Spin moment of a surface wave at the interface between hyperbolic and topological insulators

Commonly, a surface wave traveling along the interface between isotropic media has a spin moment that lies in the plane of the interface and is perpendicular to the direction of propagation. Here, we show that, if one of the two media is a topological insulator, the spin moment vector has a component that is normal to the surface of the interface and is proportional to odd integers. The appearance of the normal component of the spin moment is associated with the topological magnetoelectric effect, as a result of which the polarization of the wave changes upon passage through the interface.