Crossover from few-to many-body physics: The Efimov effect and the Bose polaron
Simple few-body physics can give rise to interesting emergent many-body effects. Even three particles can already display interesting emergent behavior: they can form a special type of trimer states even when dimer states are not yet possible. In our work we study how this so-called Efimov effect can in turn strongly influence the properties of larger many-body systems, in particular Bose-Einstein condensates (BECs). To this end, we variationally study the Bose polaron problem of an impurity immersed in a BEC [1,2]. We show that the attractive interactions between the bosons mediated by the impurity through the Efimov effect can lead to a collapse of the BEC onto the impurity. The onset of this instability can be interpreted as a shifted Efimov resonance, showing a remarkable connection between few-and many-body physics. When the bosons have an intrinsic repulsion this will compete with the impurity-mediated attraction. This leads to a transition from the regime of polaronic instability to a regime where the polaron will experience a smooth crossover into a small, but special cluster .
 A Christianen, JI Cirac, R Schmidt, Physical Review Letters 128 (18), 183401
 A Christianen, JI Cirac, R Schmidt, Physical Review A 105 (5), 053302
 A Christianen, JI Cirac, R Schmidt, in preparation