Mainz 2017 – scientific programme

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Q: Fachverband Quantenoptik und Photonik

Q 39: Ultracold atoms and BEC - IV (with A)

Q 39.8: Talk

Wednesday, March 8, 2017, 16:15–16:30, N 1

Superfluidity and relaxation dynamics of a laser-stirred 2D Bose gas — •Vijay Pal Singh^{1, 2}, Christof Weitenberg², Jean Dalibard³, and Ludwig Mathey^{1, 2} — ¹Zentrum für Optische Quantentechnologien, Universität Hamburg, 22761 Hamburg, Germany — ²Institut für Laserphysik, Universität Hamburg, 22761 Hamburg, Germany — ³Laboratoire Kastler Brossel, Collège de France, ENS-PSL Research University, CNRS, UPMC-Sorbonne Universités, 11 place Marcelin Berthelot, 75005 Paris, France

We study the superfluid behavior of a two-dimensional (2D) Bose gas of ⁸⁷Rb atoms. In the experiment by R. Desbuquois et al., Nat. Phys. 8, 645 (2012) a 2D quasicondensate in a trap is stirred by a blue-detuned laser beam along a circular path around the trap center. Here, we study this experiment from a theoretical perspective. The heating induced by stirring increases rapidly above a velocity v_c, which we define as the critical velocity. We identify the superfluid, the crossover, and the thermal regime by a finite, a sharply decreasing, and a vanishing critical velocity, respectively. A direct comparison of our results to the experiment shows good agreement, if a systematic shift of the critical phase space density is included. We relate this shift to the absence of thermal equilibrium between the condensate and the thermal wings in the experiment, which were used to extract the temperature. We expand on this observation by studying the full relaxation dynamics between the condensate and the thermal cloud. Analytical results on the vortex formation are also discussed.