  For the fermionic Hubbard model at strong coupling, we demonstrate that
directional transport of localized doublons (repulsively bound pairs of two
particles occupying the same site of the crystal lattice) can be achieved by
applying an unbiased ac field of time-asymmetric (sawtooth-like) shape. The
mechanism involves a transition to intermediate states of virtually zero double
occupation which are reached by splitting the doublon by fields of the order of
the Hubbard interaction. The process is discussed on the basis of numerically
exact calculations for small clusters, and we apply it to more complex states
to manipulate the charge order pattern of one-dimensional systems.
