  Using the axially-symmetric time-dependent mean-field Gross-Pitaevskii
equation we study the Josephson oscillation in a repulsive Bose-Einstein
condensate trapped by a harmonic plus an one-dimensional optical-lattice
potential to describe the experiments by Cataliotti et. al. [Science 293 (2001)
843, New J. Phys. 5 (2003) 71.1]. After a study of the formation of matter-wave
interference upon releasing the condensate from the optical trap, we directly
investigate the alternating atomic superfluid Josephson current upon displacing
the harmonic trap along the optical axis. The Josephson current is found to be
disrupted upon displacing the harmonic trap through a distance greater than a
critical distance signaling a superfluid to a classical insulator transition in
the condensate.
