# Collective oscillation modes of a superfluid Bose-Fermi mixture

## TL;DR

This paper provides a theoretical analysis of collective oscillation modes in superfluid Bose-Fermi mixtures across the BCS-BEC crossover, highlighting how interactions and trap geometry influence mode frequencies and ground state phases.

## Contribution

It introduces explicit formulas for collective mode frequencies based on density overlaps and tests their validity with numerical simulations under experimental conditions.

## Key findings

- Frequencies of collective modes increase with repulsive Bose-Fermi interactions.
- Mode frequency shifts vary across the BCS-BEC crossover, indicating different ground state phases.
- Analytical expressions are validated by numerical calculations for realistic traps.

## Abstract

In this work, we present a theoretical study for the collective oscillation modes, i.e. quadrupole, radial and axial mode, of a mixture of Bose and Fermi superfluids in the crossover from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a molecular Bose-Einstein condensate (BEC) in harmonic trapping potentials with cylindrical symmetry of experimental interest. To this end, we start from the coupled superfluid hydrodynamic equations for the dynamics of Bose-Fermi superfluid mixtures and use the scaling theory that has been developed for a coupled system. The collective oscillation modes of Bose-Fermi superfluid mixtures are found to crucially depend on the overlap integrals of the spatial derivations of density profiles of the Bose and Fermi superfluids at equilibrium. We not only present the explicit expressions for the overlap density integrals, as well as the frequencies of the collective modes provided that the effective Bose-Fermi coupling is weak, but also test the valid regimes of the analytical approximations by numerical calculations in realistic experimental conditions. In the presence of a repulsive Bose-Fermi interaction, we find that the frequencies of the three collective modes of the Bose and Fermi superfluids are all upshifted, and the change speeds of the frequency shifts in the BCS-BEC crossover can characterize the different groundstate phases of the Bose-Fermi superfluid mixtures for different trap geometries.