Correlated Insulating States in Twisted Double Bilayer Graphene

Burg, G. William; Zhu, Jihang; Taniguchi, Takashi; Watanabe, Kenji; MacDonald, Allan H.; Tutuc, Emanuel

doi:10.1103/physrevlett.123.197702

Cited by 271 publications

(226 citation statements)

References 28 publications

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“…Figure 1D shows a uMIM map of a tDBG sample, where we resolve the moiré period from the contrasts of signal arising from the different local stacking. The measured moiré period of this sample is (10.9 ± 0.7) nm, corresponding to a twist angle of = (1.3 ± 0.1)°, the "magic angle" in tDBG for which field-tunable correlated insulating and superconducting states have been reported (7)(8)(9)(10).…”

Section: Resultsmentioning

confidence: 83%

Ultrahigh-resolution scanning microwave impedance microscopy of moiré lattices and superstructures

et al. 2020

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Two-dimensional heterostructures composed of layers with slightly different lattice vectors exhibit new periodic structure known as moiré lattices, which, in turn, can support novel correlated and topological phenomena. Moreover, moiré superstructures can emerge from multiple misaligned moiré lattices or inhomogeneous strain distributions, offering additional degrees of freedom in tailoring electronic structure. High-resolution imaging of the moiré lattices and superstructures is critical for understanding the emerging physics. Here, we report the imaging of moiré lattices and superstructures in graphene-based samples under ambient conditions using an ultrahigh-resolution implementation of scanning microwave impedance microscopy. Although the probe tip has a gross radius of ~100 nm, spatial resolution better than 5 nm is achieved, which allows direct visualization of the structural details in moiré lattices and the composite super-moiré. We also demonstrate artificial synthesis of novel superstructures, including the Kagome moiré arising from the interplay between different layers.

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Section: Resultsmentioning

confidence: 83%

Ultrahigh-resolution scanning microwave impedance microscopy of moiré lattices and superstructures

et al. 2020

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“…The perpendicular electric field has a profound effect on the band structure of TDBG [7][8][9][10][11] . As depicted in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…2b, we plot the measured nonlocal resistance which is large only at the gaps. Apart from the resistance peak at the gaps, there are other high-resistance regions in the local resistance, characteristics to small-angle TDBG [7][8][9][10][11] . Such examples are the cross-like feature originating at D = 0 in the hole side and the ring-like regions in the electron side for |D|/ϵ 0~0 .3 V nm −1 .…”

Section: Resultsmentioning

confidence: 99%

“…T he advancement in twistronics has opened up new avenues to study electron correlations physics, such as Mott insulator states [1][2][3] , superconductivity 3,4 , and orbital ferromagnetism 5,6 in twisted bilayer graphene (TBG). Recent experiments in twisted double bilayer graphene (TDBG) [7][8][9][10][11] and trilayer graphene aligned to hexagonal boron nitride (hBN) [12][13][14] also reveal correlation effects. While low-energy flat bands enhance electronic correlations [15][16][17] , such moiré systems support topological bands with nonzero valley Chern number 5,6,14,18,19 .…”

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confidence: 99%

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Bulk valley transport and Berry curvature spreading at the edge of flat bands

et al. 2020

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2D materials based superlattices have emerged as a promising platform to modulate band structure and its symmetries. In particular, moiré periodicity in twisted graphene systems produces flat Chern bands. The recent observation of anomalous Hall effect (AHE) and orbital magnetism in twisted bilayer graphene has been associated with spontaneous symmetry breaking of such Chern bands. However, the valley Hall state as a precursor of AHE state, when time-reversal symmetry is still protected, has not been observed. Our work probes this precursor state using the valley Hall effect. We show that broken inversion symmetry in twisted double bilayer graphene (TDBG) facilitates the generation of bulk valley current by reporting experimental evidence of nonlocal transport in a nearly flat band system. Despite the spread of Berry curvature hotspots and reduced quasiparticle velocities of the carriers in these flat bands, we observe large nonlocal voltage several micrometers away from the charge current path — this persists when the Fermi energy lies inside a gap with large Berry curvature. The high sensitivity of the nonlocal voltage to gate tunable carrier density and gap modulating perpendicular electric field makes TDBG an attractive platform for valley-twistronics based on flat bands.

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“…With this we conclude the rigorous arguments showing that the spontaneously polarized Ising Chern magnets are exact unique zero energy ground states of the class of Hamiltonians introduced in Eq. (19). Now, when the intravalley interactions are not δ functions, the Ising magnets (0, N φ ), (N φ , 0) can still be the absolute ground states, even though it is harder to make rigorous statements in this case.…”

Section: Ising Chern Magnets: Ideal Hamiltonians and Exact Groundmentioning

confidence: 99%

Excitonic Laughlin states in ideal topological insulator flat bands and their possible presence in moiré superlattice materials

Stefanidis

Sodemann

2020

Phys. Rev. B

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We investigate few-and many-body states in half-filled ideal topological insulator flat bands realized by two degenerate Landau levels which experience opposite magnetic fields. This serves as a toy model of flat bands in moiré materials in which valleys have Chern numbers C = ±1. We argue that although the spontaneously polarized Ising Chern magnet is a natural ground state for repulsive Coulomb interactions, it can be in reasonable energetic competition with correlated Laughlin states of excitons when short-distance corrections to interactions are included. This is because charge neutral excitons in these bands behave effectively as charged particles in ordinary Landau levels. In particular, the Ising Chern magnet is no longer the ground state once the strength of a short-range intravalley repulsion is about 30% larger than the intervalley repulsion. Remarkably, these excitonic Laughlin states feature valley number fractionalization but no charge fractionalization and a quantized charge Hall conductivity identical to the Ising magnet, σ xy = ±e 2 /h, and thus cannot be distinguished from it by ordinary charge transport measurements. The Laughlin state with the highest density of excitons that can be constructed in these bands is an analog of ν = 1/4 bosonic Laughlin state and has no valley polarization even though it spontaneously breaks time reversal symmetry.

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Correlated Insulating States in Twisted Double Bilayer Graphene

Cited by 271 publications

References 28 publications

Ultrahigh-resolution scanning microwave impedance microscopy of moiré lattices and superstructures

Ultrahigh-resolution scanning microwave impedance microscopy of moiré lattices and superstructures

Bulk valley transport and Berry curvature spreading at the edge of flat bands

Excitonic Laughlin states in ideal topological insulator flat bands and their possible presence in moiré superlattice materials

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