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Superfluidity in ultracold fermionic gases is a fascinating phenomenon. It allows for persistent currents, where a fluid flows without any energy losses. However, not every atom is allowed to participate in the "superflow" - only those which are able to find a partner with an opposite spin. Recently, physicists from Warsaw University of Technology revealed how the superfluid dynamics looks like in the gas where numbers of spin-up and spin-down atoms are imbalanced. As it might be experienced in your life – when you participate in a folk dance party where the numbers of men and women are imbalanced, someone would be frustrated as he/she cannot form a pair. So, what happens with unpaired atoms? The physicists found that they like to be sucked into various solitonic structures - objects that can maintain their shape while propagate in the system. Depending on amount of unpaired atoms in the system, decay patterns of solitonic objects changes dramatically. They end up with different stages, ``dark soliton'', ``vortex ring'' or ``vortex line'', determined by the spin polarization. Exploring a new degree of freedom of atomic gases: spin-polarization, brought scientists from Warsaw into a terra incognita - a field where new phases can appear and qualitative new effects emerge.

Link to publication: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.253002