Probing superfluidity of Bose-Einstein condensates via laser stirring

Abstract

We investigate the superfluid behavior of a Bose-Einstein condensate of 6 Li 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, 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.

Superfluidity and relaxation dynamics of a laser-stirred two-dimensional Bose gas

“…The observed critical velocity is v c ≈ 0.4 v B . This is notably different from the case of an attractive stirring potential, where v c ≈ v B [28]. We explain this reduction of v c for a repulsive stirring potential in Sec.…”

“…We note that in the crossover region the decrease of v c is as sharp as the size of the stirrer allows. Furthermore, we note that the observed almost constant v c for R < r BKT can be due to the accelerated circular motion and the large strength of the stirring potential [28].…”

“…We demonstrate that a blue-detuned laser of intensity comparable to the mean-field energy causes dissipation due to the creation of vortex-antivortex pairs. This is in contrast to laser stirring with a red-detuned laser [11], where dissipation occurs via phonons [28]. Furthermore, we study the relaxation dynamics of the stirred gas following the stirring process, which shows a slow energy transport between the condensate and the thermal cloud.…”

Critical behavior of a chiral superfluid in a bipartite square lattice

“…As m L increases or T L decreases, the condensed region becomes larger. smoothly varying trap [43][44][45][46]. Here, the phonon velocity will vary as the phonon pulse approaches the interface of the condensate and the thermal gas.…”

Anisotropic Superfluid Behavior of a Dipolar Bose-Einstein Condensate

“…An obvious effect lowering the measured v c is the inhomogeneuos density distribution both along the beam and transversally [25]. Vortex formation is a dominant effect for repulsive obstacles lowering the density, but should be supressed in our experiment with an attractive beam [26]. Yet, the macrosocopic size of the beam can influence the measured critical velocity as well [?…”