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Q: Fachverband Quantenoptik und Photonik

Q 9: Cold atoms II - interactions (joint session A/Q)

Q 9.4: Talk

Monday, March 5, 2018, 11:15–11:30, K 2.019

Observation of the Higgs mode in a strongly interacting fermionic superfluid — •Martin Link¹, Alexandra Behrle¹, Timothy Harrison¹, Johannes Kombe², Kuiyi Gao¹, Jean-Sebastien Bernier², Corinna Kollath², and Michael Köhl¹ — ¹Physikalisches Institut, University of Bonn, Wegelerstrasse 8, 53115 Bonn, Germany — ²HISKP, University of Bonn, Nussallee 14-16, 53115 Bonn, Germany

Higgs and Goldstone modes are possible collective modes of an order parameter upon spontaneously breaking a continuous symmetry. Whereas the low-energy Goldstone (phase) mode is always stable, additional symmetries are required to prevent the Higgs (amplitude) mode from rapidly decaying into low-energy excitations. In the realm of condensed-matter physics, particle-hole symmetry can play this role and a Higgs mode has been observed in weakly-interacting superconductors. However, whether the Higgs mode is also stable for strongly-correlated superconductors in which particle-hole symmetry is not precisely fulfilled or whether this mode becomes overdamped has been subject of numerous discussions. Here, we observe the Higgs mode in a strongly-interacting superfluid Fermi gas. By inducing a periodic modulation of the amplitude of the superconducting order parameter Δ, we observe an excitation resonance at frequency 2Δ/h. For strong coupling, the peak width broadens and eventually the mode disappears when the Cooper pairs turn into tightly bound dimers signalling the eventual instability of the Higgs mode.