Cyclotron Resonance at the Dirac Point

We present high field infrared spectroscopy data on the n = -1 $\rightarrow$ 0 and n = 0 $\rightarrow$ 1 Landau Level (LL) transitions in graphene. At high magnetic fields, up to 31 T, measurements of single layer graphene show large shifts in the transition energies, which are suggestive of a gap in the n = 0 LL. The magnitude of these shifts are strongly enhanced over the bare Zeeman splitting naively expected for a spin-split state. We compare the field dependence of the shifts with proposed degeneracy-breaking mechanisms at the Dirac point.