Intraband Landau level transitions in monolayer graphene

We study the cyclotron resonance of electrons and holes in monolayer graphene, via infrared transmission measurements in a magnetic field, $B$, up to 18 T. We find that, instead of having a single resonance energy as in a traditional two-dimensional system, a wide range of transitions between different sets of Landau levels (LLs) can be uniquely distinguished in monolayer graphene. We have observed intraband transitions between neighboring LLs up to $n=7$, where $n$ is the LL index. As expected from the unusual linear dispersion of the low-energy electronic band of monolayer graphene, we show that the corresponding energies of all observed LL transitions are proportional to $\sqrt{B}$. In addition, beyond such a simple linear dispersion, we find that the measured band velocity near the charge-neutral Dirac point ($E=0$) is $\sim$$12\%$ larger than that at higher energies. The LL transitions in the electron and hole bands of monolayer graphene show a considerable asymmetric behavior.