Self-bound quantum droplets of atomic mixtures observed

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We report on the observation of quantum liquid droplets in a bosonic mixture. While ultracold atomic systems are commonly found in a gas phase, recent theoretical and experimental results have surpringly pointed out that under special circumstances condensed atoms can form self-bound liquid-like droplets. At the origin of this new phase is the coexistence of repulsive and attractive forces that perfectly balance to generate the self-binding mechanism. The two competing energies are provided by the mean-field interaction and the first beyond mean-field correction, the so-called Lee-Huang-Yang term. We observe the existence of such self-bound ensembles in a bosonic mixture of K-39 atoms and we characterize their equilibrium properties. Quantum droplets are predicted to be macroscopic zero-temperature objects, due to their peculiar energy spectrum, where no discrete modes are expected below the particle emission threshold. The observation reported in this work certainly opens the way to further studies of the exotic properties of this new phase, which also constitutes the only known quantum liquid together with helium nanodroplets.

G. Semeghini, et al.,
Self-Bound Quantum Droplets of Atomic Mixtures in Free Space
Phys. Rev. Lett. 120, 235301 (2018)