Repulsive long-range forces between anisotropic atoms and dielectrics

TL;DR

This paper explores long-range electromagnetic forces between anisotropic atoms and dielectrics, revealing conditions for repulsive interactions and analyzing their dependence on configuration and orientation.

Contribution

It provides the first analytic expressions for anisotropic atom-dielectric interactions and demonstrates the existence of stable repulsive configurations in the weak-coupling regime.

Findings

Repulsive configurations between anisotropic atoms are constructed.

Analytic expressions for complex configurations are derived.

Non-monotonic energy dependence on separation is explained.

Abstract

We investigate long-range forces between atoms with anisotropic electric polarizability interacting with dielectrics having anisotropic permittivity in the weak-coupling approximation. Unstable configurations in which the force between the objects is repulsive are constructed. Such configurations exist for three anisotropic atoms as well as for an anisotropic atom above a dielectric plate with a hole whose permittivity is anisotropic. Apart from the absolute magnitude of the force, the dependence on the configuration is qualitatively the same as for metallic objects for which the anisotropy is a purely geometric effect. In the weak limit closed analytic expressions for rather complicated configurations are obtained. The non-monotonic dependence of the interaction energy on separation is related to the fact that the electromagnetic Green's dyadic is not positive definite. The analysis in the weak limit is found to also semi-quantitatively explain the dependence of Casimir forces on the orientation of anisotropic dielectrics observed experimentally. Contrary to the scalar case, irreducible electromagnetic three-body energies can change sign. We trace this to the fact that the electromagnetic Green's dyadic is not positive definite.