Self-stabilized Bose polarons

TL;DR

This paper introduces a stabilized model for Bose polarons that includes boson repulsion, preventing instability at strong interactions and providing insights into their energy, size, and universal behavior near resonance.

Contribution

It presents a beyond-Bogoliubov solution for Bose polarons that incorporates boson repulsion, stabilizing the system at strong coupling and near resonance.

Findings

Polaron energy remains bounded across the resonance.

Polaron size stays finite even at strong attraction.

The effective range universally influences polaron energy at resonance.

Abstract

The mobile impurity in a Bose-Einstein condensate (BEC) is a paradigmatic many-body problem. For weak interaction between the impurity and the BEC, the impurity deforms the BEC only slightly and it is well described within the Fr\"ohlich model and the Bogoliubov approximation. For strong local attraction this standard approach, however, fails to balance the local attraction with the weak repulsion between the BEC particles and predicts an instability where an infinite number of bosons is attracted toward the impurity. Here we present a solution of the Bose polaron problem beyond the Bogoliubov approximation which includes the local repulsion between bosons and thereby stabilizes the Bose polaron even near and beyond the scattering resonance. We show that the Bose polaron energy remains bounded from below across the resonance and the size of the polaron dressing cloud stays finite. Our results demonstrate how the dressing cloud replaces the attractive impurity potential with an effective many-body potential that excludes binding. We find that at resonance, including the effects of boson repulsion, the polaron energy depends universally on the effective range. Moreover, while the impurity contact is strongly peaked at positive scattering length, it remains always finite. Our solution highlights how Bose polarons are self-stabilized by repulsion, providing a mechanism to understand quench dynamics and nonequilibrium time evolution at strong coupling.