The origin of the Rashba–Edelstein effect relies on the presence of spin-split surface or interface states, which can arise for a structural inversion asymmetry or because the material exhibits a topologically protected surface, being a topological insulator. In both cases, the material surface displays the spin polarization locked to the momentum, meaning that these two quantities are univocally linked and orthogonal one to the other (this is clearly visible from the Fermi countours). It is worth noticing that also a bulk inversion asymmetry could be present, which would result in the Dresselhaus effect. In fact, if, in addition to the spatial inversion asymmetry or to the topological insulator band structure, also a bulk inversion asymmetry is present, the spin and momentum are still locked but their relative orientation is not straightforwardly determinable (since also the orientation of the charge current with respect to the crystallographic axes plays a relevant role). In the following discussion, the Dresselhaus effect will be neglected, for simplicity.