  We study the dynamics of bosonic atoms in a tilted one-dimensional optical
lattice and report on the first direct observation of density-induced
tunneling. We show that the interaction affects the time evolution of the
doublon oscillation via density-induced tunneling and pinpoint its density- and
interaction-dependence. The experimental data for different lattice depths are
in good agreement with our theoretical model. Furthermore, resonances caused by
second-order tunneling processes are studied, where the density-induced
tunneling breaks the symmetric behavior for attractive and repulsive
interactions predicted by the Hubbard model.
