We investigate the origin of the translational symmetry breaking in epitaxially grown single-layer graphene. Despite the surface morphology of graphene films influenced by the presence of mutually parallel SiC surface terraces, the far-infrared magnetoplasmon absorption is almost independent of the angle between the probing light polarization and the orientation of terraces. Based on a detailed analysis of the plasmon absorption line shape and its behavior in the magnetic field, supported by confocal Raman mapping and atomic force microscopy, we explain this discrepancy by spontaneously formed graphene microflakes. We further support our conclusions using data collected on artificially created graphene nanoribbons: we recognize similar plasmon origin in artificial ribbons and naturally formed grains. An unexpectedly large plasmon resonance redshift was observed in nanoribbons. In a hydrogen-intercalated sample (which does not contain the buffer), this redshift is quantitatively considered by a plasmon-plasmon interaction. This redshift is due to an interplay between the plasmon-plasmon coupling and the Coulomb screening by the buffer-induced interface states in nonintercalated samples featuring a buffer layer. This model determines the density of interface states in good agreement with experimentally reported values.

• Received 13 March 2023
• Revised 23 June 2023
• Accepted 6 July 2023

DOI:https://doi.org/10.1103/PhysRevB.108.045308