Periodic Reporting for period 1 - DipInQuantum (Dipolar quantum gases of Dysprosium)
Période du rapport: 2016-04-01 au 2018-03-31
Our project was aimed at investigating dipolar superfluids. A superfluid is a fluid which shows extraordinary properties like the ability to flow without friction. These properties arise at very low temperatures, because these fluids are not described by the usual laws of room temperature but obey quantum mechanics. In a dipolar superfluid, the particles composing the fluid are magnetic. They then interact with each other like magnets, which modify the properties of the fluid. The effects of magnetic interactions on normal fluids have been studied in the past, these magnetic fluids are called ferrofluids. Our project was then aimed at exploring how the magnetic interaction can modify the properties of superfluids which one can then call "quantum ferrofluids", and how these interactions can lead to new effects and new phases of matter. A famous example in ferrofluids is the Rosensweig instability, where an external magnetic field induces a stable pattern of peaks and valleys at the surface of a ferrofluid. In the context of superfluids, this spontaneous appearance of structure raises a very fundamental question: Can a system become ordered, and still remain superfluid in the sense that matter can flow without friction? This quest for this state that is called a "supersolid" is was one of the motivations for our project. Specifically we have used Dysprosium atoms which have very strong magnetic properties and can be cooled to very low temperatures where they form a superfluid. This basic research is driven by the general goal of harnessing quantum effects in future application. Understanding superfluidity creates a strong theoretical knowledge. This knowledge will then serve as a basis in the search for applications were the extraordinary properties studied could be used in settings amenable to real-life applications. In addition this research is very competitive and technically challenging, as a consequence, the training received both by the Experienced Researcher (ER) and graduate students working on the project will clearly benefit the European society and economy.
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.
Scientifically, our results extend far beyond the state of the art prior to our work in the project. The search for superfluidity in dipolar superfluids was ignited by our work, and the new liquid that we observed has attracted high attention and established a new field of research since several independent teams reproduced and extended our observations to other systems. Our work led us to collaborate with researchers in different European countries reinforcing the research links within Europe in our field. The ER could build a strong network of collaborations which will be highly beneficial for European research in the future and allowed him to secure a permanent position in academic research. The benefits from the training received through the project will be measurable in the long term in future research and teaching. Several graduate student that were trained performing research on the project and mentored by the ER went on to start successful careers in the private sector in high-tech companies, thus benefiting the European society and economy in a broader sense.