  An acoustic plasmon is predicted to occur, in addition to the conventional
two-dimensional (2D) plasmon, as the collective motion of a system of two types
of electronic carriers coexisting in the very same 2D band of extrinsic (doped
or gated) graphene. The origin of this novel mode resides in the strong
anisotropy that is present in the graphene band structure near the Dirac point.
This anisotropy allows for the coexistence of carriers moving with two distinct
Fermi velocities along the Gamma-K direction, which leads to two modes of
collective oscillation: one mode in which the two types of electrons oscillate
in phase with one another [this is the conventional 2D graphene plasmon, which
at long wavelengths (q->0) has the same dispersion, q^1/2, as the conventional
2D plasmon of a 2D free electron gas], and the other mode found here
corresponding to a low-frequency acoustic oscillation [whose energy exhibits at
long wavelengths a linear dependence on the 2D wavenumber q] in which the two
types of electrons oscillate out of phase. If this prediction is confirmed
experimentally, it will represent the first realization of acoustic plasmons
originated in the collective motion of a system of two types of carriers
coexisting within the very same band.
