REPORTS Observation of Plasmarons in

A hallmark of graphene is its unusual conical band structure that leads to a zero-energy band gap at a single Dirac crossing point. By measuring the spectral function of charge carriers in quasi-freestanding graphene with angle-resolved photoemission spectroscopy, we showed that at finite doping, this well-known linear Dirac spectrum does not provide a full description of the charge-carrying excitations. We observed composite “plasmaron ” particles, which are bound states of charge carriers with plasmons, the density oscillations of the graphene electron gas. The Dirac crossing point is resolved into three crossings: the first between pure charge bands, the second between pure plasmaron bands, and the third a ring-shaped crossing between charge and plasmaron bands. Electrons in metals and semiconductorsundergo many complex interactions, andmost theoretical treatments make use of the quasiparticle approximation, in which independent electrons are replaced by electron- and hole-like quasiparticles interacting through a dynamically screened Coulomb force. The details of the screen-ing are determined by the valence band structure