Roton Excitations in an Oblate Dipolar Quantum Gas
Jan-Niklas Schmidt, Jens Hertkorn, Mingyang Guo, Fabian Böttcher, Matthias Schmidt, K. S. H. Ng, Sean Graham, Tim Langen, Martin W. Zwierlein, Tilman Pfau
Abstract
We observe signatures of radial and angular roton excitations around a droplet crystallization transition in dipolar Bose-Einstein condensates. In situ measurements are used to characterize the density fluctuations near this transition. The static structure factor is extracted and used to identify the radial and angular roton excitations by their characteristic symmetries. These fluctuations peak as a function of the interaction strength indicating the crystallization transition of the system. We compare our observations to a theoretically calculated excitation spectrum allowing us to connect the crystallization mechanism with the softening of the angular roton modes.