Higgs oscillations in a unitary Fermi superfluid

Ultracold atomic gases with tunable interactions provide a versatile setting to explore quantum many-body systems out of equilibrium. Here, we study the dynamics in a two-component strongly interacting Fermi gas following a quench of the inter-atomic interactions. Small quenches within the superfluid phase excite Higgs oscillations of the superfluid order parameter. We observe these directly using Bragg spectroscopy which measures the imaginary part of the density-density response function. Our measurements at unitarity show a strong temperature dependence, and the oscillations decay according to a power law with a damping exponent approximately midway between the well-known Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein Condensate (BEC) limits. As the interactions are tuned through the BCS-BEC crossover regime, we observe a monotonic change of the oscillation frequency reflecting the evolution of the pair-breaking energy, as well as the damping exponent. Our data show good qualitative agreement with the time-dependent BCS theory.