Self bound droplets of a dilute magnetic quantum liquid
Self bound many body systems are formed through a balance of attractive and repulsive forces and occur in many physical scenarios. Liquid droplets are an example of a self bound system, formed by a balance of the mutual attractive and repulsive forces that derive from different components of the inter particle potential. It has been suggested that self bound ensembles of ultracold atoms should exist for atom number densities that are 108 times lower than in a helium droplet, which is formed from a dense quantum liquid. However, such ensembles have been elusive up to now because they require forces other than the usual zero range contact interaction, which is either attractive or repulsive but never both. On the basis of the recent finding that an unstable bosonic dipolar gas can be stabilized by a repulsive many body term, it was predicted that three dimensional self bound quantum droplets of magnetic atoms should exist. Here we report the observation of such droplets in a trap free levitation field. We find that this dilute magnetic quantum liquid requires a minimum, critical number of atoms, below which the liquid evaporates into an expanding gas as a result of the quantum pressure of the individual constituents. Consequently, around this critical atom number we observe an interaction driven phase transition between a gas and a self bound liquid in the quantum degenerate regime with ultracold atoms. These droplets are the dilute counterpart of strongly correlated self bound systems such as atomic nuclei and helium droplets.
Nature