structures and related quantities [20,25].…”

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confidence: 99%

Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene

et al. 2016

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Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moiré physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moiré wave vector.

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“…1(b) the optical micrograph of an hBN encapsulated WSe 2 sample with bottom Pt contacts, and separate local top-and back-gates. The mono-and bilayer WSe 2 Hall bar samples were fabricated using a modified van der Waals assembly technique [13,14]. The bottom Pt electrodes in combination with a large, negative topgate bias (V T G ) ensure Ohmic hole contacts to the WSe 2 [10,13].…”

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Density-Dependent Quantum Hall States and Zeeman Splitting in Monolayer and Bilayer WSe2

et al. 2017

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We report a study of the quantum Hall states (QHSs) sequence of holes in mono-and bilayer WSe2. The QHSs sequence transitions between predominantly even and predominantly odd filling factors as the hole density is tuned in the range 1.6 − 12 × 10 12 cm −2 . The QHSs sequence is insensitive to the transverse electric field, and tilted magnetic field measurements reveal an insensitivity of the QHSs sequence to the in-plane magnetic field, evincing that the hole spin is locked perpendicular to the WSe2 plane. These observations imply that the QHSs sequence is controlled by the Zeemanto-cyclotron energy ratio, which remains constant as a function of perpendicular magnetic field at a fixed carrier density, but changes as a function of density due to strong electron-electron interaction.The strong spin-orbit coupling and broken inversion symmetry in 2H transition metal dichalcogenide (TMD) monolayers leads to coupled spin and valley degrees of freedom [1]. Breaking the time reversal symmetry by applying a perpendicular magnetic field further lifts the valley degeneracy, thanks to the spin (valley) Zeeman effect [2,3]. Insights into the Zeeman effect, a fundamental property of TMDs, have been provided by magnetooptical measurements of TMD monolayers, which report the exciton g-factors from luminescence shifts in perpendicular magnetic fields [4,5]. While magnetotransport measurements have been traditionally used to determine the effective g-factor (g * ) in several two-dimensional electron systems (2DESs) [6][7][8], the lack of reliable low temperature Ohmic contacts, combined with a moderate mobility, have hampered a similar progress in TMDs. Recent advances in sample fabrication have now facilitated more detailed studies of the electron physics in TMDs [9][10][11]. Tungsten diselenide (WSe 2 ) is of particular interest because of a large spin-orbit splitting in the valence band [12], high-mobility [10], and low temperature Ohmic contacts [13].In this work, we report on the magnetotransport of holes in mono-and bilayer WSe 2 in the quantum Hall regime. An examination of the Shubnikov-de Haas (SdH) oscillations, and the quantum Hall states (QHSs) sequence reveals interesting carrier density-dependent transitions between predominantly even and predominantly odd filling factors (FFs) as the hole density is tuned. Measurements in tilted magnetic fields reveal an insensitivity of the QHSs sequence to the in-plane magnetic field, indicating that the hole spin is locked perpendicular to the WSe 2 plane. The QHSs sequence is also found to be insensitive to the applied transverse electric field. These observations can be explained by a Zeeman-tocyclotron energy ratio which remains constant as a function of perpendicular magnetic field at a fixed carrier density, but changes as a function of density because of strong electron-electron interaction.Figure 1(a) shows the schematic cross section, and Fig. 1(b) the optical micrograph of an hBN encapsulated WSe 2 sample with bottom Pt contacts, and separate local top-and back-gates. ...

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van der Waals Heterostructures with High Accuracy Rotational Alignment

Cited by 591 publications

References 34 publications

Tunable moiré bands and strong correlations in small-twist-angle bilayer graphene

Tunable moiré bands and strong correlations in small-twist-angle bilayer graphene

Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene

Density-Dependent Quantum Hall States and Zeeman Splitting in Monolayer and Bilayer WSe2

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