  In this work, we study the spin Hall effect and Rashba-Edelstein effect of a
2D Weyl fermion system in the clean limit using the Kubo formalism. Spin
transport is solely due to the spin-torque current in this strongly spin-orbit
coupled (SOC) system, and chiral spin-flip scattering off non-SOC scalar
impurities, with potential strength $V$ and size $a$, gives rise to a
skew-scattering mechanism for the spin Hall effect. The key result is that the
resultant spin-Hall angle has a fixed sign, with $\theta^{SH} \sim O
\left(\tfrac{V^2}{v_F^2/a^2} (k_F a)^4 \right)$ being a strongly-dependent
function of $k_F a$, with $k_F$ and $v_F$ being the Fermi wave-vector and Fermi
velocity respectively. This, therefore, allows for the possibility of tuning
the SHE by adjusting the Fermi energy or impurity size.
