The collapse of the Fermi surface for strongly interacting fermions in the BCS-BEC crossover

The Fermi surface - a discontinuity in the momentum distribution - is a hallmark of fermionic ensembles. Its existence even for interacting systems forms the basis for Landau’s celebrated Fermi liquid theory. Understanding under what circumstances the Fermi surface collapses and Fermi liquid theory breaks down is one of the long-standing challenges in condensed matter physics. We study this question experimentally with a strongly interacting ultracold Fermi gas in the BCS-BEC crossover regime. The nature of the normal state of the gas in the BCS-BEC crossover is an intriguing and controversial topic, both experimentally and theoretically. While the many-body ground state is always a superfluid 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. We measure the distribution of single-particle energies and momenta (spectral function) in a homogeneous gas above Tc. Fits to data taken for different interaction strengths reveal the onset of pairing and decreasing spectral weight (or quasiparticle residue, Z) for the Fermi liquid. We extract the effective mass m, pair correlation length, and Tan’s contact. Surprisingly, we find that Z vanishes abruptly and not gradually with increasing interaction, which signals a sudden breakdown of a Fermi liquid description and the disappearance of the Fermi surface.