The topological insulator case is easier to visualize due to the presence of a single Fermi contour, therefore the topological insulator case is discussed first. Topological insulators display spin-split surface states where spin-momentum locking is present. Indeed, when a charge current flows in the surface states of the topological insulator, it can also be seen as a well-defined momentum shift Δ Δ k in the reciprocal space, resulting in a different occupation of the spin-polarized branches of the Dirac cone. This unbalance, accordingly to the structure of the topological insulator band dispersion relation, produces a spin accumulation in the investigated material, i.e., a charge-to-spin conversion occurs. The spin accumulation is orthogonal to the injected charge current, accordingly to spin-momentum locking. Due to the fact that these materials display a conductive behaviour on their surface while being insulating on their bulk, the charge current is only allowed to flow on the topological insulator surfaces: this is the origin of the bidimensionality of this charge-to-spin conversion mechanism.