  We consider an overdamped Brownian particle, exposed to a two-dimensional,
square lattice potential and a rectangular ac-drive. Depending on the driving
amplitude, the linear response to a weak dc-force along a lattice symmetry axis
consist in a mobility in basically any direction. In particular, motion exactly
opposite to the applied dc-force may arise. Upon changing the angle of the
dc-force relatively to the square lattice, the particle motion remains
predominantly opposite to the dc-force. The basic physical mechanism consists
in a spontaneous symmetry breaking of the unbiased deterministic particle
dynamics.
