We study the infrared cyclotron resonance of high-mobility monolayer graphene encapsulated in hexagonal boron nitride, and simultaneously observe several narrow resonance lines due to interband Landau-level transitions. By holding the magnetic field strength $B$ constant while tuning the carrier density $n$, we find the transition energies show a pronounced nonmonotonic dependence on the Landau-level filling factor, $\nu \propto n/B$. This constitutes direct evidence that electron-electron interactions contribute to the Landau-level transition energies in graphene, beyond the single-particle picture. Additionally, a splitting occurs in transitions to or from the lowest Landau level, which is interpreted as a Dirac mass arising from coupling of the graphene and boron nitride lattices.

• Received 8 August 2017
• Revised 12 November 2017

DOI:https://doi.org/10.1103/PhysRevLett.120.047401