Observation of Interband Collective Excitations in Twisted Bilayer Graphene

Abstract

The single-particle and many-body properties of twisted bilayer graphene (TBG) can be dramatically 1 different from those of a single graphene layer, in particular when the two layers are rotated relative 2 to each other by a small angle ��≈��∘1–6. Here, we probe for the first time collective excitations of TBG 3 graphene with 20 nanometer spatial resolution, by applying mid-infrared (MIR) near-field optical 4 microscopy. We unveil a propagating plasmon mode in charge-neutral TBG with ��=��.��−��.��∘, which 5 is dramatically different from the ordinary single-layer graphene intraband plasmon7,8. We interpret it 6 as an interband plasmon associated with the optical transitions between minibands originating from 7 the moiré superlattice9,10. The details of the plasmon dispersion are directly related to the motion of 8 electrons in the moiré superlattice and offer invaluable insight into a plethora of physical properties, 9 such as the band nesting between flat band and remote band10, local interlayer coupling, losses etc. We 10 find a strongly reduced interlayer coupling in the regions with AA-stacking, pointing at screening due 11 to electron-electron (e-e) interactions. Optical nano-imaging studies of TBG pave the way to spatially 12 probe interactions effects at the nanoscale11, it could potentially elucidate the contribution of collective excitations to many-body ground states12, and it unveils itself as a new platform for strong light-matter 14 interactions and quantum plasmonic studies and devices13.