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Making sound with strongly interacting matter

Article chosen as Editors Suggestion and highlighted in a Viewpoint in Physcial Review letters.

15.04.2020 | Line Lindskov Lassen

Figure Caption: A sound wave propagating in a Fermi gas, where strong interactions between two spin components lead to the formation of Cooper pairs and hence a superfluid phase.

An interactional collaboration involving Georg M. Bruun have explored the propagation of sound in a strongly Fermi gas. By carefully comparing experimental results with theoretical calculations, they were able to infer detailed information concerning the properties of the system as it undergoes a phase transition between a normal and a superfluid phase. Their article was chosen as Editors Suggestion and highlighted in a Viewpoint in Physcial Review letters.

The collaboration explored sound in a Fermi gas consisting of Li-atoms with an interaction as strong as allowed by quantum mechanics. Such a system is particularly interesting to study, since interaction effects make its properties “universal” in the sense that they are very similar to other types of seemingly very different strongly interacting matter including liquid Helium, electrons in solids, neutron stars, and quark-gluon matter. Sound modes in the gas were excited by standing waves created by laser beams and their frequency and damping were subsequently measured. The damping was observed to decrease abruptly with temperature as the system entered a superfluid phase. This was explained theoretically from the fact that its friction is dramatically reduced in the superfluid phase due to Cooper pairing. The results will guide scientists in the exploration of transport in strongly interacting quantum matter – a topic of great importance across many energy scales in physics.

Original publication: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.150401

Viewpoint article: https://physics.aps.org/articles/v13/53

Institut for Fysik og Astronomi, Offentligheden / Pressen, Medarbejdere