Rotation of Quantum Impurities in the Presence of a Many-Body Environment

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.

Theory of excitation of Rydberg polarons in an atomic quantum gas

“…Polarons play important roles in the conduction in ionic crystals and polar semiconductors [1], spin-current transport in organic semiconductors [2], dynamics of molecules in superfluid helium nanodroplets [3][4][5], and collective excitations in strongly interacting fermionic and bosonic ultracold gases [6][7][8].…”

Libration of Strongly‐Oriented Polar Molecules inside a Superfluid

“…This results in renormalization of the molecular rotational constantthe effect previously observed for several molecules inside superfluid helium nanodroplets [9]. The angulon theory allows for a simple interpretation of such a renormalization in terms of the rotational Lamb shift [1][2][3], which is an exact phononic analogue of the photonic Lamb shift of quantum electrodynamics [10]. It was shown that even in the context of weakly-interacting ultracold gases, the magnitude of this shift is large enough to be accessible in modern experiments [11].…”

“…We consider a linear rotor molecule immersed into a superfluid, as described by the angulon Hamiltonian [1], in the presence of an external electrostatic field:…”

“…The angulon represents a quantum rotor dressed by a field of many-particle excitations [1][2][3], and in principle can be thought of as a rotational analogue of the well-studied polaron quasiparticle [4][5][6][7][8]. It was demonstrated, however, that the non-Abelian algebra used to describe quantum rotations, as well as the discrete spectrum of the rotor's eigenvalues, render the angulon physics substantially different from that of any other impurity problem [1][2][3].…”

Bose-Einstein-condensate polaron in harmonic trap potentials in the weak-coupling regime: Lee-Low-Pines–type approach

“…The cost we have to pay is that an interaction among bosons newly appears in the transformed Hamiltonian H ′ . Here we should note that a more general transformation than (14) is presented in the literature [44] for the description of a rotating impurity, so-called angulon [45], which is characterized by the transfer of the angular momentum between the impurity and the environmental bosonic degrees of freedom and by structural deformations of the bosonic distribution around the impurity. Although the system of an angulon assumes an infinite background space in contrast to our case of trapped atoms, emphasis is commonly put on the conserved quantity of the system, i.e.…”