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Plasmons in graphene moiré superlattices

Abstract

Moiré patterns are periodic superlattice structures that appear when two crystals with a minor lattice mismatch are superimposed. A prominent recent example is that of monolayer graphene placed on a crystal of hexagonal boron nitride. As a result of the moiré pattern superlattice created by this stacking, the electronic band structure of graphene is radically altered, acquiring satellite sub-Dirac cones at the superlattice zone boundaries. To probe the dynamical response of the moiré graphene, we use infrared (IR) nano-imaging to explore propagation of surface plasmons, collective oscillations of electrons coupled to IR light. We show that interband transitions associated with the superlattice mini-bands in concert with free electrons in the Dirac bands produce two additive contributions to composite IR plasmons in graphene moiré superstructures. This novel form of collective modes is likely to be generic to other forms of moiré-forming superlattices, including van der Waals heterostructures.

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Figure 1: AFM and plasmon nano-imaging with s-SNOM.
Figure 2: Plasmonic line-profiles for both moiré-patterned graphene (MPG) and plain graphene (PG) at different carrier densities.
Figure 3: Extraction of optical conductivity and plasmon damping.
Figure 4: Electronic band structure and the optical conductivity of graphene moiré superlattices.

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Acknowledgements

Work at the University of California, San Diego (UCSD), on optical phenomena in vdW materials is supported by DOE-BES DE-FG02-00ER45799. Research at UCSD on tunable plasmonic reflectors is supported by ONR. D.N.B. is funded by the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4533. The development of scanning plasmon interferometry is supported by DOE-BES and ARO. G.X.N., B.Ö. and A.H.C.N. acknowledge the National Research Foundation, Prime Minister Office, Singapore, under its Medium Sized Centre Program and CRP award ‘Novel 2D materials with tailored properties: beyond graphene’ (R-144-000-295-281).

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All authors were involved in designing the research, performing the research, and writing the paper.

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Correspondence to D. N. Basov.

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F.K. is one of the co-founders of Neaspec, producer of the s-SNOM apparatus used in this study.

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Ni, G., Wang, H., Wu, J. et al. Plasmons in graphene moiré superlattices. Nature Mater 14, 1217–1222 (2015). https://doi.org/10.1038/nmat4425

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