Flavour Hund’s coupling, Chern gaps and charge diffusivity in moiré graphene

Park, Jeong Min; Cao, Yuan; Watanabe, Kenji; Taniguchi, Takashi; Jarillo‐Herrero, Pablo

doi:10.1038/s41586-021-03366-w

Cited by 162 publications

(87 citation statements)

References 42 publications

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“…3 do not follow a simple progression with ν and B, reflecting the rich competition between the gapped single-particle Hofstadter subbands and the tendency towards spontaneous flavor polarization into a subset of these bands (see Methods for additional discussion). This is similar to recent observations in tBLG [27][28][29][30][31][32] , however, here we observe a more complicated sequence of symmetry breaking in which various flavor-polarized and unpolarized ground states closely compete. For example, in Fig.…”

Section: Resultssupporting

confidence: 92%

Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

Zhang

et al. 2021

Nat Commun

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Flat band moiré superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin ‘flavor’ symmetries. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system owing to its low crystal symmetry and the tunability of its bandwidth and topology with an external electric field. Here, we find that orbital magnetism is abundant within the correlated phase diagram of tMBG, giving rise to the anomalous Hall effect in correlated metallic states nearby most odd integer fillings of the flat conduction band, as well as correlated Chern insulator states stabilized in an external magnetic field. The behavior of the states at zero field appears to be inconsistent with simple spin and valley polarization for the specific range of twist angles we investigate, and instead may plausibly result from an intervalley coherent (IVC) state with an order parameter that breaks time reversal symmetry. The application of a magnetic field further tunes the competition between correlated states, in some cases driving first-order topological phase transitions. Our results underscore the rich interplay between closely competing correlated ground states in tMBG, with possible implications for probing exotic IVC ordering.

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Section: Resultssupporting

confidence: 92%

Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

Zhang

et al. 2021

Nat Commun

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“…Nevertheless, the sign of the excursion in our measurements, the peak followed by the dip, is opposite to that in previous measurements on the zeroth Landau level in graphene 42 , which requires further theoretical investigation. However, peak-dip patterns with negative compressibility have been seen in the phase transitions between correlated states in recent measurements of the flat-band system twisted bilayer graphene 43 .…”

Section: Resultsmentioning

confidence: 99%

Edge channels of broken-symmetry quantum Hall states in graphene visualized by atomic force microscopy

et al. 2021

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The quantum Hall (QH) effect, a topologically non-trivial quantum phase, expanded the concept of topological order in physics bringing into focus the intimate relation between the “bulk” topology and the edge states. The QH effect in graphene is distinguished by its four-fold degenerate zero energy Landau level (zLL), where the symmetry is broken by electron interactions on top of lattice-scale potentials. However, the broken-symmetry edge states have eluded spatial measurements. In this article, we spatially map the quantum Hall broken-symmetry edge states comprising the graphene zLL at integer filling factors of $${{\nu }}={{0}},\pm {{1}}$$ ν = 0 , ± 1 across the quantum Hall edge boundary using high-resolution atomic force microscopy (AFM) and show a gapped ground state proceeding from the bulk through to the QH edge boundary. Measurements of the chemical potential resolve the energies of the four-fold degenerate zLL as a function of magnetic field and show the interplay of the moiré superlattice potential of the graphene/boron nitride system and spin/valley symmetry-breaking effects in large magnetic fields.

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“…Experiments on different twisted bilayer graphene (TBG) devices, all close to the first magic angle, have produced a broad variety of different low-temperature phase diagrams. For example, at the charge neutrality point (CNP), both semimetallic [1][2][3][4][5][6] and insulating [7][8][9][10][11] states have been observed. The insulating devices are thought to be divided into two groups.…”

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confidence: 99%

“…[9] as well as the jνj ¼ 2 insulators in general, but it cannot explain the semimetals observed in Refs. [1,[3][4][5][6]. Moreover, self-consistent Hartree-Fock (SCHF) predicts a K-IVC gap of ∼20 meV [22], while experiments measure a global transport gap of only ∼1 meV [9].…”

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confidence: 99%

Strain-Induced Quantum Phase Transitions in Magic-Angle Graphene

et al. 2021

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We investigate the effect of uniaxial heterostrain on the interacting phase diagram of magic-angle twisted bilayer graphene. Using both self-consistent Hartree-Fock and density-matrix renormalization group calculations, we find that small strain values (ϵ ∼ 0.1%-0.2%) drive a zero-temperature phase transition between the symmetry-broken "Kramers intervalley-coherent" insulator and a nematic semimetal. The critical strain lies within the range of experimentally observed strain values, and we therefore predict that strain is at least partly responsible for the sample-dependent experimental observations.

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Flavour Hund’s coupling, Chern gaps and charge diffusivity in moiré graphene

Cited by 162 publications

References 42 publications

Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

Edge channels of broken-symmetry quantum Hall states in graphene visualized by atomic force microscopy

Strain-Induced Quantum Phase Transitions in Magic-Angle Graphene

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