Local spectroscopy of moiré-induced electronic structure in gate-tunable twisted bilayer graphene

Wong, Dillon; Wang, Yang; Jung, Jeil; Pezzini, Sergio; DaSilva, Ashley; Tsai, Hsin‐Zon; Jung, Han Sae; Khajeh, Ramin; Kim, Youngkyou; Lee, Juwon; Kahn, Salman; Tollabimazraehno, Sajjad; Rasool, Haider I.; Watanabe, Kenji; Taniguchi, Takashi; Zettl, A.; Adam, Shaffique; MacDonald, Allan H.; Crommie, Michael F.

doi:10.1103/physrevb.92.155409

Cited by 136 publications

(116 citation statements)

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“…Because we expect a strong dependence of electronic properties on twist angle, it is critically important that the angle achieved by the rotation process be directly measured. To this end, we use scanning tunneling microscopy and spectroscopy to examine the topography of the longwavelength moiré patterns and their local electronic properties (11,(24)(25)(26)(27). The SPM samples are realized using the techniques of Fig.…”

Section: Resultsmentioning

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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527

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: Resultsmentioning

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.

Self Cite

527

408

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“…These hybridizations at the overlap of the Dirac cones produce the VHS 28 , which have already been investigated by experimental STM measurements [29][30][31][32] .…”

Section: Bilayer Graphenementioning

confidence: 99%

Twistronics: Manipulating the electronic properties of two-dimensional layered structures through their twist angle

et al. 2017

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The ability in experiments to control the relative twist angle between successive layers in twodimensional (2D) materials offers a new approach to manipulating their electronic properties; we refer to this approach as "twistronics". A major challenge to theory is that, for arbitrary twist angles, the resulting structure involves incommensurate (aperiodic) 2D lattices. Here, we present a general method for the calculation of the electronic density of states of aperiodic 2D layered materials, using parameter-free hamiltonians derived from ab initio density-functional theory. We use graphene, a semimetal, and MoS2, a representative of the transition metal dichalcogenide (TMDC) family of 2D semiconductors, to illustrate the application of our method, which enables fast and efficient simulation of multi-layered stacks in the presence of local disorder and external fields. We comment on the interesting features of their Density of States (DoS) as a function of twist-angle and local configuration and on how these features can be experimentally observed.

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“…In fact, signatures of TIBB are observed for the lightly hole-doped oxychloride Ca 2 CuO 2 Cl 2 [3], and poor electronic screening effects around charged impurities are observed for Fe dopants in the topological insulator Bi 2 Se 3 [17], for Co adatoms in graphene [18], and possibly for chiral defects in Sr 3 Ir 2 O 7 [19]. TIBB has also been discussed for twodimensional (2D) transition metal dichalcogenides [20] and for graphene systems [21]. We expect that TIBB could affect measurements or could even be used for gating of topical materials with poor electronic screening, including iron-based superconductors, transition metal dichalcogenides, van der Waals heterostructures, or new topological materials.…”

Section: Introductionmentioning

confidence: 99%

Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy

et al. 2017

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The effective Mott gap measured by scanning tunneling microscopy (STM) in the lightly doped Mott insulator (Sr 1−x La x ) 2 IrO 4 differs greatly from values reported by photoemission and optical experiments. Here we show that this is a consequence of the poor electronic screening of the tip-induced electric field in this material. Such effects are well known from STM experiments on semiconductors and go under the name of tip-induced band bending (TIBB). We show that this phenomenon also exists in the lightly doped Mott insulator (Sr 1−x La x ) 2 IrO 4 and that, at doping concentrations of x 4%, it causes the measured energy gap in the sample density of states to be bigger than the one measured with other techniques. We develop a model able to retrieve the intrinsic energy gap leading to a value which is in rough agreement with other experiments, bridging the apparent contradiction.At doping x ≈ 5% we further observe circular features in the conductance layers that point to the emergence of a significant density of free carriers in this doping range and to the presence of a small concentration of donor atoms. We illustrate the importance of considering the presence of TIBB when doing STM experiments on correlated-electron systems and discuss the similarities and differences between STM measurements on semiconductors and lightly doped Mott insulators.

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Local spectroscopy of moiré-induced electronic structure in gate-tunable twisted bilayer graphene

Cited by 136 publications

References 50 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

Twistronics: Manipulating the electronic properties of two-dimensional layered structures through their twist angle

Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy

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