Cyclotron Resonance in Dual-Gated Bilayer Graphene*

We report observations of the far-infrared cyclotron transitions involving the quasi-zero energy Landau level octet in a dual-gated, encapsulated graphene bilayer. Working at a fixed magnetic field of 13 tesla, we systematically study the effects of varying the Landau level filling factor, ν, and the electric displacement field applied perpendicular to the sheet. Varying the displacement field yields a direct measurement of the valley splitting in the lowest Landau level at ν = 4, and signatures of a transition between an interaction-driven and layer polarized ground state at ν = 0. At zero displacement field, weak electron-hole asymmetry and multiple resonances are observed as the filling is tuned between ν = -6 and +6. Numerical calculations of the single particle Landau level energies including all non-leading band parameters fail to account for several of our observations.