Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture

Abstract

We investigate the properties of a strongly interacting imbalanced mixture of bosonic K41 impurities immersed in a Fermi sea of ultracold Li6 atoms. This enables us to explore the Fermi polaron scenario 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 and interspecies interactions are widely tunable by means of a well-characterized 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 a theoretical prediction, based on Landau's quasiparticle theory, of weak effective interaction between the polarons. The condensed fraction of the bosonic K41 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 Li6 fermions inside the K41 condensate. A closer investigation of the behavior of the condensate by means of Rabi oscillation measurements supports this observation, indicating that we have realized Fermi and Bose polarons, two fundamentally different quasiparticles, in one cloud.