  Spatiotemporal evolution of a confined Bose-Einstein condensate is studied by
numerically integrating the time-dependent Gross-Pitaevskii equation.
Self-interference between the successively expanding and reflecting nonlinear
matter waves results in spiral atomic density profile, which subsequently
degenerates into an embedding structure: The inner part preserves memory of the
initial states while the outer part forms a sequence of necklacelike rings. The
phase plot reveals a series of discrete concentric belts. The large gradients
between adjacent belts indicate that the ring density notches are dark
solitons. In the dynamical process, a scenario of vortex-antivortex pairs are
spontaneously created and annihilated, whereas the total vorticity keeps
invariant.
