Graphene & 2D Materials

Although the fundamental nature of quantum Hall effect (QHE) involves a constant flow of carriers in topologicallyprotected edge states where backscattering is supressed, the QHE has been found to persist at terahertz (THz, ~4 meV) frequencies in various 2D systems such as graphene and semiconductor 2D electron gases. We measure the Hall conductivity of a two-dimensional electron gas formed at a GaAs/AlGaAs heterojunction in the THz regime close to the cyclotron resonance (CR) frequency using highly sensitive Faraday rotation measurements. The sample is electrically gated, allowing the electron density to be changed continuously by more than a factor of 3. We observe clear plateau like and steplike features in the Faraday

rotation angle vs. electron density and magnetic field (Landau-level filling factor) even at fields or frequencies within half a linewidth of the CR absorption. It is surpising to see QHE plateaus where the conductivity is dominated by inter-Landau level optical absorption. We have also looked for signatures of the QHE near CR in graphene at mid-infrared energies (~100 meV).