Fermion-Mediated Interactions Between Bosonic Atoms

In our ultracold mixture of atomic gases, the Cs atoms form a Bose-Einstein condensate (BEC) which is completely immersed in a much larger degenerate Fermi gas of Li atoms. Theorists have long predicted that such a system should permit an interesting new type of interaction, where bosons interact through density fluctuations of the Fermi sea. We have recently been able to show for the first time that these fermion-mediated interactions indeed exist in a quantum gas mixture!

 

We performed the experiment by very carefully measuring the size of our BEC as a function of the Cs-Cs interaction strength. We were able to measure a small additional attraction felt between the Cs atoms only when they were immersed in the Fermi sea. This effect is consistent with the existence of  fermion-mediated interactions.  Finally, we showed that the Li atoms are capable of rendering the Cs BEC unstable and causing it to fracture into previously unobserved "Bose-Fermi solitons". 

 

These fermion-mediated interactions are predicted to be long-ranged and oscillatory, analogous to the RKKY (Ruderman-Kittel-Kasuya-Yosida) interaction in magnetic materials. We are excited about the prospect of exploring such a novel interaction in ultracold atomic systems, and are currently working to learn more about the way these fermion-mediated interactions behave.

 

The details of this experiment can be found in our recent Nature paper here. For more reading, check out the Nature News and Views article here and the UChicago News article here.