Figure 2
From: Single-particle properties of topological Wannier excitons in bismuth chalcogenide nanosheets
Momentum-space skyrmion textures of an electron and a hole. This idealized illustration corresponds to the configuration in a weakly bound exciton. Due to rotational symmetry the two circles actually represent only a radial slice of the two spheres onto which the complete momentum plane is mapped with a unit winding number. The south (north) pole of the top (bottom) sphere corresponds to the origin \(Q = 0\), whereas the opposite pole corresponds to \(Q \rightarrow \infty\). Note that in a normal semiconductor like CdSe the pseudospins always point in the same direction, independent of Q, and thus have a zero winding number.
