  The quantum self-trapping phenomenon of a Bose-Einstein condensate (BEC)
represents a remarkable nonlinear effect of wide interest. By considering a
purely dipolar BEC in a double-well potential, we study how the dipole
orientation affects the ground state structure and the transition between
self-trapping and Josephson oscillation in dynamics. Three-dimensional
numerical results and an effective two-mode model demonstrate that the onset of
self-trapping of a dipolar BEC can be radically modified by the dipole
orientation. We also analyze the failure of the two-mode model in predicting
the rate of Josephson oscillations. We hope that our results can motivate
experimental work as well as future studies of self-trapping of ultracold
dipolar gases in optical lattices.
