Superfluidity of a laser-stirred Bose-Einstein condensate

We study superfluidity of a cigar-shaped Bose-Einstein condensate by stirring it with a Gaussian potential oscillating back and forth along the axial dimension of the condensate, motivated by experiments of Raman et al. [Phys. Rev. Lett. 83, 2502 (1999)]. Using classical-field simulations and perturbation theory, we examine the induced heating rate, based on the total energy of the system, as a function of the stirring velocity $v$. We identify the onset of dissipation by a sharply increasing heating rate above a velocity ${v}_{c}$, which we define as the critical velocity. We show that ${v}_{c}$ is influenced by the oscillating motion, the strength of the stirrer, the temperature, and the inhomogeneous density of the cloud. This results in a vanishing ${v}_{c}$ for the parameters similar to the experiments, which is inconsistent with the measurement of nonzero ${v}_{c}$. However, if the heating rate is based on the thermal fraction after a $100\phantom{\rule{4pt}{0ex}}\mathrm{ms}$ equilibration time, our simulation recovers the experimental observations. We demonstrate that this discrepancy is due to the slow relaxation of the stirred cloud and dipole mode excitation of the cloud.

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