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

By Xiaobo Lu, P. Stepanov, Wei Yang, M. Xie, M. Aamir, Ipsita Das, C. Urgell, Kenji Watanabe, Takashi Taniguchi, Guangyu Zhang and 3 more. Nature, 2019

1,559 citations recorded by the source corpus.

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. The fabrication of magic-angle twisted bilayer graphene devices with highly uniform twist angles enables the observation of new superconducting domes, orbital magnets and Chern insulating states.

Subfields: Atomic and Molecular Physics, and Optics, Condensed Matter Physics, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/4ee96637d4f67fa582b8f7815c1ae2b85dce5424

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Citations in the corpus

Flat bands, strange metals and the Kondo effect (2023, neutral): “In heavy-fermion compounds, it is attributed to a Kondo destruction transition, also referred to as f -orbital- selective Mott transition.”
Spontaneous fractional Chern insulators in transition metal dichalcogenide moiré superlattices (2021, background): “[5] 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.”
Electrically tunable correlated and topological states in twisted monolayer–bilayer graphene (2020, background): “In graphene-based moiré heterostructures, the correlated phase diagram and band topology depend strongly on the number of graphene layers [1–17], their relative stacking arrangement [15–17], and details”
Correlated states in twisted double bilayer graphene (2019, background): “7 Lu, X. et al. Superconductors, orbital magnets and correlated states in magic-angle bilayer graphene. Nature 574, 653-657(2019).”