Measurement of the excitation spectrum of a dipolar gas in the macrodroplet regime

TL;DR

This study uses Bragg spectroscopy to investigate the excitation spectrum of a dipolar quantum gas transitioning into a macrodroplet, revealing stiffening of modes and multi-band responses consistent with theoretical predictions.

Contribution

First experimental observation of excitation spectrum features in a dipolar gas near the macrodroplet regime, confirming theoretical models with finite size effects.

Findings

Upward shift of low momentum excitation energies

Multi-band response at high momenta

Confirmation of mode stiffening near the macrodroplet regime

Abstract

The excitation spectrum of a cigar-shaped strongly dipolar quantum gas at the crossover from a Bose-Einstein condensate to a trapped macrodroplet is predicted to exhibit peculiar features - a strong upward shift of low momentum excitation energies together with a strong multi-band response for high momenta. By performing Bragg spectroscopy over a wide range of momenta, we observe both key elements and also confirm the predicted stiffening of excitation modes when approaching the macrodroplet regime. Our measurements are in good agreement with numerical calculations taking into account finite size effects.