Bose polaron and the Efimov effect: A Gaussian-state approach

Since the Efimov effect was introduced, a detailed theoretical understanding of Efimov physics has been developed in the few-body context. However, it has proven challenging to describe the role Efimov correlations play in many-body systems such as quenched or collapsing Bose-Einstein condensates (BECs). To study the impact the Efimov effect has in such scenarios, we consider a light impurity immersed in a weakly interacting BEC, forming a Bose polaron. In this case, correlations are localized around the impurity, making it more feasible to develop a theoretical description. Specifically, we employ a variational Gaussian state Ansatz in the reference frame of the impurity, capable of capturing both the Efimov effect and the formation of a polaron cloud consisting of a macroscopic number of particles. We find that the Efimov effect entails cooperative binding of bosons to the impurity, leading to the formation of large clusters. These many-particle Efimov states exist for a wide range of scattering lengths, with energies significantly below the polaron energy. As a result, the polaron is not the ground state, but rendered a metastable excited state which can decay into these clusters. While this decay is slow for small interaction strengths, it becomes more prominent as the attraction increases, up to a point where the polaron becomes completely unstable. We show that the critical scattering length where this happens can be interpreted as a many-body shifted Efimov resonance, where the scattering of two excitations of the bath with the polaron can lead to polaron-cloud assisted bound-state formation. Compared to the few-body case, the resonance is shifted to weaker attraction due to the participation of the polaron cloud in the cooperative binding process. This represents an intriguing example of chemistry in a quantum medium [A. Christianen et al., Phys. Rev. Lett. 128, 183401 (2022)], where many-body effects lead to a shift in the resonances of the chemical recombination, which can be directly probed in state-of-the-art experiments.