学术报告（6月23日）

主持人：李朝红 教授

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摘要：

Recently the Bose-Fermi superfluid mixture has been first realized by ENS group. In the experiment, the dipole oscillation is measured. The dipole oscillation exhibits a damping behavior with critical velocity. We interpret this damping effect as the Landau-Beliaev damping. We consider the damping process of  a quasiparticle of the Bose superfluid due to its interaction with quasiparticles in the Fermi superfluid. At zero temperature, We find that the Beliaev damping rate, i.e., the inverse of the lifetime, has quite a different threshold behavior at the BCS and the BEC side of the Fermi superfluid. The damping rate is a constant near the threshold momentum in the BCS side, while it increases rapidly in the BEC side. This is because, in the BCS side, the decay process is restricted by the constraint that the fermion quasiparticle is located near the Fermi surface, while such a restriction does not exist in the BEC side where the damping process is dominated by bosonic quasiparticles of the Fermi superfluid. While at finite temperature, we find that the Landau damping rate is exponential suppressed at the relative low temperature both at BCS side and BEC side. This suppression make it possible to observed Beliaev damping with critical velocity in the finite temperature experiment.