Filling-factor-dependence of the magnetoplasmon spectrum in graphene

We present cyclotron resonance measurements of a graphite-gated, hexagonal boron nitride-encapsulated monolayer graphene device. We observe a remarkable progression of the CR lineshape as a function of the Landau level filling factor, ν, from a single peak at ν=0 to four peaks at ν=1 to two peaks for ν>2, giving an unprecedented spectroscopic view of the evolution of the magnetoplasmon spectrum arising from the broken spin and valley symmetries in the N=0 Landau level. By comparing to many-particle theories of graphene CR, we extract precise values for the renormalized Fermi velocity and a Dirac mass due to sublattice-symmetry breaking by the hBN. A simple model of LL shifts due to interactions allows extraction of the effective g values for Zeeman gaps in the N=0 Landau level both at and below the Fermi energy. Additionally we find evidence for a small yet clear electron-hole asymmetry.