Quantum kinetic equation for the Wigner function and reduction to the Boltzmann transport equation under discrete impurities

We derive a quantum kinetic equation under discrete impurities for the Wigner function from the quantum Liouville equation. To attain this goal, the electrostatic Coulomb potential is separated into the long- and short-range parts, and the self-consistent coupling with Poisson's equation is explicitly taken into account within the analytical framework. It is shown that the collision integral associated with impurity scattering as well as the usual drift term is derived on an equal footing. As a result, we find that the conventional treatment of impurity scattering under the Wigner function scheme is inconsistent in the sense that the collision integral is introduced in an ad hoc way and, thus, the short-range part of the impurity potential is double-counted. The Boltzmann transport equation (BTE) is then derived without imposing the assumption of random impurity configurations over the substrate. The derived BTE would be applicable to describe the discrete nature of impurities such as potential fluctuations.