Excitations of a Superfluid in a 3D Optical Lattice

TL;DR

This paper investigates the excitation spectrum of a superfluid Bose-Einstein condensate in a 3D optical lattice, revealing interaction-dependent responses and an unexpected resonance at stronger interactions.

Contribution

It introduces a method to study superfluid excitations in a 3D optical lattice with tunable interactions and reports novel resonance phenomena.

Findings

Weak interactions show low susceptibility to excitations.

Stronger interactions lead to an unexpected resonance.

Coherent fraction measurements relate to condensate depletion.

Abstract

We prepare a Bose-Einstein condensed gas in a three-dimensional optical lattice and study the excitation spectrum of the superfluid phase for different interaction strengths. We probe the response of the system by modulating the depth of the optical lattice along one axis. The interactions can be controlled independently by varying the tunnel coupling along the other two lattice axes. In the weakly interacting regime we observe a small susceptibility of the superfluid to excitations, while for stronger interactions an unexpected resonance appears in the excitation spectrum. In addition we measure the coherent fraction of the atomic gas, which determines the depletion of the condensate.