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Interacting Polarons: Simulations of Interacting Quasiparticles in Ultracold Quantum Gas


 Quasiparticles are an essential tool to describe the complex processes resulting from strong interactions in solids. These quasiparticles can be accurately simulated and studied in a radically different system, namely an ultracold quantum gas. Now scientists have been able to observe in a real experiment how quasiparticles called Fermi polarons interact with each other. Their findings have been published in Nature Physics.

An electron moving through a solid generates a polarization of its environment due to its electric charge. The famous Russian physicist Lev Landau introduced the concept of quasiparticles in the late 1950s to describe the joint entity formed by the electron and the surrounding cloud of excitations. More than ten years ago, a collaboration between theorists Pietro Massignan of the Department of Physics of the Universitat Politècnica de Catalunya (UPC), Georg Bruun of the Department of Physics and Astronomy of the University of Aarhus and the experimental team led by Rudolf Grimm at the Institute for Quantum Optics and Quantum Information (IQQOI) in Innsbruck successfully generated quasiparticles with both attractive and repulsive interactions with their environment. For this purpose, the team used a quantum gas of lithium and potassium atoms cooled to nanoKelvin temperatures in a high vacuum chamber. Using radio frequency pulses, they transferred very few potassium atoms into a state that experienced arbitrarily strong attractive or repulsive interactions with the surrounding lithium atoms. In this way, the researchers generated complex states similar to that produced by conduction electrons in a solid.

Now, the same team, together with theorist Miguel-Àngel Bastarrachea of the Universidad Autónoma Metropolitana-Iztapalapa, has been able to produce several quasiparticles simultaneously and observe their interactions with each other. The quantum statistics of impurities play a crucial role, and although one would assume that polarons always attract each other, the theory predicts attractive interactions between bosonic impurities but repulsive interactions between fermionic ones, as has now been shown clearly in a real study. experience this surprising change in behavior, which follows as a consequence of Landau's theory. The effect is extremely small and experimental uncertainties prevented its observation in previous experiments.

These investigations provide us with insights into very fundamental mechanisms of nature and offer us excellent opportunities to study them in detail.

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