Scientific Results

Critical velocity for a toroidal Bose-Einstein condensate flowing through a barrier

Year: 2013

Authors: Piazza, F., Collins L.A., Smerzi A.

Autors Affiliation: Physik Department, Technische Universitat Munchen, James-Franck-Strae, D-85748 Garching, Germany; Theoretical Division, Los Alamos National Laboratory, Mail Stop B214, Los Alamos, NM87545, USA; QSTAR, INO-CNR and LENS, Largo Enrico Fermi 2, I-50125 Firenze, Italy

Abstract: We consider the setup employed in a recent experiment (Ramanathan et al 2011 Phys. Rev. Lett. 106 130401) devoted to the study of the instability of the superfluid flow of a toroidal Bose-Einstein condensate in the presence of a repulsive optical barrier. Using the Gross-Pitaevskii mean-field equation, we observe, consistently with what we found in Piazza et al (2009 Phys. Rev. A 80 021601), that the superflow with one unit of angular momentum becomes unstable at a critical strength of the barrier and decays through the mechanism of phase slippage performed by pairs of vortex-antivortex lines annihilating. While this picture qualitatively agrees with the experimental findings, the measured critical barrier height is not very well reproduced by the Gross-Pitaevskii equation, indicating that thermal fluctuations can play an important role (Mathey et al 2012 arXiv: 1207.0501). As an alternative explanation of the discrepancy, we consider the effect of the finite resolution of the imaging system. At the critical point, the superfluid velocity in the vicinity of the obstacle is always of the order of the sound speed in that region, v(barr) = C-1. In particular, in the hydrodynamic regime (not reached in the above experiment), the critical point is determined by applying the Landau criterion inside the barrier region. On the other hand, the Feynman critical velocity v(f) is much lower than the observed critical velocity. We argue that this is a general feature of the Gross-Pitaevskii equation, where we have v(f) = epsilon C-1 with epsilon being a small parameter of the model. Given these observations, the question still remains open about the nature of the superfluid instability.

Journal/Review: JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS

Volume: 46 (9)      Pages from: 095302  to: 095302

More Information: We would like to thank A Recati for fruitful discussions. FP acknowledges support from the Alexander Von Humboldt foundation. AS acknowledges support from the EU-STREP Project QIBEC. This work was supported the Los Alamos National Laboratory, operated by Los Alamos National Security, LLC for the National Nuclear Security Administration of the US Department of Energy under contract no. DE-AC52-06NA25396, with computer resources provided by an LANL institutional computing grant.
DOI: 10.1088/0953-4075/46/9/095302

Citations: 23
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