Electrical switching of magnetic order in an orbital Chern insulator

Polshyn, Hryhoriy; Zhu, Jihang; Kumar, Manish; Zhang, Y.; Yang, Fangyuan; Tschirhart, Charles; Serlin, Marec; Watanabe, Kenji; Taniguchi, Takashi; MacDonald, Allan H.; Young, Andrea

doi:10.1038/s41586-020-2963-8

Cited by 259 publications

(239 citation statements)

References 34 publications

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“…A new era of band engineering and novel electronic phase design is promised by the discovery of moiré superlattices in 2d van der Waals heterostructures. Recent experiments on graphene-based moiré systems [1][2][3][4][5][6][7][8][9][10][11][12][13] unfolded a brand new world of strongly interacting electron phases, including correlated insulating states, unconventional superconductivity as well as quantum anomalous hall states. Moiré superlattices based on transition metal dichalcogenides (TMD) [14][15][16][17][18][19] lately emerge as a new host of novel electronic phases of matter.…”

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confidence: 99%

Excitonic density wave and spin-valley superfluid in bilayer transition metal dichalcogenide

2021

Nat Commun

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Artificial moiré superlattices in 2d van der Waals heterostructures are a new venue for realizing and controlling correlated electronic phenomena. Recently, twisted bilayer WSe2 emerged as a new robust moiré system hosting a correlated insulator at moiré half-filling over a range of twist angle. In this work, we present a theory of this insulating state as an excitonic density wave due to intervalley electron–hole pairing. We show that exciton condensation is strongly enhanced by a van Hove singularity near the Fermi level. Our theory explains the remarkable sensitivity of the insulating gap to the vertical electric field. In contrast, the gap is weakly reduced by a perpendicular magnetic field, with quadratic dependence at low field. The different responses to electric and magnetic field can be understood in terms of pair-breaking versus non-pair-breaking effects in a BCS analog of the system. We further predict superfluid spin transport in this electrical insulator, which can be detected by optical spin injection and spatial-temporal imaging.

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confidence: 99%

Excitonic density wave and spin-valley superfluid in bilayer transition metal dichalcogenide

2021

Nat Commun

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“…In magicangle twisted bilayer graphene (tBLG), certain IVC states have been proposed as the parent ground state out of which superconductivity emerges 10,12,13 , although direct experimental identification of the ground state order is challenging. Twisted monolayer-bilayer graphene (tMBG) features lower crystal symmetry, and consequentially the lattice degeneracy is strongly lifted at the single-particle level in an external displacement field, D. The lowest moiré conduction band has four remaining degenerate copies corresponding to spin and valley, and is flat enough to host correlated states over a small range of twist angles [14][15][16] . The bandwidth, W, and valley Chern number, C v , additionally depend on the orientation of D (Supplementary Fig.…”

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confidence: 99%

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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“… 60 , 61 , 62 It will be intriguing to investigate the UMR effect in the presence of superconductivity for TBG. Furthermore, in experiment magic-angle TBG can exhibit orbital ferromagnetism 6 , 7 , 8 , 14 , 63 , 64 , 65 due to the electron-electron interaction. 30 , 56 , 66 , 67 , 68 , 69 , 70 We point out that the orbital magnetism also leads to UMR where it plays the same role as the magnetic field.…”

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confidence: 97%

Chirality-Induced Giant Unidirectional Magnetoresistance in Twisted Bilayer Graphene

2021

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Summary Twisted bilayer graphene (TBG) exhibits fascinating correlation-driven phenomena like the superconductivity and Mott insulating state, with flat bands and a chiral lattice structure. We find by quantum-transport calculations that the chirality leads to a giant unidirectional magnetoresistance (UMR) in TBG, where the unidirectionality refers to the resistance change under the reversal of the direction of current or magnetic field. We point out that flat bands significantly enhance this effect. The UMR increases quickly upon reducing the twist angle, and reaches about 20% for an angle of 1.5° in a 10 T in-plane magnetic field. We propose the band structure topology (asymmetry), which leads to a direction-sensitive mean free path, as a useful way to anticipate the UMR effect. The UMR provides a probe for chirality and band flatness in the twisted bilayers.

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Electrical switching of magnetic order in an orbital Chern insulator

Cited by 259 publications

References 34 publications

Excitonic density wave and spin-valley superfluid in bilayer transition metal dichalcogenide

Excitonic density wave and spin-valley superfluid in bilayer transition metal dichalcogenide

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

Chirality-Induced Giant Unidirectional Magnetoresistance in Twisted Bilayer Graphene

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