Theory of correlated insulating behaviour and spin-triplet superconductivity in twisted double bilayer graphene

Two graphene monolayers twisted by a small magic angle exhibit nearly flat bands, leading to correlated electronic states. Here we study a related but different system with reduced symmetry - twisted double bilayer graphene (TDBG), consisting of two Bernal stacked bilayer graphenes, twisted with res...

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Bibliographic Details
Published in:Nature communications Vol. 10; no. 1; pp. 5333 - 10
Main Authors: Lee, Jong Yeon, Khalaf, Eslam, Liu, Shang, Liu, Xiaomeng, Hao, Zeyu, Kim, Philip, Vishwanath, Ashvin
Format: Journal Article
Language:English
Published: London Nature Publishing Group UK 25.11.2019
Nature Publishing Group
Nature Portfolio
Subjects:
639/301/119/997
639/766/119/1003
639/766/119/2793
Bilayers
Correlation analysis
Electron states
Ferromagnetism
Graphene
Hartree approximation
Humanities and Social Sciences
Insulators
Monolayers
multidisciplinary
Phase diagrams
Science
Science (multidisciplinary)
Superconductivity
Variation
ISSN:2041-1723, 2041-1723
Online Access:Get full text
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Summary:Two graphene monolayers twisted by a small magic angle exhibit nearly flat bands, leading to correlated electronic states. Here we study a related but different system with reduced symmetry - twisted double bilayer graphene (TDBG), consisting of two Bernal stacked bilayer graphenes, twisted with respect to one another. Unlike the monolayer case, we show that isolated flat bands only appear on application of a vertical displacement field. We construct a phase diagram as a function of twist angle and displacement field, incorporating interactions via a Hartree-Fock approximation. At half-filling, ferromagnetic insulators are stabilized with valley Chern number C v = ± 2 . Upon doping, ferromagnetic fluctuations are argued to lead to spin-triplet superconductivity from pairing between opposite valleys. We highlight a novel orbital effect arising from in-plane fields plays an important role in interpreting experiments. Combined with recent experimental findings, our results establish TDBG as a tunable platform to realize rare phases in conventional solids. Twisted bilayer graphene exhibits correlated electronic phases and superconductivity, but its precise nature is under debate. Here, Lee and Khalaf et al. study a twisted double bilayer graphene, where ferromagnetic insulator and spin triplet superconducting phases can be stabilized.
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ISSN:2041-1723
2041-1723
DOI:10.1038/s41467-019-12981-1