  We report the long-time nonlinear dynamical evolution of ultracold atomic
gases in the P-band of an optical lattice. A Bose-Einstein condensate (BEC) is
fast and efficiently loaded into the Pband at zero quasi-momentum with a
non-adiabatic shortcut method. For the first one and half milliseconds, these
momentum states undergo oscillations due to coherent superposition of different
bands, which are followed by oscillations up to 60ms of a much longer period.
Our analysis shows the dephasing from the nonlinear interaction is very
conducive to the long-period oscillations induced by the variable force due to
the harmonic confinement.
