Rotation of quantum impurities in the presence of a many-body environment

TL;DR

This paper introduces the 'angulon' quasiparticle concept to describe quantum impurities with rotational degrees of freedom interacting with a many-body environment, predicting spectral shifts and fine structures.

Contribution

It develops a microscopic theory of rotational quantum impurities in many-body systems, revealing the angulon quasiparticle and its spectral properties.

Findings

Prediction of rotational Lamb shift in impurity spectra

Discovery of many-body-induced fine structure in rotational spectra

Theoretical explanation of experimental observations in superfluid helium droplets

Abstract

We develop a microscopic theory describing a quantum impurity whose rotational degree of freedom is coupled to a many-particle bath. We approach the problem by introducing the concept of an 'angulon' - a quantum rotor dressed by a quantum field - and reveal its quasiparticle properties using a combination of variational and diagrammatic techniques. Our theory predicts renormalisation of the impurity rotational structure, such as observed in experiments with molecules in superfluid helium droplets, in terms of a rotational Lamb shift induced by the many-particle environment. Furthermore, we discover a rich many-body-induced fine structure, emerging in rotational spectra due to a redistribution of angular momentum within the quantum many-body system.