Fate of an impurity strongly interacting with a thermal Bose gas

We spectroscopically study mobile impurities immersed in a homogeneous bosonic bath (a box-trapped Bose gas), varying the bath temperature and the strength of impurity-bath interactions. We compare our results to those for a quasipure Bose-Einstein condensate (BEC), and find that for strong impurity-bath interactions, the spectra narrow with increasing temperature, while the impurity energy shift is suppressed. Near the critical temperature for condensation, many-body effects still play an important role, and only for a nondegenerate bath, the system approaches the classical Boltzmann-gas behavior. The key spectral features are reproduced within the theory of an ideal Bose polaron.