I will present our recent investigations on ultracold Fermi gases with strong repulsive interactions employing reverse radio-frequency spectroscopy.
In a first experiment, recently published in Ref., we focus on a spin-imbalanced mixture, studying the properties of the so called repulsive polaron. In the regime of strong interactions, above a critical value of repulsion, the polaron energy is found to exceed the Fermi energy of the bath while the polaron effective mass diverges and even turns negative. Our findings indicate that the repulsive Fermi liquid state becomes energetically and thermodynamically unstable.
In a second ongoing experiment we focus on a balanced Fermi mixture. We use rapid radio-frequency transfers to quench repulsive interactions on a timescale faster than the pairing one. We probe the spectral response of the many-body system investigating both the growth of spin-correlation in the repulsive gas and the decay processes into a paired state.
 Scazza, F. et al. Phys. Rev. Lett. 118, 083602 (2017).