Higgs and Goldstone modes in cold atom systems

Abstract

Many superfluid and superconducting systems can be well characterized by a complex order parameter that obtains a nonzero expectation value after spontaneous U(1) symmetry breaking. The complex order parameter can exhibit oscillations in phase or in amplitude, corresponding to Goldstone and Higgs excitations, respectively. These modes are also expected to be present in superfluid atomic Bose and Fermi gases. The excellent level of experimental control over interaction strength, atom numbers, and trapping geometry motivated theoretical and experimental efforts to study the Goldstone and Higgs modes in these cold atom systems. In atomic Fermi superfluids in particular, there exists a tunable coupling between these modes, as well as a coupling to a nearby continuum of single-particle excitations, and this leads to a much richer physics than can be initially expected from simple field-theoretical models. In this contribution, the recent theoretical and experimental developments for the collective modes in superfluid atomic systems are reviewed and placed in the context of earlier research both in particle physics and solid-state physics. The focus is on superfluid Fermi gases, as they include the case of the Bose gas in the limit of strong pairing. For this system, the state-of-the-art theoretical understanding of these modes is provided and compared to recent experimental observations of the collective modes.