Thermal Activation of Vortex-Antivortex Pairs in Quasi-Two-Dimensional Bose-Einstein Condensates

Here we show, by performing ab initio classical field simulations that two distinct superfluid phases, separated by thermal vortex-antivortex pair creation, exist in experimentally producible quasi-2D Bose gas. These results resolve the debate on the nature of the low temperature phase(s) of a trapped interacting 2D Bose gas.

Figure 1

Low temperature phase diagram for quasi-2D Bose gas. The markers denote different vortex-pair excitation probabilities $p=0$ (○), $0<p<0.25$ ($\otimes$), and $p>0.25$ (●) inside a circular region whose diameter equals the Thomas-Fermi radius. The lines are analytical estimates using Eq. (1) with $ϵ=0$ (solid line) and $ϵ=1/2$ (dashed line).

Figure 2

Internal energy for $N=1\times {10}^4$ particles (main frame) and the vortex excitation probability for a range of $N$ (inset) as functions of the fractional temperature. The solid line is a fourth order polynomial fit to the data. The dashed line is the ideal-gas result.

Figure 3

Interference pattern (a) produced by two independent classical fields (b) and (c) at temperature $T=0.86T_0$. The relevant particle numbers are $N_{\mathrm{cl}}=3.0\times {10}^3$ and $N=4.0\times {10}^4$. The zipper structure in (a) is the telltale signature of the phase singularity associated with the central vortex in (b). The locations of vortices and antivortices are marked by + and − signs, respectively.