Correlated states in twisted double bilayer graphene

Shen, Cheng; Chu, Yanbang; Wu, Quansheng; Li, Na; Wang, Shuopei; Zhao, Yanchong; Tang, Jian; Liu, Jieying; Tian, Jinpeng; Watanabe, Kenji; Taniguchi, Takashi; Yang, Rong; Meng, Zi Yang; Shi, Dongxia; Yazyev, Oleg V.; Zhang, Guangyu

doi:10.1038/s41567-020-0825-9

Cited by 483 publications

(368 citation statements)

References 38 publications

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“…From a quantum engineering point of view, applying a perpendicular bias between layers [18] provides a versatile way of tuning correlated states in twisted graphene multilayers. This has been demonstrated in paradigmatic examples of correlated states in twisted tetralayers (double bilayers) [19,20] and twisted trilayers (monolayers/bilayers) [16,17]. Moreover, interlayer bias is known to generate internal valley currents in twisted graphene bilayers [5,6,21,22], creating topological networks at low angles [5,6,21] and generating valley fluxes in flat band regimes [22].…”

Section: Introductionmentioning

confidence: 99%

Electrical band flattening, valley flux, and superconductivity in twisted trilayer graphene

López-Bezanilla

Lado

2020

Phys. Rev. Research

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Twisted graphene multilayers have been demonstrated to yield a versatile playground to engineer controllable electronic states. Here, by combining first-principles calculations and low-energy models, we demonstrate that twisted graphene trilayers provide a tunable system where Van Hove singularities can be controlled electrically. In particular, it is shown that besides the band flattening, bulk valley currents appear, which can be quenched by local chemical dopants. We finally show that in the presence of electronic interactions, a nonuniform superfluid density emerges whose nonuniformity gives rise to spectroscopic signatures in dispersive higher-energy bands. Our results put forward twisted trilayers as a tunable van der Waals heterostructure displaying electrically controllable flat bands and bulk valley currents.

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Section: Introductionmentioning

confidence: 99%

Electrical band flattening, valley flux, and superconductivity in twisted trilayer graphene

López-Bezanilla

Lado

2020

Phys. Rev. Research

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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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“…Recently a different platform for the appearance of extremely flat bands with nontrivial topology has emerged in the form of moiré superlattices of two-dimensional materials. Following the prediction of the appearance of flat bands at small twist-angle in graphene moiré superlattices [4,5], experiments have observed a variety of superconducting and correlated insulating states in these [6][7][8][9][10][11][12][13][14] and other moiré superlattice materials [15][16][17][18][19][20][21][22]. The mechanisms behind these phenomena have been strongly debated [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38].…”

Section: Introductionmentioning

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 states in twisted double bilayer graphene

Cited by 483 publications

References 38 publications

Electrical band flattening, valley flux, and superconductivity in twisted trilayer graphene

Electrical band flattening, valley flux, and superconductivity in twisted trilayer graphene

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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