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Mardi 19 novembre 2019 - 14:00

Spectre d’excitations d’un gaz de Fermi superfluide

Hadrien Kurkjian (Université d’Anvers)¡ Attn : salle 20 au RdC !

par Revaz Ramazashvili - 19 novembre 2019

I will discuss the excitation spectrum of an ultracold gas of paired fermions in its superfluid phase. There remains important open questions on this fundamental subject of many-body quantum physics, and cold atom experiments have now the capacity to answer them. Like in other superfluids, the lowest-energy excitations are phonons : collective modes of density fluctuations. But unlike most superfluids, the phonon dispersion may become subsonic in a Fermi gas, which has important consequences on the interaction mechanisms between phonons, and thus on the system’s dissipative properties.

At higher energy, I will show that another collective branch, related to excitations of the order-parameter amplitude, exists inside the continuum of fermionic pair-breaking excitations. The observability and dispersion relation of this branch (sometimes called ``Higgs’’ branch) is still controversial in the theoretical literature while several experimental results are available. I will explain that to identify clearly the collective mode resonance in the continuum background, one should locate the poles of the order-parameter Green’s function in its analytic continuation through the branch cut associated to the continuum.

Finally, in these paired systems, there exists a gapped spectrum of fermionic quasiparticles. I will explain how to describe this fermionic branch beyond the mean-field approximation by viewing the quasiparticles as a polaronic impurities embedded in a sea of phonons.

Post-scriptum :

contact : Z. Ristivojevic