Repulsive Fermi polarons in a resonant mixture of ultracold ${}^6$Li atoms

TL;DR

This study uses radio-frequency spectroscopy to explore repulsive Fermi polarons in a strongly interacting ultracold ${}^6$Li mixture, revealing quasiparticle properties and instabilities at high interaction strengths.

Contribution

It provides the first detailed characterization of repulsive Fermi polarons near a broad Feshbach resonance in ultracold ${}^6$Li, including energy, effective mass, residue, and decay rate measurements.

Findings

Well-defined quasiparticles observed at strong repulsive interactions

Polaron energy exceeds Fermi energy at high interactions

Effective mass diverges and becomes negative, indicating instability

Abstract

We employ radio-frequency spectroscopy to investigate a polarized spin-mixture of ultracold ${}^6$Li atoms close to a broad Feshbach scattering resonance. Focusing on the regime of strong repulsive interactions, we observe well-defined coherent quasiparticles even for unitarity-limited interactions. We characterize the many-body system by extracting the key properties of repulsive Fermi polarons: the energy $E_+$, the effective mass $m^*$, the residue $Z$ and the decay rate $\Gamma$. Above a critical interaction, $E_+$ is found to exceed the Fermi energy of the bath while $m^*$ diverges and even turns negative, thereby indicating that the repulsive Fermi liquid state becomes energetically and thermodynamically unstable.