From a polaron into a cluster: the fate of an impurity in a Bose Einstein condensate

Ultracold atomic gases can be used to simulate phenomena from condensed matter physics, such as the formation of polaron quasiparticles. However, at strong coupling a Bose polaron formed by an impurity atom in a Bose Einstein condensate (BEC) displays fascinating behavior quite distinct from the common condensed matter scenario. This is due to the possibility of bound state formation and the Efimov effect, which leads to attractive impurity-mediated interactions between the bosons from the BEC. In particular, the intricate competition between this impurity-mediated attraction and the intrinsic interboson repulsion plays an important role. In our work [1] we fully incorporate both of these processes and compare different variational methods to form a complete theoretical picture of the strong coupling Bose polaron. We find two parameter regimes with qualitatively different behavior. For light impurities and weak repulsion, the impurity-mediated interactions dominate and we find a polaronic instability [2,3] due to the formation of large Efimov clusters. For strong repulsion or heavier impurities, we find that the formation of large clusters and the polaronic instability are prevented, and that there is a smooth crossover into a small cluster or molecule instead.