Superfluid Fraction of a 2D Bose-Einstein Condensate in a Triangular Lattice

TL;DR

This paper experimentally measures the superfluid fraction of a 2D Bose-Einstein condensate in a triangular lattice using two novel methods, confirming results with simulations and theoretical bounds.

Contribution

It introduces two independent methods to determine the superfluid fraction in a 2D BEC within an optical lattice, validated by experiments and simulations.

Findings

Consistent superfluid fraction results from hydrodynamic and dynamical methods.

Agreement with Gross-Pitaevskii simulations.

Superfluid fraction respects Leggett bounds.

Abstract

We experimentally investigate the superfluid properties of a two-dimensional, weakly interacting Bose-Einstein condensate in the zero-temperature regime, when it is subjected to a triangular optical lattice potential. We implement an original method, which involves solving the hydrodynamic continuity equation to extract the superfluid fraction tensor from the measured in situ density distribution of the fluid at rest. In parallel, we apply an independent dynamical approach that combines compressibility and sound velocity measurements to determine the superfluid fraction. Both methods yield consistent results in good agreement with simulations of the Gross-Pitaevskii equation as well as with the Leggett bounds determined from the measured density profiles.