# Breathing Mode of a BEC Repulsively Interacting with a Fermionic Reservoir

## TL;DR

This study explores how the breathing mode frequency of a small Bose-Einstein condensate changes when coupled to a large Fermi sea, especially during phase separation caused by strong interspecies repulsion.

## Contribution

It provides experimental observations and theoretical models explaining the frequency shift of the BEC's breathing mode in a coupled Bose-Fermi mixture during phase separation.

## Key findings

- Significant frequency shift observed at phase separation
- Maximum shift depends on atom number ratio
- Models successfully explain the experimental results

## Abstract

We investigate the fundamental breathing mode of a small-sized elongated Bose-Einstein condensate coupled to a large Fermi sea of atoms. Our observations show a dramatic shift of the breathing frequency when the mixture undergoes phase separation at strong interspecies repulsion. We find that the maximum frequency shift in the full phase-separation limit depends essentially on the atom number ratio of the components. We interpret the experimental observations within a model that assumes an adiabatic response of the Fermi sea, or within another model that considers single fermion trajectories for a fully phase-separated mixture. These two complementary models capture the observed features over the full range of interest.