Self-bound liquid droplets in free space

March 2018. The LENS team observed the formation of liquid-like droplets in a quantum gas of ultracold atoms in free space. These atomic systems result to be confined on a finite volume even in the absence of any external potential, due to a self-binding mechanism coming from the competition of attractive and repulsive forces in the system. Similarly to the work performed at ICFO, the experimental team at LENS studied quantum droplets in a homonuclear mixture of bosonic atoms, this time removing any external confinement thanks to a novel technique for the levitation of the cloud. In this condition the self-bound clusters that form are spherical in shape, differently also from the dipolar droplets studied in recent experiments [2,3]. The isotropic geometry makes mixture droplets in free space a unique system in the quantum realm. There exists indeed a regime where droplets are predicted to be zero temperature objects, meaning that they cannot live in an excited state, but they are able to dissipate energy by expelling atoms, a phenomenon called self evaporation. In this experiment the LENS group characterizes the properties of spherical droplets as well as the conditions for they formation, finding a very good agreement with the theoretical study that predicted the existence of this quantum phase [4]. This work sets the stage for future studies on spherical quantum droplets, including the predicted phenomenon of self evaporation and the investigation of the droplet excitation spectrum as well as its superfluid properties.

[1] G. Semeghini, G. Ferioli, L. Masi, C. Mazzinghi, L. Wolswijk, F. Minardi, M. Modugno, G. Modugno, M. Inguscio, and M. Fattori, Self-bound quantum droplets in atomic mixtures, arXiv:1710.10890 (2017).

[2] I. Ferrier-Barbut, H. Kadau, M. Schmitt, M. Wenzel, and T. Pfau, Observation of quantum droplets in a strongly dipolar Bose gas, Phys. Rev. Lett. 116, 215301 (2016).

[3] L. Chomaz, S. Baier, D. Petter, M. J. Mark, F. Wächtler, L. Santos, F. Ferlaino, Quantum-fluctuation-driven crossover from a dilute Bose-Einstein condensate to a macrodroplet in a dipolar quantum fluid, Phys. Rev. X 6, 041039 (2016).

[4] D. S. Petrov, Quantum mechanical stabilization of a collapsing Bose-Bose mixture, Phys. Rev. Lett. 115, 155302 (2015).

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