Observation of Thermally Activated Vortex Pairs in a Quasi-2D Bose Gas

TL;DR

This study observes thermally activated vortex pairs in a quasi-2D Bose gas, revealing a transition from a vortex-pair phase to a vortex-free condensate as temperature decreases, using enhanced vortex core visibility and correlation analysis.

Contribution

It provides direct experimental evidence of vortex pairing and the crossover from BKT phase to Bose-Einstein condensate in a finite-size 2D system.

Findings

Vortex density decreases with temperature.

Vortex-free region appears below a critical temperature.

Vortex pairing is confirmed by spatial correlation functions.

Abstract

We measure the in-plane distribution of thermally activated vortices in a trapped quasi-2D Bose gas, where we enhance the visibility of density-depleted vortex cores by radially compressing the sample before releasing the trap. The pairing of vortices is revealed by the two-vortex spatial correlation function obtained from the vortex distribution. The vortex density decreases gradually as temperature is lowered, and below a certain temperature, a vortex-free region emerges in the center of the sample. This shows the crossover from a Berezinskii-Kosterlitz-Thouless phase containing vortex-pair excitations to a vortex-free Bose-Einstein condensate in a finite-size 2D system.