Critical Point of an Interacting Two-Dimensional Atomic Bose Gas

TL;DR

This study measures the critical atom number in 2D Bose gases and finds it significantly exceeds ideal predictions, highlighting the importance of interactions and the Berezinskii-Kosterlitz-Thouless theory in understanding 2D superfluidity.

Contribution

The paper provides experimental evidence that the critical atom number in 2D Bose gases deviates from ideal theory and introduces a heuristic model based on BKT theory to explain this.

Findings

Critical atom number is about five times higher than ideal gas prediction.

Conventional BEC theory does not apply to interacting 2D gases.

BKT-based heuristic model explains the experimental results.

Abstract

We have measured the critical atom number in an array of harmonically trapped two-dimensional (2D) Bose gases of rubidium atoms at different temperatures. We found this number to be about five times higher than predicted by the semi-classical theory of Bose-Einstein condensation (BEC) in the ideal gas. This demonstrates that the conventional BEC picture is inapplicable in an interacting 2D atomic gas, in sharp contrast to the three-dimensional case. A simple heuristic model based on the Berezinskii-Kosterlitz-Thouless theory of 2D superfluidity and the local density approximation accounts well for our experimental results.