Linear rotor in an ideal Bose gas near the threshold for binding

TL;DR

This paper investigates how a linear rotor interacts with a bosonic bath near the formation of a shallow bound state, revealing a many-body instability that disrupts angulon dynamics, especially with anisotropic interactions.

Contribution

It introduces a beyond-linear-coupling angulon Hamiltonian to analyze impurity-boson interactions near bound-state formation, highlighting the instability caused by anisotropic shallow bound states.

Findings

Polaron formalism describes the system with isotropic interactions.

Anisotropic shallow bound states lead to many-body instability.

Angulon dynamics are washed out by the instability.

Abstract

We study a linear rotor in a bosonic bath within the angulon formalism. Our focus is on systems where isotropic or anisotropic impurity-boson interactions support a shallow bound state. To study the fate of the angulon in the vicinity of bound-state formation, we formulate a beyond-linear-coupling angulon Hamiltonian. First, we use it to study attractive, spherically symmetric impurity-boson interactions for which the linear rotor can be mapped onto a static impurity. The well-known polaron formalism provides an adequate description in this limit. Second, we consider anisotropic potentials, and show that the presence of a shallow bound state with pronounced anisotropic character leads to a many-body instability that washes out the angulon dynamics.