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

TL;DR

This study explores the properties of Fermi and Bose polarons in a strongly interacting Fermi-Bose mixture, revealing their coexistence and the breakdown of the Fermi polaron model at high impurity densities.

Contribution

It demonstrates the simultaneous realization of Fermi and Bose polarons in a single ultracold mixture and investigates their behaviors across different impurity concentrations.

Findings

Fermi polaron energy remains insensitive to impurity concentration.

Breakdown of Fermi polaron description in the Bose-Einstein condensate.

Observation of a new spectral branch indicating Bose polaron formation.

Abstract

We investigate the properties of a strongly interacting imbalanced mixture of bosonic $^{41}$K impurities immersed in a Fermi sea of ultracold $^6$Li 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 a weak effective interaction between the polarons. The condensed fraction of the bosonic $^{41}$K 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 $^{6}$Li fermions inside the $^{41}$K 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.