Observation of flat bands in twisted bilayer graphene

Spectroscopic measurements using nano-ARPES on twisted bilayer graphene directly highlight the presence of the flat bands. Transport experiments in twisted bilayer graphene have revealed multiple superconducting domes separated by correlated insulating states(1-5). These properties are generally associated with strongly correlated states in a flat mini-band of the hexagonal moire 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 electronic compressibility measurements(14), which report two or more sharp peaks in the density of states that may be associated 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 spectral weight that is highly localized in momentum space. The flat band is separated from the dispersive Dirac bands, which show multiple moire hybridization gaps. These data establish the salient features of the twisted bilayer graphene band structure.