where q is the elementary charge, τ τ m is the momentum scattering time, k F and v F are, respectively, the Fermi wavevector and the Fermi velocity and ℏ ℏ is the reduced Planck constant. The spin current density can be also analytically computed by integrating across the Fermi surface the product of the spin polarization and the corresponding distribution function. In the Edelstein effect case, this quantity results in: