title: Observation of thermally activated vortex pairs in a quasi-2D Bose gas.
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.
authors.affiliation: Center for Subwavelength Optics and Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea.
authors.forename: Jae-yoon
authors.initials: JY
authors.lastname: Choi
authors.forename: Sang Won
authors.initials: SW
authors.lastname: Seo
authors.forename: Yong-il
authors.initials: YI
authors.lastname: Shin