Small polarons in dilute gas Bose-Einstein condensates
  A neutral impurity atom immersed in a dilute Bose-Einstein condensate (BEC) can have a bound ground state in which the impurity is self-localized. In this small polaron-like state, the impurity distorts the density of the surrounding BEC, thereby creating the self-trapping potential minimum. We describe the self-localization in a strong coupling approach. 
Fernando M. Cucchietti and Eddy Timmermans