Observation of the Presuperfluid Regime in a Two-Dimensional Bose Gas

TL;DR

This paper reports the first observation of non-meanfield pre-superfluid behavior in a two-dimensional Bose gas, highlighting the divergence of compressibility and the emergence of a momentum-space spike as the system approaches degeneracy.

Contribution

It provides experimental evidence of pre-superfluid phenomena in a 2D Bose gas, revealing non-meanfield physics before superfluidity sets in.

Findings

Compressibility diverges from bare-atom model predictions as density increases.

A spike at zero momentum appears in the momentum distribution.

Non-meanfield effects are observed prior to superfluid transition.

Abstract

In complementary images of coordinate-space and momentum-space density in a trapped 2D Bose gas, we observe the emergence of pre-superfluid behavior. As phase-space density $\rho$ increases toward degenerate values, we observe a gradual divergence of the compressibility $\kappa$ from the value predicted by a bare-atom model, $\kappa_{ba}$. $\kappa/\kappa_{ba}$ grows to 1.7 before $\rho$ reaches the value for which we observe the sudden emergence of a spike at $p=0$ in momentum space. Momentum-space images are acquired by means of a 2D focusing technique. Our data represent the first observation of non-meanfield physics in the pre-superfluid but degenerate 2D Bose gas.