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Abstract
We measured the dispersion of the graphite optical phonons in the in-plane Brillouin
zone by inelastic x-ray scattering. The longitudinal and transverse optical branches
cross along the Gamma-K as well as the Gamma-M direction. The dispersion of the optical
phonons was, in general, stronger than expected from the literature. At the K point
the transverse optical mode has a minimum and is only approximately 70 cm(-1) higher
in frequency than the longitudinal mode. We show that first-principles calculations
describe very well the vibrational properties of graphene once the long-range character
of the dynamical matrix is taken into account.