Dipolar interactions are fundamentally different from the usual van der Waals forces in real gases. Besides the anisotropy the dipolar interaction is nonlocal and as such allows for self organized structure formation . In 2015 we could observe the formation of a stable droplet crystal and found that this unexpected stability is due to beyond mean-field quantum corrections of the Lee-Huang-Yang type. When arranged in a 1D array also phase coherence between the droplets was observed, which was first evidence for a supersolid state of matter. Upon crossing the transition to the dipolar supersolid a Goldstone mode appears, which we have observed. The existence of this mode proofs the superfluid stiffness or the so-called phase rigidity of this new state of matter. We have also studied the static structure factor across the transition which allows to show that the characteristic fluctuations correspond to elementary excitations such as the roton modes, and that the supersolid state supports both superfluid as well as crystal phonons. A recent review on the discovery of quantum droplets and dipolar supersolids can be found in ref. .
 review: T. Lahaye, et al., Rep. Prog. Phys. 72, 126401 (2009)
 review: F. Böttcher et al. Reports on Progress in Physics 84, 012403 (2021)
|Presenter name||Tilman Pfau|
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