Roton Excitations in an Oblate Dipolar Quantum Gas*

Elementary excitations leave a characteristic footprint in the fluctuations of a quantum mechanical system. We report on first signatures of radial and angular roton excitations around a droplet crystallization transition in oblate dipolar Bose-Einstein condensates by direct in situ measurements of the density fluctuations near this transition. This approach allows for a direct extraction of the static structure factor simultaneously at all momenta, which we use to identify the radial and angular excitations by the characteristic symmetries of their spatial patterns. The fluctuations peak as a function of interaction strength indicating the crystallization transition of the system. We connect the crystallization mechanism with the softening of the angular roton modes by comparing our observations to a theoretically calculated excitation spectrum. This understanding is an important step towards the realization of a dipolar supersolid in two-dimensional oblate trapping geometries.