Towards Bragg spectroscopy of boson-boson density correlations, due to fermion-mediated interactions, in a quantum degenerate Bose-Fermi mixture

Bose–Fermi mixtures provide a controlled setting for studying matter-wave mediated interactions [1]. We report progress toward Bragg spectroscopy of the ⁸⁷Rb component in a quantum-degenerate ⁸⁷Rb⁴⁰K Bose–Fermi mixture with tunable interspecies interactions. Two-photon Bragg pulses are applied to the bosonic component (⁸⁷Rb) to directly probe the dynamic structure factor S_B(q,ω). The momentum transfer is chosen near q≈k_F. The interspecies interaction length a_BF​ is varied near the 546 G ⁸⁷Rb⁴⁰K Feshbach resonance, into the strongly interacting regime. Our central goal is to test how RKKY-like, fermion-mediated interactions [2] modify boson–boson density correlations. This modification is probed through the bosonic density response measured by Bragg spectroscopy. Our mean-field calculations predict a roton-like reduction of the bosonic response that grows with (a_BF)², and eventually leads to instabilities near unitarity. Our measurements will provide a quantitative benchmark for mediated interactions in resonant Bose–Fermi mixtures and help constrain theoretical descriptions beyond the mean-field description.

[1] Hagai Edri, Boaz Raz, Noam Matzliah, Nir Davidson, and Roee Ozeri, Phys. Rev. Lett. 124, 163401 (2020)

[2] Henry Ando, Geyue Cai, Cheng Chin, Tilman Enss, arXiv:2511.09745