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

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 $m^{\ast }$, the residue $Z$ and the decay rate $\mathrm{\Gamma }$. Above a critical interaction, $E_{+}$ is found to exceed the Fermi energy of the bath while $m^{\ast }$ diverges and even turns negative, thereby indicating that the repulsive Fermi liquid state becomes energetically and thermodynamically unstable.