Physics

Dr. Vijay Singh
University of Hamburg, Germany

Abstract 

We investigate the superfluid behavior of a Bose-Einstein condensate of 6Li molecules. In the experiment by Weimer et al., Phys. Rev. Lett. 114, 095301 (2015) a condensate is stirred by a weak, red-detuned laser beam along a circular path around the trap center. The rate of induced heating increases steeply above a velocity vc, which we define as the critical velocity. Below this velocity, the moving beam creates almost no heating. In this paper [1], we demonstrate a quantitative understanding of the critical velocity. Using both numerical and analytical methods, we identify the non-zero temperature, the circular motion of the stirrer, and the density profile of the cloud as key factors influencing the magnitude of vc. A direct comparison to the experimental data shows excellent agreement.
We then explore superfluidity of a two-dimensional (2D) Bose gas of 87Rb atoms. In the exper- iment 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. In Ref. [2], we study this experiment from a theoretical perspective. 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.

References:
[1] V. P. Singh, W. Weimer, K. Morgener, J. Siegl, K. Hueck, N. Luick, H. Moritz, and L. Mathey, Phys. Rev. A 93, 023634 (2016).
[2] V. P. Singh, C. Weitenberg, J. Dalibard, and L. Mathey, Phys. Rev. A 95, 043631 (2017).