Observation of flat bands in twisted bilayer graphene

Lisi, S.; Lu, X.; Benschop, T.; Jong, T.A. de; Stepanov, P.; Duren, J.R.; Margot, F.; Cucchi, I.; Cappelli, E.; Hunter, A.; Tamai, A.; Kandyba, V.; Giampietri, A.; Barinov, A.; Jobst, J.; Stalman, V.; Leeuwenhoek, M.; Watanabe, K.; Taniguchi, T.; Rademaker, L.; Molen, S.J. van der; Allan, M.P.; Efetov, D.K.; Baumberger, F.

doi:doi:10.1038/s41567-020-01041-x

Transport experiments in twisted bilayer graphene have
revealed multiple superconducting domes separated by cor-
related insulating states 1–5 . These properties are generally
associated with strongly correlated states in a flat mini-band
of the hexagonal moiré superlattice as was predicted by band
structure calculations 6–8 . Evidence for the existence of a flat
band comes from local tunnelling spectroscopy 9–13 and elec-
tronic compressibility measurements 14 , which report two or
more sharp peaks in the density of states that may be asso-
ciated with closely spaced Van Hove singularities. However,
direct momentum-resolved measurements have proved to be
challenging 15 . Here, we combine different imaging techniques
and angle-resolved photoemission with simultaneous real- and
momentum-space resolution (nano-ARPES) to directly map
the band dispersion in twisted bilayer graphene devices near
charge neutrality. Our experiments reveal...Show moreTransport experiments in twisted bilayer graphene have
revealed multiple superconducting domes separated by cor-
related insulating states 1–5 . These properties are generally
associated with strongly correlated states in a flat mini-band
of the hexagonal moiré superlattice as was predicted by band
structure calculations 6–8 . Evidence for the existence of a flat
band comes from local tunnelling spectroscopy 9–13 and elec-
tronic compressibility measurements 14 , which report two or
more sharp peaks in the density of states that may be asso-
ciated with closely spaced Van Hove singularities. However,
direct momentum-resolved measurements have proved to be
challenging 15 . Here, we combine different imaging techniques
and angle-resolved photoemission with simultaneous real- and
momentum-space resolution (nano-ARPES) to directly map
the band dispersion in twisted bilayer graphene devices near
charge neutrality. Our experiments reveal large areas with a
homogeneous twist angle that support a flat band with a spec-
tral weight that is highly localized in momentum space. The flat
band is separated from the dispersive Dirac bands, which show
multiple moiré hybridization gaps. These data establish the
salient features of the twisted bilayer graphene band structure.
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Observation of flat bands in twisted bilayer graphene