Superconductors, orbital magnets and correlated states in magic-angle bilayer graphene

Xiaobo Lu, Petr Stepanov, Wei Yang, Ming Xie, Mohammed Ali Aamir, Ipsita Das, Carles Urgell, Kenji Watanabe, Takashi Taniguchi, Guangyu Zhang, Adrian Bachtold, Allan H. MacDonald and Dmitri K. Efetov ()
Additional contact information
Xiaobo Lu: The Barcelona Institute of Science and Technology
Petr Stepanov: The Barcelona Institute of Science and Technology
Wei Yang: The Barcelona Institute of Science and Technology
Ming Xie: University of Texas at Austin
Mohammed Ali Aamir: The Barcelona Institute of Science and Technology
Ipsita Das: The Barcelona Institute of Science and Technology
Carles Urgell: The Barcelona Institute of Science and Technology
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Guangyu Zhang: Chinese Academy of Sciences
Adrian Bachtold: The Barcelona Institute of Science and Technology
Allan H. MacDonald: University of Texas at Austin
Dmitri K. Efetov: The Barcelona Institute of Science and Technology

Nature, 2019, vol. 574, issue 7780, 653-657

Abstract: Abstract Superconductivity can occur under conditions approaching broken-symmetry parent states1. In bilayer graphene, the twisting of one layer with respect to the other at ‘magic’ twist angles of around 1 degree leads to the emergence of ultra-flat moiré superlattice minibands. Such bands are a rich and highly tunable source of strong-correlation physics2–5, notably superconductivity, which emerges close to interaction-induced insulating states6,7. Here we report the fabrication of magic-angle twisted bilayer graphene devices with highly uniform twist angles. The reduction in twist-angle disorder reveals the presence of insulating states at all integer occupancies of the fourfold spin–valley degenerate flat conduction and valence bands—that is, at moiré band filling factors ν = 0, ±1, ±2, ±3. At ν ≈ −2, superconductivity is observed below critical temperatures of up to 3 kelvin. We also observe three new superconducting domes at much lower temperatures, close to the ν = 0 and ν = ±1 insulating states. Notably, at ν = ± 1 we find states with non-zero Chern numbers. For ν = −1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field greater than 3.6 tesla is applied, which is consistent with a field-driven phase transition. Our study shows that broken-symmetry states, interaction-driven insulators, orbital magnets, states with non-zero Chern numbers and superconducting domes occur frequently across a wide range of moiré flat band fillings, including close to charge neutrality. This study provides a more detailed view of the phenomenology of magic-angle twisted bilayer graphene, adding to our evolving understanding of its emergent properties.

Date: 2019
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DOI: 10.1038/s41586-019-1695-0

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