ISBN 9783843950091

978-3-8439-5009-1, Reihe Physik

Jan-Niklas Schmidt
Density Fluctuations in a Dipolar Quantum Gas - from Superfluids to Supersolids

169 Seiten, Dissertation Universität Stuttgart (2022), Softcover, A5

Zusammenfassung / Abstract

In this thesis, we investigate density fluctuations in dipolar quantum gases across the phase transition from a superfluid Bose-Einstein condensate to a supersolid. A supersolid is a paradoxical state of matter that combines the frictionless flow of a superfuid with the crystalline order of a solid. This elusive state was recently found in an array of dipolar quantum droplets. These quantum droplets exist in dipolar quantum gases due to a precise interplay between attractive binary interactions and repulsive quantum fluctuations. The quantum droplets are self-bound and can self-assemble into elongated chains in the presence of a harmonic confinement. Within such a elongated chain of quantum droplet the density overlap between neighbors can establish a global common phase. We study density fluctuations in this system and identify softening roton modes as the key driver for the crystallization mechanism that induces a dipolar supersolid. Our study reveals the supersolid state to simultaneously support excitation modes that are characteristic for a superfluid and for a crystal, highlighting its dual nature.

In addition, we generalize the geometry and study the spectrum of collective excitations in circularly symmetric traps by investigating the excitations' characteristic footprint on the density fluctuations. We observe a significantly enriched spectrum featuring angular and radial roton modes and reveal their key role in the nature of the crystallization transition to a two-dimensional droplet array.