  We discuss the mechanism and the conditions for the appearance of
synchronized charge oscillations which have been observed experimentally and
theoretically after strong photoexcitation of dimerized systems. In the Hubbard
model with on-site repulsion, the Bloch equations for a wave-number-dependent
pseudospin -- whose components describe the charge-density difference, current
density, and bond density between the two sublattices -- involve an
alternatingly tilted pseudomagnetic field, which assists the synchronization of
pseudospins with different wave numbers, irrespective of the initial condition.
This fact is numerically confirmed by the dynamics in finite lattices based on
the exact diagonalization method. In the presence of nearest-neighbor
repulsion, however, the synchronization can be hindered by excitons. Therefore,
the excitation of a sufficiently large density of free electron-hole pairs, but
low density of excitons, is needed to achieve synchronization.

Strongly Correlated Electrons