The Landau critical velocity for a particle in a Fermi superfluid

Abstract

We determine {à} la Landau the critical velocity $v\_c^L$ of a moving impurity in a Fermi superfluid, that is by restricting to the minimal excitation processes of the superfluid. $v\_c^L$ is then the minimal velocity at which these processes are energetically allowed. The Fermi superfluid actually exhibits two excitation branches~: one is the fermionic pair-breaking excitation, as predicted by BCS theory; the other one is bosonic and sets pairs into motion, as predicted by Anderson's RPA. $v\_c^L$ is the smallest of the two corresponding critical velocities $v\_{c,f}^L$ and $v\_{c,b}^L$. In the parameter space (superfluid interaction strength, fermion-to-impurity mass ratio), we identify two transition lines, corresponding to a discontinuity of the first-order and second-order derivatives of $v\_c^L$. These two lines meet in a triple point and split the plane in three domains. We briefly extend this analysis to the very recently realized case at ENS, where the moving object in the Fermi superfluid is a weakly interacting Bose superfluid of impurities, rather than a single impurity. For a Bose chemical potential much smaller than the Fermi energy, the topology of the transition lines is unaffected; a key result is that the domain $v\_c^L=c$, where $c$ is the sound velocity in the Fermi superfluid, is turned into a domain $v\_c^L=c+c\_B$, where $c\_B$ is the sound velocity in the Bose superfluid, with slightly shifted boundaries.

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References in corpus (5)

• Theory of ultracold Fermi gases
• Vortices and Superfluidity in a Strongly Interacting Fermi Gas
• Pure Gas of Optically Trapped Molecules Created from Fermionic Atoms
• Precision Measurements of Collective Oscillations in the BEC-BCS Crossover
• Critical velocity for superfluid flow across the BEC-BCS crossover

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• Andreev-Bashkin effect in superfluid cold gases mixture
• Perpetual motion and driven dynamics of a mobile impurity in a quantum fluid
• Dipole dynamics of an interacting bosonic mixture
• Dynamics of a degenerate Cs-Yb mixture with attractive interspecies interactions
• One-dimensional Fermi polaron after a kick: two-sided singularity of the momentum distribution, Bragg reflection and other exact results