Roton Excitations in an Oblate Dipolar Quantum Gas

TL;DR

This paper reports the experimental observation of radial and angular roton excitations in dipolar Bose-Einstein condensates, linking their softening to the crystallization transition through in situ measurements and theoretical comparison.

Contribution

It provides the first experimental characterization of roton excitations and their role in the crystallization process in dipolar quantum gases.

Findings

Identification of radial and angular roton modes via structure factor analysis

Observation of peak fluctuations indicating crystallization transition

Theoretical validation of roton mode softening as a mechanism for crystallization

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 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.