Electromagnetic response of two interacting topological insulator spheres in external fields
We study the static electromagnetic response of two spherical topological insulators embedded in a dielectric medium and subjected to a uniform external electric field. The gapped surface states are described by a piecewise constant axion field, which induces a topological magnetoelectric coupling localized at the spherical interfaces. {More generally, the same formalism applies to isotropic magnetoelectric media characterized by an effective scalar magnetoelectric response.} The electrostatic problem is solved at zeroth order using bispherical coordinates, allowing for an exact treatment of both parallel and perpendicular orientations of the external field relative to the center-to-center axis. The resulting mode expansions are determined by three-term recurrence relations, which are solved perturbatively for nonoverlapping spheres. The { magnetoelectric}-induced response is then computed to leading order in the fine-structure constant {(or, more generally, in the effective coupling strength)}. The induced sources are purely interfacial and generate distinct magnetostatic field configurations in the parallel and perpendicular geometries. Closed-form series representations for the induced vector potential and magnetic field are obtained in terms of the zeroth-order electrostatic coefficients. These results provide an analytically controlled description of {interaction-induced magnetostatics in coupled spherical magnetoelectric systems}.