Repulsive Fermi polarons in the universal mass-balanced broad-resonance case

The Fermi polaron represents a fundamental problem in many-body physics. In particular, repulsive Fermi polarons are centrally important for understanding the whole phase diagram of the repulsive Fermi gas and for realizing repulsive many-body states. We employ radio-frequency spectroscopy to investigate spin-mixtures of ultracold Li-6 atoms with tunable polarization in the vicinity of a broad Feshbach resonance. We report on the observation of well-defined coherent quasiparticles up to unitarity-limited interactions. We characterize the many-body system by extracting the key properties of repulsive Fermi polarons: the energy E$_{\mathrm{+}}$, the effective mass m$^{\mathrm{\ast }}$, the residue $Z$ and the decay rate $\Gamma $. Above a critical interaction, we find E$_{\mathrm{+}}$ to exceed the Fermi energy of the bath while m$^{\mathrm{\ast }}$ diverges and even turns negative, revealing an instability of the repulsive Fermi liquid.