High-energy excitonic effects in single-layer graphene

Using high-energy polarization-dependent reflectivity, we discover high-energy resonant excitons at room temperature in single-layer graphene. The resonant excitons occur at $\ensuremath{\sim}10.04$ and $\ensuremath{\sim}12.29$ eV arising from transitions between parallel $\ensuremath{\sigma}$ and $\ensuremath{\pi}*$ states along $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}K$ as well as $\ensuremath{\sigma}$ and $\ensuremath{\sigma}*$ states at the $M$ point, respectively. The observation of spectral weight transfer signifies electronic corelation and that the two-dimensional nature and low-energy properties are strongly coupled with high-energy bands. Our result shows the importance of electron-electron and electron-hole interactions in determining electronic and optical properties of graphene and two-dimensional systems.