Quantum transport theory with nonequilibrium coherent potentials

Since any realistic electronic device has some degree of disorder, predicting disorder effects in quantum transport is a critical problem. Here, we report the theory of nonequilibrium coherent potential approximation (NECPA) for analyzing disorder effects in nonequilibrium quantum transport of nanoelectronic devices. The NECPA is formulated by contour-ordered nonequilibrium Green's function where the disorder average is carried out within the coherent potential approximation on the complex-time contour. We have derived a set of rules that supplement the celebrated Langreth theorem and, as a whole, the generalized Langreth rules allow us to derive NECPA equations for real-time Green's functions. The solution of NECPA equations provides the disorder-averaged nonequilibrium density matrix as well as other relevant quantities for quantum transport calculations. We establish the excellent accuracy of NECPA by comparing its results to brute force numerical calculations of disordered tight-binding models. Moreover, the connection of NECPA equations which are derived on the complex-time contour to the nonequilibrium vertex correction theory which is derived on the real-time axis is made. As an application, we demonstrate that NECPA can be combined with density functional theory to enable analysis of nanoelectronic device physics from atomistic first principles.