Repulsive polarons in a strongly interacting Fermi mixture

Ultracold Fermi gases with tunable interactions represent a unique test bed to explore the many-body physics of strongly interacting quantum systems. In the past decade, experiments have mainly focused on the ground-state properties but metastable states in Fermi gases with strong repulsive interactions represent an exciting new frontier in the field. Here, we exploit radio-frequency spectroscopy to measure the complete excitation spectrum of fermionic $^{40}$K impurities resonantly interacting with a Fermi sea of $^{6}$Li atoms. In particular, we show that a well-defined quasiparticle exists for strongly repulsive interactions. For this ``repulsive polaron'' we measure its energy and its lifetime against decay. We also probe its coherence properties by measuring the quasiparticle residue. The results are well described by a theoretical approach that takes into account the finite effective range of the interaction in our system. We find that a non-zero range of the order of the interparticle spacing results in a substantial lifetime increase. This major benefit for the stability of the repulsive branch opens up new perspectives for investigating novel phenomena in metastable, repulsively interacting fermion systems.