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We have studied the time evolution of a harmonically trapped BEC loaded into a 1D moving optical lattice [1]. The lifetime of the condensate in such a potential exhibits a strong dependence on the quasimomentum state q, which we attribute to the onset of dynamical instability. This kind of instability, driven by nonlinearities in a periodic potential, occurs when the excitation spectrum of the system presents complex frequencies, so that arbitrary small initial perturbations may grow exponentially in time [2-4]. The observed dynamics, featuring a nontrivial behavior near the band edges and the appearance of complex structures in the expanded atomic density profile, is compared with the numerical solution of the GPE, showing the validity of this interpretation.
[1] L. Fallani, L. De Sarlo, J. E. Lye, M. Modugno, R. Saers, C. Fort, M. Inguscio, "Observation of dynamical instability for a Bose-Einstein condensate in a moving 1D optical lattice", accepted for publication in Phys. Rev. Lett. (2004), arXiv:cond-mat/0404045 (2004) [2] B. Wu and Q. Niu, Phys. Rev. A 64, 061603R (2001); B. Wu and Q. Niu, New Journ. Phys. 5, 104 (2003). [3] F. S. Cataliotti, L. Fallani, F. Ferlaino, C. Fort, P. Maddaloni, M. Inguscio, New Journ. Phys. 5, 71 (2003). [4] A. Smerzi, A. Trombettoni, P. G. Kevrekidis and A. R. Bishop, Phys. Rev. Lett. 89, 170402 (2002); C. Menotti, A. Smerzi and A. Trombettoni, New Journ. Phys. 5, 112 (2003). |