Motion of a Gaussian Through a 2D Dilute BEC Droplet

University essay from Lunds universitet/Matematisk fysik; Lunds universitet/Fysiska institutionen

Abstract: In the last decades, Bose-Einstein Condensates (BECs) have been a research topic of great interest. In 2015, a new type of liquid was found - dilute self-bound BEC droplets that have orders of magnitude lower density than air. These droplets are not predicted by classical van der Waals theory, but are stabilised by quantum fluctuations. In this bachelor thesis, these droplets were numerically studied when colliding with a Gaussian obstacle in two-dimensions. This is interesting because below a certain velocity, dilute BECs can behave like superfluids. This is called Landau's criterion. Several droplet velocities and Gaussian widths were tested with special focus on two cases: one where the droplet starts outside the Gaussian and one where the droplet starts with the Gaussian inside of it. The droplet was then propagated until the Gaussian was approximately at the centre of the droplet. In none of the collisions simulated, laminar- attached vortices- or vortex street flow patterns were observed. These are flow patterns one can see in similar classical examples and/or with trapped dilute BECs. However, it appeared that in all the cases the droplet did not fully behave like a superfluid. It might have been quantum fluctuations that created a drag force. Further, a large deformation of the droplet could be seen when the droplet started outside a broad Gaussian. This is believed to have been caused by the Gaussian piercing the surface of the droplet. Finally, suggestions for future research are given at the end of this thesis.

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