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arXiv · 1903.08685

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·Ashvin Vishwanath
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Abstract

Two monolayers of graphene twisted by a small `magic' angle exhibit nearly flat bands leading to correlated electronic states and superconductivity, whose precise nature including possible broken symmetries, remain under debate. Here we theoretically study a related but different system with reduced symmetry - twisted {\em double} bilayer graphene (TDBLG), consisting of {\em two} Bernal stacked bilayer graphene sheets, 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 $D$. We construct a phase diagram as a function of twist angle and $D$, incorporating interactions via a Hartree-Fock approximation. At half filling, ferromagnetic insulators are stabilized, typically with valley Chern number $C_v=2$. Ferromagnetic fluctuations in the metallic state are argued to lead to spin triplet superconductivity from pairing between electrons in opposite valleys. Response of these states to a magnetic field applied either perpendicular or parallel to the graphene sheets is obtained, and found to compare favorably with a recent experiment. We highlight a novel orbital effect arising from in-plane fields that can exceed the Zeeman effect and plays an important role in interpreting experiments.

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Jong Yeon Lee, Eslam Khalaf, Shang Liu, Xiaomeng Liu, Zeyu Hao, Philip Kim, Ashvin Vishwanath. 2019-03-20. Theory of correlated insulating behaviour and spin-triplet superconductivity in twisted double bilayer graphene. https://doi.org/10.1038/s41467-019-12981-1

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