Breakdown of the Fermi Liquid Description for Strongly Interacting Fermions

The nature of the normal state of an ultracold Fermi gas in the BCS-BEC crossover regime is an intriguing and controversial topic. While the many-body ground state remains a condensate of paired fermions, the normal state must evolve from a Fermi liquid to a Bose gas of molecules as a function of the interaction strength. How this occurs is still largely unknown. We explore this question with measurements of the distribution of single- particle energies and momenta in a nearly homogeneous gas above T-c. The data fit well to a function that includes a narrow, positively dispersing peak that corresponds to quasiparticles and an incoherent background that can accommodate broad, asymmetric line shapes. We find that the quasiparticle's spectral weight vanishes abruptly as the strength of interactions is modified, which signals the breakdown of a Fermi liquid description. Such a sharp feature is surprising in a crossover.