I will present experimental results on magnetic quantum fluids. These consist of a dilute Bose-Einstein condensate of dysprosium atoms, the most magnetic stable element. They allow to study the many-body consequences of the anisotropic and long-range dipole-dipole interaction, benefitting from the control tools of ultracold atomic physics.
First, we have observed in this system an unanticipated phase-transition between a gas and a liquid, characterized by the formation of self-bound droplets [1-3]. It forms in a parameter region where the existing theory, based on the mean-field approximation, predicted a mechanical collapse of the gas. We showed that the repulsive beyond meanfield corrections prevent the collapse and are responsible for the stabilization of the liquid [2]. These corrections arise from quantum fluctuations (zero-point motion) of the collective modes (Bogolyubov sound modes) in the quantum fluid.
In recent work we show that in constrained geometries, the ground-state is selforganized (left image). Studying these geometries experimentally, we indeed observe stable self-organized ‘stripe’ phases (right image), likely in metastable excited states. I will discuss the prospects for a strange kind of supersolidity in this system. In other experiments we study the effect of a rotating magnetic field on a quantum droplet, as a tool for the study of the different low-lying collective modes of the system.
[1] Observing the Rosensweig instability of a quantum ferrofluid, H. Kadau, M. Schmitt, M. Wenzel, C. Wink, T. Maier, I. Ferrier-Barbut, and T. Pfau, Nature 530, 194 (2016).
[2] Observation of quantum droplets in a strongly dipolar Bose gas, I. Ferrier-Barbut, H. Kadau, M. Schmitt, M. Wenzel, and T. Pfau, Phys. Rev. Lett. 116, 215301 (2016).
[3] Self-bound droplets of a dilute magnetic quantum liquid, M. Schmitt, M. Wenzel, F. Böttcher, I. Ferrier-Barbut and T. Pfau, Nature 539, 259 (2016).
