Title: Crossover from few-to many-body physics: The Efimov effect and the Bose polaron

Abstract: Simple few-body physics can give rise to interesting emergent many-body effects. One of the smallest emergent effects one can imagine is the Efimov effect, which describes how three particles with pairwise interactions help each other to form a special type of three-body bound states. Studying the Bose polaron problem of an impurity immersed in a Bose-Einstein condensate, we demonstrate how Efimov physics can strongly influence the properties of a many-body system. We use a variational method based on Gaussian States to show that the formation of large Efimov clusters can lead to an instability of the polaron. The onset of this instability can be interpreted as a many-body shifted Efimov resonance, where the shift originates from the participation of the polaron cloud in the bound state formation. Since Efimov resonances and the formation of Efimov clusters lead to a strongly enhanced recombination rate, this represents a remarkable example of how chemistry can be modified by a quantum medium, an effect accessible for experimental study with ultracold atoms. Finally, I will discuss how adding interboson repulsion into the model can lead to an interesting interplay between this repulsion and the attractive interactions mediated by the impurity. For strong enough repulsion the polaronic instability can be prevented and instead a smooth crossover into a small molecule is predicted.

References:
[1] A. Christianen, J.I. Cirac, R. Schmidt, arxiv:2108.03174
[2] A. Christianen, J.I. Cirac, R. Schmidt, arxiv:2108.03175