Jae-yoon Choi, Sang Won Seo, Yong-il Shin
Understanding the emerging mechanisms of superfluidity has been a central theme in many-body physics. In particular, the superfluid state in a two-dimensional (2D) system is intriguing because large thermal fluctuations prohibit the formation of long-range order and consequently the picture of Bose-Einstein condensation is not applicable to the phase transition. The Berezinskii-Kosterlitz-Thouless (BKT) theory provides a microscopic mechanism for the superfluid phase transition, where vortices with opposite circulation are paired below the critical temperature, establishing quasi-long-range phase coherence. While this mechanism has been tested experimentally, there has been no direct observation of thermal vortex pairs in a 2D superfluid. Here we report the observation of a BKT superfluid in a trapped quasi-2D atomic gas by measuring the spatial distribution of thermally activated vortices and revealing their pair correlations. The vortex population concentrates in the low-density superfluid region of the trapped sample and decreases gradually as the temperature is lowered, showing a crossover from a BKT phase to a Bose-Einstein condensate with no thermal vortices. These observations clarify the nature of the superfluid state of a trapped 2D Bose gas.
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http://arxiv.org/abs/1211.5649
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