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arXiv:cond-mat/0212040 · Physical Review A

Effective-mass analysis of Bose-Einstein condensates in optical lattices: Stabilization and levitation

Han Pu, Leslie O. Baksmaty, W. Zhang, N. P. Bigelow, Pierre Meystre

Published Apr 11, 2003• 58 citations
arXivHTMLPDF

Abstract

We investigate the time evolution of a Bose-Einstein condensate in a periodic optical potential. Using an effective mass formalism, we study the equation of motion for the envelope function modulating the Bloch states of the lattice potential. In particular, we show how the negative effective-mass affects the dynamics of the condensate.
DOI
10.1103/physreva.67.043605

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We consider stationary and propagating solutions for a Bose-Einstein condensate in a periodic optical potential with an additional confining optical or magnetic potential. Using an effective mass approximation we express the condensate wavefunction in terms of slowly-varying envelopes modulating the Bloch modes of the optical lattice. In the limit of a weak nonlinearity, we derive a nonlinear Schrödinger equation for propagation of the envelope function which does not contain the rapid oscillation of the lattice. We then consider the ground state solutions in detail in the regime of weak, moderate and strong nonlinear interactions. We describe the form of solution which is appropriate in each regime, and place careful limits on the validity of each type of solution.
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It is known that solitons of Bose-Einstein condensates with positive scattering lengths can be produced in optical lattices. The key of this process is the negative sign of the effective mass of the wavepacket as it comes closer to the edge of the first Brillouin zone. However, the underlying assumption of the effective mass approach is the slow variation of the wavefunction envelope between consecutive lattice cells. Previous experiments and computations on this type of solitons show that this is not the usual case. In this contribution we will go beyond the slowly varying assumption to demonstrate that the equations governing the dynamics of gap solitons contain a third derivative in the dispersion. As a result, the dynamics of the stabilized wavepackets is found different that the soliton's. In particular, a radiative component is present, and the stability under collisions is only partially achieved.
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A one-band effective-mass (EFM) theory was developed to study the properties of the localized states of a Bose-Einstein condensate (BEC) in a periodic optical potential with either attractive or repulsive atom interactions. The localized states of the BEC are solitons in the gaps or gap solitons. The analytic solutions of the gap solitons are a Bloch function from periodicity, modulated by a soliton envelope function of the nonlinear Schrödinger equation. The soliton width is determined by the detuning chemical potential from the band edge. The detuning chemical potential in the nonlinear Schrödinger equation indicates that not all the atoms in the condensate are used for forming gap solitons, and that some atoms form the background of nonlinear modes. The background atoms are determined by the chemical potential at the band edge. The gap solitons form on top of the background atoms. PACS numbers: 03.75.Lm, 03.75.Kk, 05.30.Jp I.