We are interested in the many-body physics of impurities in a fermionic environment under conditions of strong interactions. Our experimental model system consists of fermions (K-40) or bosons (K-41) in an optically trapped sea of fermions (Li-6). Interaction tuning is provided by interspecies Feshbach resonances. We prepare Fermi polarons as microscopic impurities or small Bose-Einstein condensates as mescoscopic impurities, and study the fundamental interactions and the static and dynamic properties using various experimental tools.

Our work is funded by the Austrian Science Fund FWF within SFB FoQuS (project part P04).

**Fermi impurities: Polarons and beyond**

*Mediated interactions between Fermi polarons and the role of impurity quantumstatistics
*C. Baroni, B. Huang (黄博), I. Fritsche, E. Dobler, G. Anich, E. Kirilov, R. Grimm, M. A. Bastarrachea-Magnani, P. Massignan, G. Bruun

arXiv:2305.04915

*Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture
*I. Fritsche, C. Baroni, E. Dobler, E. Kirilov, B. Huang (黄博), R. Grimm, G. Bruun, P. Massignan

Phys. Rev. A

**103**, 053314 (2021); arXiv:2103.03625

*Ultrafast many-body interferometry of impurities coupled to a Fermi sea*

M. Cetina, M. Jag, R. S. Lous, I. Fritsche, J. T. M. Walraven, R. Grimm, J. Levinsen, M. M. Parish, R. Schmidt, M. Knap, E. Demler

Science **354**, 96 (2016); arXiv:1604.07423

*Decoherence of Impurities in a Fermi Sea of Ultracold Atoms*

M. Cetina, M. Jag, R. S. Lous, J. T. M. Walraven, R. Grimm, R. S. Christensen, G. M. Bruun

Phys. Rev. Lett. **115**, 135302 (2015); arXiv:1505.00738

*Metastability and coherence of repulsive polarons in a strongly interacting Fermi mixture*

C. Kohstall, M. Zaccanti, M. Jag, A. Trenkwalder, P. Massignan, G. Bruun, F. Schreck, R. Grimm

Nature **485**, 615 (2012); arXiv:1112.0020

**BEC in a Fermi sea**

*BEC immersed in a Fermi sea: Theory of static and dynamic behavior across phase separation
*B. Huang (黄博)

*Phys. Rev. A*

**101**, 063618 (2020); arXiv:2004.09227

*Breathing Mode of a BEC Repulsively Interacting with a Fermionic Reservoir
*B. Huang (黄博), I. Fritsche, R. S. Lous, C. Baroni, J. T. M. Walraven, E. Kirilov, R. Grimm

Phys. Rev. A 99, 041602(R) (2019); arXiv:1812.06445

*Probing the Interface of a Phase-Separated State in a Repulsive Bose-Fermi Mixture*

R. S. Lous, I. Fritsche, M. Jag, F. Lehmann, E. Kirilov, B. Huang (黄博), R. Grimm

Phys. Rev. Lett. **120**, 243403 (2018); arXiv:1802.01954

**Ultracold molecules and few-body phenomena**

*Lifetime of Feshbach dimers in a Fermi-Fermi mixture of Li-6 and **K-40*

M. Jag, M. Cetina, R. S. Lous, R. Grimm, J. Levinsen, D. S. Petrov

Phys. Rev. A **94**, 062706 (2016); arXiv:1608.07425

*Observation of a Strong Atom-Dimer Attraction in a Mass-Imbalanced Fermi-Fermi Mixtur*e

M. Jag, M. Zaccanti, M. Cetina, R. S. Lous, F. Schreck, R. Grimm, D. S. Petrov, J. Levinsen

Phys. Rev. Lett. **112**, 075302 (2014); arXiv:1311.5146