Angular self-localization of impurities rotating in a bosonic bath

TL;DR

This paper demonstrates a finite-coupling self-localization transition of angulons, rotational analogues of polarons, involving symmetry breaking and energy discontinuities, with potential experimental realizations in cold and solid-state systems.

Contribution

It reveals the self-localization transition of angulons at finite coupling, including symmetry breaking and state diversity, extending polaron concepts to rotational quasiparticles.

Findings

Self-localization occurs at finite impurity-bath coupling.

Transition involves spherical-symmetry breaking of the ground state.

Discontinuity in the first derivative of the ground-state energy.

Abstract

The existence of a self-localization transition in the polaron problem has been under an active debate ever since Landau suggested it 83 years ago. Here we reveal the self-localization transition for the rotational analogue of the polaron -- the angulon quasiparticle. We show that, unlike for the polarons, self-localization of angulons occurs at finite impurity-bath coupling already at the mean-field level. The transition is accompanied by the spherical-symmetry breaking of the angulon ground state and a discontinuity in the first derivative of the ground-state energy. Moreover, the type of the symmetry breaking is dictated by the symmetry of the microscopic impurity-bath interaction, which leads to a number of distinct self-localized states. The predicted effects can potentially be addressed in experiments on cold molecules trapped in superfluid helium droplets and ultracold quantum gases, as well as on electronic excitations in solids and Bose-Einstein condensates.