Observation of a 2D Bose-gas: from thermal to quasi-condensate to superfluid

TL;DR

This paper reports experimental observations of a quasi-2D Bose gas near the BKT transition, identifying the transition from thermal to quasi-condensate and superfluid states through density and coherence measurements.

Contribution

First experimental evidence of the superfluid transition in a quasi-2D Bose gas near the BKT transition, using in situ density and coherence measurements.

Findings

Observation of bimodal distribution without long-range order

Longer coherence length in the quasi-condensed state

Experimental identification of the BKT superfluid transition

Abstract

We present 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, \textit{in situ} and after time of flight, and the coherence length, 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, the gas presents a longer coherence length than the thermal cloud; it is quasi-condensed but is not superfluid. Experimental evidence indicates that we observe the superfluid transition (BKT transition).