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 $m^*$, 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. 
F. Scazza, G. Valtolina, P. Massignan, A. Recati, A. Amico, A.   Burchianti, C. Fort, M. Inguscio, M. Zaccanti, G. Roati
cond-mat.quant-gas physics.atom-ph quant-ph