This paper presents experimental results on a Bose gas in a quasi-2D geometry near the Berezinskii,Kosterlitz and Thouless (BKT) transition temperature. By measuring the density profile, in situ and after time of flight, and the coherence length of the gas we identify different states of the gas. In particular, we observe that the gas develops a bimodal distribution without long range order. In this state, which we call a fractured quasi- condensate, the gas is not superfluid, but presents a longer coherence length than the thermal cloud. Experimental evidence seems to indicate that we are observing the transition from the fractured quasi-condensate towards superfluid (BKT transition).
Citation: Physical Review Letters
Pub Type: Journals
BEC (Bose-Einstein Condensate), BKT (Berezinskii Kosterlitz Thouless), Bose-gas, coherence length, crossover transition, quasi-condensate, superfluid, transition, 2-D (two-dimensional)