Bose polarons in the strongly interacting regime
  When an impurity is immersed in a Bose-Einstein condensate, impurity-boson interactions are expected to dress the impurity into a quasiparticle, the Bose polaron. We superimpose an ultracold atomic gas of $^{87}$Rb with a much lower density gas of fermionic $^{40}$K impurities. Through the use of a Feshbach resonance and RF spectroscopy, we characterize the energy, spectral width and lifetime of the resultant polaron on both the attractive and the repulsive branches in the strongly interacting regime. The width of the polaron in the attractive branch is narrow compared to its binding energy, even as the two-body scattering length formally diverges. 
Ming-Guang Hu, Michael J. Van de Graaff, Dhruv Kedar, John P. Corson,   Eric A. Cornell, Deborah S. Jin
cond-mat.quant-gas quant-ph