When time-reversal symmetry is broken on its surface, topological insulators exhibit a magnetoelectric response which is described by axion electrodynamics. A direct consequence of this theory is the appearance of a magnetic field that resembles the one produced by a magnetic image monopole when a point-like electric charge is located near the surface of the material. In this paper we investigate the more realistic problem when the point-like charge is replaced by a finite size sphere at constant potential. We calculate the electromagnetic fields using the potential formulation in a particular bispherical coordinate system. We find that the electromagnetic fields can be interpreted in terms of point electric and image magnetic charges as if the medium were the vacuum. As a manifestation of the magnetoelectric effect, we highlight the resulting magnetic field, which we analyze in detail along the symmetry axis, since such estimates could be useful in evaluating the experimental possibility of its measurement via sensible magnetometers. Our numerical estimates show that the proposed setup provides a magnetic field strength in the range of 10-100 mG, which is attainable with present day sensitivities in NV center-diamond magnetometers, for example.
I. INTRODUCTIONGeneral magnetoelectric (ME) media are characterized by additional relations between the magnetic (electric) field and the polarization (magnetization), aside from the standard connection found in conventional dielectrics [1][2][3][4]. A linear ME material is described by the ME term θ ij E i B j in the free enthalpy of the system, where θ ij is the ME tensor which can be either symmetric or antisymmetric [5]. In the simplest case, when θ ij = θδ ij , we recover the ME coupling θ E · B, which we recognize as that of axion electrodynamics, with θ being the axion field [6]. In the context of particle physics, the axion is an additional pseudoscalar degree of freedom which gives a solution to the strong CP problem [7]. Linear magnetoelectrics can be realized in topological materials, such as topological insulators (TIs) [8][9][10][11] and Weyl semimetals (WSMs) [12].Topological phases are an emerging class of materials which have attracted much attention in condensed matter physics. Among them, the most studied are the TIs, which are time-reversal-symmetric materials characterized by a fully insulating bulk with protected conducting surface states [9,10]. This behavior was first predicted in two-dimensional HgTe/CdTe quantum wells [13][14][15] and less than one year after, it was experimentally observed [16]. The generalization to three-dimensional compounds came shortly afterwards [17][18][19]. In particular, Fu and Kane [20] predicted that the alloy Bi 1−x Sb x would be a three-dimensional (3D) TI in a special range of x, and it was experimentally confirmed one year later [21]. A second generation of 3D TIs was predicted to occur in the stoichiometric crystals Bi 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 [22], and they were experimentally discovered in 2009 [22,23]...