报告题目: |
Observation of thermally activated vortex pairs in a quasi-2D Bose gas |
报告人: |
Prof. Yong-il Shin |
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Seoul National University
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报告时间: |
2013-05-08 15:15 |
报告地点: |
Conference Hall 322, Science Building, Tsinghua University |
主办单位: |
高等研究院 |
简介: |
The Berezinskii-Kosterlitz-Thouless (BKT) theory provides a microscopic mechanism for the 2D phase transition, where vortices with opposite circulation are paired below a critical temperature. The BKT mechanism has been experimentally tested in many 2D systems, but there has been no direct observation of the vortex pairing in a 2D superfluid. In this talk, I will report on the observation of thermally excited vortex pairs in a trapped quasi-2D Bose gas. We measure the in-plane distribution of thermally activated vortices in a trapped quasi-2D Bose gase, where the visibility of density-depleted vortex cores is enhanced 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 represents a crossover from a Berezinskii-Kosterlitz-Thouless phase with vortex-pair excitations to a vortex-free Bose-Einstein condensate in a finite-size 2D system.
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