Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms

TL;DR

This paper reports the experimental observation of Fermi polarons in ultracold atoms, characterizing their energy, quasiparticle residue, and a phase transition from polaronic to molecular states near a Feshbach resonance.

Contribution

First direct observation of Fermi polarons in a tunable ultracold Fermi liquid, including measurements of their properties and phase transition behavior.

Findings

Identification of a narrow peak indicating polarons in rf spectra

Measurement of polaron energy and quasiparticle residue across interactions

Observation of a phase transition from polaronic to molecular binding at a critical interaction

Abstract

We have observed Fermi polarons, dressed spin down impurities in a spin up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities' rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid coexisting with a Fermi sea.