Measurement of the Excitation Spectrum of a Dipolar Gas in the Macrodroplet Regime

The excitation spectrum of a cigar-shaped strongly dipolar quantum gas at the crossover from a BoseEinstein condensate to a trapped macrodroplet is predicted to exhibit peculiar features-a strong upward shift of low momentum excitation energies together with a strong multiband 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.