Multi-impurity polarons in a dilute Bose-Einstein condensate
  We describe the ground state of a large, dilute, neutral atom Bose- Einstein condensate (BEC) doped with N strongly coupled mutually indistinguishable, bosonic neutral atoms (referred to as "impurity") in the polaron regime where the BEC density response to the impurity atoms remains significantly smaller than the average density of the surrounding BEC. We find that N impurity atoms (N is not one) can self-localize at a lower value of the impurity-boson interaction strength than a single impurity atom. When the 'bare' short-range impurity-impurity repulsion does not play a significant role, the self-localization of multiple bosonic impurity atoms into the same single particle orbital (which we call co-self-localization) is the nucleation process of the phase separation transition. When the short-range impurity-impurity repulsion successfully competes with co-self-localization, the system may form a stable liquid of self-localized single impurity polarons. 
D. H. Santamore and Eddy Timmermans
cond-mat.stat-mech cond-mat.quant-gas physics.atom-ph