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Q: Fachverband Quantenoptik und Photonik

Q 4: Quantum gases (Fermions) I

Q 4.4: Vortrag

Montag, 9. März 2020, 11:45–12:00, e214

Observing the Few-Body Precursor of a Higgs Mode — •Luca Bayha, Marvin Holten, Keerthan Subramanian, Ralf Klemt, Philipp Preiß, and Selim Jochim — Physics Institute, Heidelberg University, Germany

The competition between two scales gives rise to rich physics. For example, the finite size of mesoscopic systems results in single particle gaps and shell structures, well known from nuclear and atomic physics. Here I will present our progress on realizing such mesoscopic systems with ultracold fermions. We can prepare closed shells of up to 12 fermions in the ground state of a two dimensional trap. With the help of a Feshbach resonance we tune the interactions from a single–particle gap regime to a pairing regime. For filled shells there is a minimal required attraction for pairing to overcome the single–particle gap.
In the thermodynamic limit, the onset of pairing with increasing interaction gives rise to a quantum phase transition into the superfluid state. This is accompanied by an undamped Higgs mode. Remarkably, we observe a precursor of this mode already in the mesoscopic system consisting of six fermions. The lowest monopole excitation shows mode softening as function of the interaction strength. The non-monotonicity is the few body analogue of the gap closing at the phase transition. By measuring the atom number distribution of the excitation, we can show that it is a pair excitation as expected for the Higgs mode, since the order parameter is the pair density.