Semeghini, Giulia

Date:   Wed, June 20, 2018
Time:   11:45
Place:  ETH Zurich, Hönggerberg, HPL D 34
Host:   Tilman Esslinger

Quantum liquid droplets in a Bose-Bose mixture of ultracold atoms

Giulia Semeghini
LENS, University of Florence, Italy

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. In a condensed Bose-Bose mixture the two competing energies are provided by the mean-field (MF) interaction and the first beyond mean-field correction, the so-called Lee-Huang-Yang (LHY) term. When the attraction between the two atomic species becomes larger than the average repulsion in the single species, the condensate is expected to collapse according to MF theory. In this regime, instead, the positive LHY contribution arises to arrest collapse and stabilize the system. In our experiment we first 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.