Interacting polarons

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Potassium atoms (yellow) surrounded by lithium atoms (blue) form polarons that i
Potassium atoms (yellow) surrounded by lithium atoms (blue) form polarons that interact with each other. © IQOQI Innsbruck/Harald Ritsch

In physics, the complex processes in solids are often described in terms of quasiparticles. In ultracold quantum gases, these quasiparticles can be reproduced and studied. Now, for the first time, Innsbruck scientists led by Rudolf Grimm have been able to observe in experiments how Fermi polarons - a special type of quasiparticle - can interact with each other. They report on this in the journal Nature Physics.

When an electron moves through a solid, it generates a polarization in its environment due to its electric charge. In his theoretical considerations, the Russian physicist Lev Landau extended the description of such particles to include their interaction with the environment and spoke of quasiparticles. More than ten years ago, the team led by Rudolf Grimm of the Institute of Quantum Optics and Quantum Information (IQQOI) of the Austrian Academy of Sciences (ÖAW) and the Institute of Experimental Physics at the University of Innsbruck succeeded for the first time in generating such quasiparticles in a quantum gas with both attractive and repulsive interactions with the environment. For this purpose, the scientists use an ultracold quantum gas consisting of lithium and potassium atoms in a vacuum chamber. They use magnetic fields to control the interactions between the particles, and high-frequency pulses to push the potassium atoms into a state in which they attract or repel the lithium atoms surrounding them. In this way, the researchers simulate a complex state such as that produced in the solid state by a free electron.

Insights into matter

Now, scientists led by Rudolf Grimm have been able to generate several such quasiparticles simultaneously in the quantum gas and observe their interactions with each other. "In a naive notion, one would assume that polarons always attract each other, regardless of whether their interaction with the environment is attractive or repulsive," says the experimental physicist. "However, this is not the case. We always see attractive interaction in bosonic polarons, repulsive interaction in fermionic polarons. This is where quantum statistics plays a crucial role." The researchers have now been able to demonstrate this behavior, which in principle already follows as a consequence of Landau’s theory, in an experiment for the first time. The theoretical calculations for this were done by colleagues from Mexico, Spain and Denmark. "High experimental skill was required for implementation in the laboratory," explains Cosetta Baroni, the study’s first author, "because even the smallest deviations could have falsified the measurements."

"Such investigations provide us with insights into very fundamental mechanisms of nature and offer us very good opportunities to study them in detail," says ERC and Wittgenstein Award winner Rudolf Grimm enthusiastically. The results have now been published in the journal Nature Physics. The research was financially supported by the European Union.

Publication: Mediated interactions between Fermi polarons and the role of impurity quantum statistics. Cosetta Baroni, Bo Huang, Isabella Fritsche, Erich Dobler, Gregor Anich, Emil Kirilov, Rudolf Grimm, Miguel A. Bastarrachea-Magnani, Pietro Massignan, Georg Bruun. Nature Physics 2023 DOI: 10.1038/s41567’023 -02248-4 [arXiv: 2305.04915 ]