In our project, we showed that a dipolar superfluid significantly differs from a non-dipolar superfluid. We have demonstrated that the ability of the superfluid to flow without friction depends on the direction of the flow compared to an external magnetic field. The characteristic behavior of a superfluid is that it can flow without friction but only below a certain velocity, above it there is friction. What we demonstrated was that in a dipolar superfluid the maximal velocity depends on the flow direction. This is a very clear example of how magnetic interactions alter the properties of quantum ferrofluids. A second important result that we obtained was the discovery of a new liquid phase of matter. This liquid is very different from usual liquids, it arises directly from magnetic interactions and the quantum nature of the fluid. A ususal liquid exists because its constituent atoms or molecules attract each other when they are at a distance, and repel each other when they come very close. Thus a liquid is self-bound. The liquid we discovered is bound by the magnetic interactions between the dysprosium atoms: the atoms act as magnets that are placed head-to-tails, and thus attract each other. However, what was expected was a collapse due to this attraction and not the formation of a liquid because no mechanism for repulsion was known. We observed a repulsion that prevents the collapse, and showed that this repulsion comes from the fact that the fluid is behaving according to the laws of quantum mechanics. It dictates that the fluid can never be fully at rest, but "fluctuates" due to what is known as Heisenberg's uncertainty principle. These fluctuations increase as the liquid becomes denser, and eventually prevent the collapse. We have shown that the result is a liquid, which forms self-bound droplets that do not expand in free space as opposed to what a gas would do. What is remarkable is that these droplets are very elongated along the magnetic field, because this is the direction in which atoms attract each other. Finally, we aimed at understanding if spontaneous structure formation can occur in dipolar superfluids of dysprosium, and how it alters the superfluid properties. We have provided the first demonstration that indeed, magnetic interactions can induce spontaneous structure formation. We have shown that not only magnetic interaction but also external forces applied to the superfluid must be controlled. Indeed we demonstrated that the magnetic atoms must lie mostly side-by-side rather than head-to-tails in order to obtain spontaneous structure. Using external forces applied using lasers, we could control the arrangement of the atoms, and switch the structure formation on or off. Using this control, it became possible to systematically study self-structured superfluids. We then could study if superfluidity survives structure formation in order to form a supersolid. What we observed was that in fact in our experiments superfluidity is globally destroyed by spontaneous structure formation. We could show that locally, superfluidity exists, but throughout the whole sample there can be no flow without friction. Rather than bringing a definitive negative answer for the existence of a supersolid in dipolar superfluids, we explored theoretically how one could modify the system in order to obtain a supersolid. We showed that if one performs experiments tuning the external forces applied, there should be a supersolid state. We identified the difficulties in reaching this superfluid state in actual experiments, and highlighted how one might be able to circumvent them. This result is a major step in the search for a dipolar supersolid, because prior to our investigation, there was no such predictions, and it remained unknown wether such a supersolid might exist, even in theory. These results were disseminated in several scientific peer-reviewed papers in international journals (Nature, Physical Review…), and through invited presentations in various international conferences. In addition, we explored possibilities to communicate our results to the non-specialists. In particular we designed a simple exhibition experiment that was used in science outreach events by “Spiel der Kräfte”, at the 5. Physikalisches Institut.
