Dynamical theory of strongly interacting Bose-Bose mixtures and the Bose polaron

Cheng-Hsun Wu, Ibon Santiago, Jee Woo Park +3 more

Isabella Fritsche, Cosetta Baroni, Erich Dobler +5 more

The authors experimentally prepare and investigate the properties of a strongly interacting imbalanced mixture of bosonic 41K impurities immersed in a Fermi sea of ultracold 6Li atoms. Two configurations have been explored: a thermal regime described by the Fermi polaron picture and a partially Bose-Einstein-condensed regime described by coexisting Fermi and Bose polarons.

Dhruv Kedar, Ming-Guang Hu, Michael J. Van de Graaff +3 more

A. Privitera, Walter Hofstetter

We generalize the application of small polaron theory to ultracold gases [M. Bruderer, A. Klein, S. R. Clark, and D. Jaksch, New J. Phys. 10, 033015 (2008)] to the case of Bose-Fermi mixtures, where both components are loaded into an optical lattice. In a suitable range of parameters, the mixture can be described within a Bogoliubov approach in the presence of fermionic (dynamic) impurities; an effective description in terms of polarons applies. In the dilute limit of the slow-impurity regime, the hopping of fermionic particles is exponentially renormalized due to polaron formation, regardless of the sign of the Bose-Fermi interaction. This should lead to clear experimental signatures of polaronic effects, once the regime of interest is reached. The validity of our approach is analyzed in the light of currently available experiments. We provide results for the hopping renormalization factor for different values of temperature, density, and Bose-Fermi interaction for three-dimensional $^{87}\mathrm{Rb}$-$^{40}\mathrm{K}$ mixtures in an optical lattice.

Zeng-Qiang Yu, Shizhong Zhang, Hui Zhai

Jesper Levinsen, O. Bleu, Meera M. Parish

We investigate the fundamental problem of a small density of bosonic impurities immersed in a dilute Bose gas at zero temperature. Using a rigorous perturbative expansion, we show that the presence of the surrounding medium enhances the repulsion between dressed bosonic impurities (polarons) in the regime of weak interactions. Crucially, this differs from prevailing theories based on Landau quasiparticles, which neglect the possibility of quantum degenerate impurities and predict an exchange-induced attraction. We furthermore show that the polaron-polaron interactions are strongly modified if the medium chemical potential rather than the density is held fixed, such that the medium-induced attraction between thermal impurities becomes twice the expected Landau effective interaction. Our work provides a possible explanation for the differing signs of the polaron-polaron interactions observed in experiments across cold atomic gases and two-dimensional semiconductors, and it has important implications for theories of quasiparticles and quantum mixtures in general.