Cyclotron resonance study of the electron and hole velocity in graphene monolayers

We report studies of cyclotron resonance in monolayer graphene. Cyclotron resonance is detected using the photoconductive response of the sample for several different Landau level occupancies. The experiments measure an electron velocity at the $K$ (Dirac) point of $c_K^{*}=1.093\times {10}^6\mathrm{m}{\mathrm{s}}^{-1}$, which is substantially larger than in thicker graphitic systems. In addition we observe a significant asymmetry between the electron and hole bands, leading to a difference in the electron and hole velocities of 5% by energies of $125\mathrm{meV}$ away from the Dirac point.

Figure 1

(Color online) (a) Sample image with outline of the contacts used in the measurements. (b) Plot of the Landau energies as a function of magnetic field for Landau index $N=-2,\dots ,2$. Arrows indicate the resonant transitions probed in the energy range of the $\mathrm{C}{\mathrm{O}}_2$ laser. (c) Density dependence of the two-contact resistive voltage and photoconductive response of a typical graphene sample for infrared radiation of $10.23\mu \mathrm{m}$ at $10\mathrm{T}$ measured with a current of $100\mathrm{nA}$.

Figure 2

(Color online) Photoconductive response as a function of gate voltage and magnetic field for $9.25\mu \mathrm{m}$ $\left(134\mathrm{meV}\right)$. The low-field section of the map has an enhanced sensitivity to display the sharp negative resonance at zero field.

Figure 3

(Color online) Photoconductive response as a function of $\sqrt{B}$ with the carrier densities scanned to keep the occupancies constant at $\nu =-3.0$ $(1-)$, $-0.76$ $(0-)$, 0.88 $(0+)$, and 3.1 $(1+)$ for wavelengths from $9.2\text{to}10.7\mu \mathrm{m}$. The red (gray) lines show fits using Lorenzian line shapes combined with a linear background response.

Figure 4

(Color online) Resonance positions for the four resonances as a function of $\sqrt{B}$, together with a single fitted value of the electron velocity $c_K^{*}$ [red (gray) line]. The outer lines show fits to Eq. (3), with the shaded bands covering the error limits from $c_K$ and ${\beta }_0$. The individual resonance positions have errors as shown of $\sim \pm 2\%$, corresponding to $0.2\Delta \sqrt{B}$, where $\Delta \sqrt{B}$ is the half width at half maximum absorption.