I. Fritsche, C. Baroni, E. Dobler, E. Kirilov, B. Huang, R. Grimm, G. Bruun, P. Massignan Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture,
Phys. Rev. A 103 53314 (2021-05-17),
http://dx.doi.org/10.1103/PhysRevA.103.053314 doi:10.1103/PhysRevA.103.053314 (ID: 720640)
We investigate the properties of a strongly interacting imbalanced mixture of bosonic 41K impurities immersed in a Fermi sea of ultracold 6Li atoms. This enables us to explore the Fermi polaron for large impurity concentrations including the case where they form a Bose-Einstein condensate. The system is characterized by means of radio-frequency injection spectroscopy for tunable interactions using an interspecies Feshbach resonance. We find that the energy of the Fermi polarons formed in the thermal fraction of the impurity cloud remains rather insensitive to the impurity concentration, even as we approach equal densities for both species. The apparent insensitivity to high concentration is consistent with the theoretical prediction, based on Landau's quasiparticle theory, of a weak effective interaction between the polarons. The condensed fraction of the bosonic 41K gas is much denser than its thermal component, which leads to a break-down of the Fermi polaron description. Instead, we observe a new branch in the radio-frequency spectrum with a small energy shift, which is consistent with the presence of Bose polarons formed by 6Li fermions inside the 41K condensate. A closer investigation of the behavior of the condensate by means of Rabi oscillation measurements support this observation, indicating that we have realized Fermi and Bose polarons, two fundamentally different quasiparticles, in one cloud.