Imaging orbital ferromagnetism in a moiré Chern insulator

Tschirhart, Charles; Serlin, Marec; Polshyn, Hryhoriy; Shragai, Avi; Xia, Zhonghao; Zhu, Jihang; Zhang, Y.; Watanabe, Kenji; Taniguchi, Takashi; Huber, M. E.; Young, Andrea

doi:10.1126/science.abd3190

Cited by 136 publications

(97 citation statements)

References 56 publications

(77 reference statements)

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“…In case of graphene, the superconductivity likely arises due to the entanglement of finite-orbital-momentum electron states on Moire sites. Moreover, ferromagnetism observed in this system in immediate vicinity, in the parameter space, to superconductivity [15] can be understood as a consequence of orbital correlations similar to those described by Eq. ( 2), but with a triplet instead of the singlet spin component, which is stabilized by the Hund's interaction of an electron on a Moire lattice cite with another electron that virtually hops directly from the neighboring site.…”

mentioning

confidence: 77%

Orbital entanglement mechanism of superconductivity in cuprates

Mitrofanov¹,

Urazhdin²

2021

Preprint

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We utilize a 1d Hubbard model to show that the superconductivity in cuprate superconductors likely arises due to the orbital entanglement between holes in the copper oxide plane mediated by orbitally-selective charge hopping. The main role of doping required to achieve superconductivity in cuprates is to suppress the Mott correlations and orbital ordering. The proposed mechanism explains superconductivity in other unconventional superconductors, and provides guidance in the search for new high-temperature superconductors.

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confidence: 77%

Orbital entanglement mechanism of superconductivity in cuprates

Mitrofanov¹,

Urazhdin²

2021

Preprint

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“…The presence of spin textures about the QAHI at ν = +3 could be detected by measuring the degradation of magnetization for small dopings, e.g. through NMR measurements 100 , NV center magnetometry, or using a superconducting quantum interference device (SQUID) 13 . Individual spin skyrmions involve a large number of spin flips, and the spin texture lattices of Fig.…”

Section: Detecting Skyrmionsmentioning

confidence: 99%

“…12 are close to completely spin unpolarized. However there is an orbital contribution from the spontaneous Chern polarization which is likely larger 13 . A more direct probe would be spin-resolved STM near impurity sites that could pin a localized skyrmion.…”

Section: Detecting Skyrmionsmentioning

confidence: 99%

“…Correlated insulating phases arise as a result of symmetry breaking within this (almost) degenerate subspace, in a manner analogous to the formation of quantum Hall ferromagnetism in integrally-filled LLs 10 . This perspective receives support from the detection of a quantized anomalous Hall insulator (QAHI) at ν = +3 [11][12][13] and various magnetic-field stabilized Chern insulators at other fillings 4,[14][15][16][17][18][19] . While a purely strong-coupling treatment does not completely resolve the experimental puzzles surrounding TBG, it serves as a natural starting point for considering how various physically realistic corrections influence the competition between ordered states.…”

Section: Introductionmentioning

confidence: 99%

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Skyrmions in twisted bilayer graphene: stability, pairing, and crystallization

Kwan¹,

Wagner²,

Bultinck³

et al. 2021

Preprint

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We study the excitations that emerge upon doping the translationally-invariant correlated insulating states in magic angle twisted bilayer graphene at various integer filling factors ν. We identify parameter regimes where these are associated with skyrmion textures in the spin or pseudospin degrees of freedom, and explore both short-distance pairing effects and the formation of long-range ordered skyrmion crystals. We perform a comprehensive analysis of the pseudospin skyrmions that emerge upon doping insulators at even ν, delineating the regime in parameter space where these are the lowest-energy charged excitations by means of self-consistent Hartree-Fock calculations on the interacting Bistritzer-MacDonald model. We explicitly demonstrate the purely electron-mediated pairing of skyrmions, a key ingredient behind a recent proposal of skyrmion superconductivity. Building upon this, we construct hopping models to extract the effective masses of paired skyrmions, and discuss our findings and their implications for skyrmion superconductivity in relation to experiments, focusing on the dome-shaped dependence of the transition temperature on the twist angle. We also investigate the properties of spin skyrmions about the quantized anomalous Hall insulator at ν = +3. In both cases, we demonstrate the formation of robust spin/pseudospin skyrmion crystals upon doping to a finite density away from integer filling.

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“…Introduction. Near the first magic angle [1][2][3], twisted bilayer graphene (TBG) features a wealth of superconducting and correlated insulating phases, as seen in both transport [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and spectroscopy [19][20][21][22][23][24][25][26][27][28] experiments, sparking considerable theoretical effort towards their understanding . Owing to nearly dispersionless active bands, the physics of the system near an integer filling ν was argued to be in the strong coupling regime i.e.…”

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confidence: 99%

Spectroscopy of Twisted Bilayer Graphene Correlated Insulators

Călugăru,

Regnault,

et al. 2021

Preprint

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We calculate STM signatures of correlated ground-states at integer filling of the magic angle twisted bilayer graphene narrow bands. First, we compute the fully-interacting TBG spectral function at ±4 electrons/moiré unit cell and show that it can be used to experimentally validate the strong-coupling approach. Although variation exists in the data, we find experimental evidence for the strong-coupling regime. For all other integer fillings of the flat bands, we consider the spatial features of the corresponding spectral functions of many states in the large degenerate ground-state manifold, and assess the possibility of Kekulé distortion (KD) emerging at the graphene lattice scale. Remarkably, we find that coupling the two opposite graphene valleys in the intervalleycoherent (IVC) TBG insulators does not always result in KD. As an example, we show that the K-IVC state and its nonchiral U (4) rotations do not exhibit any KD, while T-IVC does. We analyze 14 different many-body correlated states and show that their combined STM/Chern number signal can be used to uniquely determine the nature of the many-body ground-state. Their STM signal and features are obtained over a large range of energies and model parameters.

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Imaging orbital ferromagnetism in a moiré Chern insulator

Cited by 136 publications

References 56 publications

Orbital entanglement mechanism of superconductivity in cuprates

Orbital entanglement mechanism of superconductivity in cuprates

Skyrmions in twisted bilayer graphene: stability, pairing, and crystallization

Spectroscopy of Twisted Bilayer Graphene Correlated Insulators

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