Temperature, Magnetic field, and Gate Bias Dependence of the Infrared Hall Effect in Graphene

In our study we probe the infrared Hall conductivity ($\sigma_ {xy}$) for single and bilayer graphene in the 120-1000 meV range as a function of gate bias at temperatures down to 7K and magnetic fields up to 7T using Faraday measurements. Unlike the longitudinal conductivity ($\sigma_{xx}$), which measures the sum of the optical responses for left and right circularly polarized light, $\sigma_ {xy}$ measures the difference and therefore is sensitive to small changes in symmetry. While $\sigma_{xx}$ and the DC Hall effect have revealed extraordinary properties of graphene (Zhang, Nature 2005; Novoselov, Nature 2005; Jiang, PRL 2007; etc...) recent calculations (Morimoto, PRL 2009) predict remarkable step-like features in the infrared $\sigma_{xy}$. We also probe the chiral response of graphene due to spatial inversion symmetry breaking. Our graphene samples are prepared using several methods, including anodically bonding graphite to pyrex, which can produce a high yield of large single layer graphene flakes ($>$100 $\mu$m) (Shukla et al., Solid State Comm. 2009), normal mechanical exfoliation of kish graphite, and grown chemical vapor deposition techniques.