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

Jong Yeon Lee, Eslam Khalaf, Shang Liu, Xiaomeng Liu, Zeyu Hao, Philip Kim and Ashvin Vishwanath ()
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Jong Yeon Lee: Harvard University
Eslam Khalaf: Harvard University
Shang Liu: Harvard University
Xiaomeng Liu: Harvard University
Zeyu Hao: Harvard University
Philip Kim: Harvard University
Ashvin Vishwanath: Harvard University

Nature Communications, 2019, vol. 10, issue 1, 1-10

Abstract: Abstract 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 $${C}_{{\rm{v}}}=\pm 2$$Cv=±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.

Date: 2019
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DOI: 10.1038/s41467-019-12981-1

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