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Superconductivity and strong correlations in moiré flat bands

Abstract

Strongly correlated systems can give rise to spectacular phenomenology, from high-temperature superconductivity to the emergence of states of matter characterized by long-range quantum entanglement. Low-density flat-band systems play a vital role because the energy range of the band is so narrow that the Coulomb interactions dominate over kinetic energy, putting these materials in the strongly-correlated regime. Experimentally, when a band is narrow in both energy and momentum, its filling may be tuned in situ across the whole range, from empty to full. Recently, one particular flat-band system—that of van der Waals heterostructures, such as twisted bilayer graphene—has exhibited strongly correlated states and superconductivity, but it is still not clear to what extent the two are linked. Here, we review the status and prospects for flat-band engineering in van der Waals heterostructures and explore how both phenomena emerge from the moiré flat bands.

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Fig. 1: Three classes of flat bands in van der Waals heterostructures.
Fig. 2: Correlation-driven phases in Hofstadter flat bands.
Fig. 3: Substrate tuning of electronic structure in twisted bilayer graphene.
Fig. 4: Superconductivity in twisted bilayer graphene.
Fig. 5: Proliferation of zero-field narrow-band systems.

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Acknowledgements

We thank K. Hejazi and C. Liu for assistance with preparation of Fig. 1c. L.B. acknowledges support by the NSF CMMT program under award no. DMR-1818533; the US Department of Energy, Office of Science, Basic Energy Sciences under award no. DE-FG02-08ER46524 and the UCSB NSF Quantum Foundry through Q-AMASE-i program award no. DMR-1906325. A.F.Y. acknowledges the support of the US Department of Energy, Office of Science, Basic Energy Sciences under award no. DE-SC0020043 and the UCSB NSF Quantum Foundry through Q-AMASE-i program award no. DMR-1906325. C.R.D. acknowledges the support of the Pro-QM EFRC funded by the US Department of Energy, Office of Science, Basic Energy Sciences under award no. DE-SC0019443. D.K.E. acknowledges support from the Ministry of Economy and Competitiveness of Spain through the ‘Severo Ochoa’ program for Centres of Excellence in R&D (SE5-0522), Fundació Privada Cellex, Fundació Privada Mir-Puig, the Generalitat de Catalunya through the CERCA program, the H2020 Programme under grant agreement no. 820378, Project: 2D·SIPC and the La Caixa Foundation.

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Correspondence to Leon Balents, Dmitri K. Efetov or Andrea F. Young.

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Balents, L., Dean, C.R., Efetov, D.K. et al. Superconductivity and strong correlations in moiré flat bands. Nat. Phys. 16, 725–733 (2020). https://doi.org/10.1038/s41567-020-0906-9

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