  A remarkable manifestation of the quantum character of electrons in matter is
offered by graphene, a single atomic layer of graphite. Unlike conventional
solids where electrons are described with the Schrodinger equation, electronic
excitations in graphene are governed by the Dirac Hamiltonian. Some of the
intriguing electronic properties of graphene, such as massless Dirac
quasiparticles with linear energy-momentum dispersion, have been confirmed by
recent observations. Here we report an infrared (IR) spectromicroscopy study of
charge dynamics in graphene integrated in gated devices. Our measurements
verify the expected characteristics of graphene and, owing to the previously
unattainable accuracy of IR experiments, also uncover significant departures of
the quasiparticle dynamics from predictions made for Dirac fermions in
idealized, free standing graphene. Several observations, including a marked
deviation of the energy bands from the simple linear dispersion, point to the
relevance of many body interactions to the rather exotic electromagnetic
response in graphene.
