Person:

Cao, Yuan

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.

AbstractFractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue towards manipulating non-Abelian excitations. Early theoretical studies1–7 have predicted their existence in systems with flat Chern bands and highlighted the critical role of a particular quantum geometry. However, FCI states have been observed only in Bernal-stacked bilayer graphene (BLG) aligned with hexagonal boron nitride (hBN)8, in which a very large magnetic field is responsible for the existence of the Chern bands, precluding the realization of FCIs at zero field. By contrast, magic-angle twisted BLG9–12 supports flat Chern bands at zero magnetic field13–17, and therefore offers a promising route towards stabilizing zero-field FCIs. Here we report the observation of eight FCI states at low magnetic field in magic-angle twisted BLG enabled by high-resolution local compressibility measurements. The first of these states emerge at 5 T, and their appearance is accompanied by the simultaneous disappearance of nearby topologically trivial charge density wave states. We demonstrate that, unlike the case of the BLG/hBN platform, the principal role of the weak magnetic field is merely to redistribute the Berry curvature of the native Chern bands and thereby realize a quantum geometry favourable for the emergence of FCIs. Our findings strongly suggest that FCIs may be realized at zero magnetic field and pave the way for the exploration and manipulation of anyonic excitations in flat moiré Chern bands.