Rapid ramps across the BEC-BCS crossover: a route to measuring the superfluid gap

We investigate the response of superfluid Fermi gases to rapid changes of the three-dimensional s-wave scattering length a by solving the time-dependent Bogoliubov–de Gennes equations. In general the magnitude of the order parameter |Δ| performs oscillations, which are sometimes called the “Higgs” mode, with the angular frequency 2Δgap/ℏ, where Δgap is the gap in the spectrum of fermionic excitations. First, we excite the oscillations with a linear ramp of 1/a and study the evolution of |Δ|. Second, we continously drive the system with a sinusoidal modulation of 1/a. In the first case, the oscillations in |Δ| damp according to a power law. In the second case, the continued driving causes revivals in the oscillations. In both cases, the excitation of the oscillations causes a reduction in the time-averaged value of |Δ|. We propose two experimental protocols, based around the two approaches, to measure the frequency and damping of the oscillations, and hence Δgap.