Few-body precursor of the Higgs mode in a superfluid Fermi gas - from theory to experiments.*

We demonstrate the presence of an undamped few-body precursor of the Higgs mode in an ultracold trapped Fermi gas.
Here, the lowest excitation mode frequency has non-monotonical dependence on the interaction strength, attaining a minimum in the crossover region.
In the many-body limit, the Higgs mode appears as a zero-energy excitation mode at the quantum phase transition point between the superfluid and the normal phase.
In the few-body limit, we observe an energy minimum that deepens with increasing particle number, reflecting the fact that we see a few-body Higgs mode analog which develops towards a zero-energy excitation.
This hallmark of the Higgs mode is persistent also for weakly anharmonic and anisotropic traps, and it can readily be observed in a new generation of microtraps where the Higgs mode is selectively excited by modulating the interaction strength via Feshbach resonances.
Using variations of the Hartree-Fock Bogoliubov method combined with exact diagonalization, we track the development of the few-body mode into the regime of N~100 particles.