## Direct observation of photo-switchable quasi-bound states in a Graphene-Azobenzene-Au multilayer    [PDF]

Emanuela Margapoti, Philip Strobel, Mahmoud M. Asmar, Max Seifert, Matthias Sachsenhauser, Jose A. Garrido, Anna Cattani-Scholz, Sergio E. Ulloa, Jonathan J. Finley
The perfect transmission of charge carriers through potential barriers in graphene (Klein tunneling) is a direct consequence of the Dirac equation that governs the low-energy carrier dynamics\cite{Miao07,Ando98,Ulloa86,Cheianov06,Katsnelson06,Beenakker08}. As a result, localized states do not exist in unpatterned graphene but quasi-bound states \emph{can} occur for potentials with closed integrable dynamics\cite{Bardarson09,Wurm11}. Here, we report the observation of optically switchable quasi-bound states in unpatterned graphene placed onto a mixed self-assembled monolayer (mSAM) of photo-switchable molecules. Conductive AFM measurements performed at room temperature reveal strong current resonances, the strength of which can be reversibly gated \textit{on-} and \textit{off-} by optically switching the molecular conformation of the mSAM. Comparisons of the voltage separation between current resonances ($\sim 70$--120 mV) with solutions of the Dirac equation indicate that the radius of the scattering potential is $\sim 7 \pm 2$ nm with a strength $\geq 0.5$ eV. Our results and methods provide a route toward \emph{optically programmable} carrier dynamics and transport in graphene nano-materials.
View original: http://arxiv.org/abs/1308.0552