Anomalous Screening of Quantum Impurities by a Neutral Environment

It is a common knowledge that an effective interaction of a quantum impurity with an electromagnetic field can be screened by surrounding charge carriers, whether mobile or static. Here we demonstrate that very strong, “anomalous” screening can take place in the presence of a neutral, weakly polarizable environment, due to an exchange of orbital angular momentum between the impurity and the bath. Furthermore, we show that it is possible to generalize all phenomena related to isolated impurities in an external field to the case when a many-body environment is present, by casting the problem in terms of the angulon quasiparticle. As a result, the relevant observables such as the effective Rabi frequency, geometric phase, and impurity spatial alignment are straightforward to evaluate in terms of a single parameter: the angular-momentum-dependent screening factor.

Figure 1

(a) The screening factor $f_1^{L,L^{\prime }}$ for selected values of $L$, $L^{\prime }$. (b) Total absorption of a free ground-state impurity (top), compared to a screened one, at bath density $\mathrm{Ln}[\tilde{n}]=-4.5$ (bottom). (c) Total absorption of a free impurity in the $L=3$ state (top), compared to a screened one, at bath density $\mathrm{Ln}[\tilde{n}]=-1.0$ (bottom). (d) Geometric phase of the screened impurity (solid line) compared to that of a free impurity (dashed line), as a function of bath density. See text.

Figure 2

(a) The screening factor $f_2^{L,L^{\prime }}$ for selected values of $L$, $L^{\prime }$. (b) Adiabatic alignment of a free ${\mathrm{CS}}_2$ molecule in a bath of selected densities, as illustrated by the time evolution of the alignment cosine. (c) Same as (b), for the case of nonadiabatic alignment. See text.