The Non-Equilibrium Green Function (NEGF) Method

TL;DR

The paper presents a simplified derivation of the Non-Equilibrium Green Function (NEGF) method from the Schrödinger equation, broadening its accessibility and applicability to quantum transport and non-equilibrium processes.

Contribution

It derives NEGF equations directly from the Schrödinger equation using elementary arguments, extending its scope beyond traditional many-body perturbation theory.

Findings

NEGF equations can be obtained without advanced quantum statistical mechanics.

The method includes phase-breaking interactions within a self-consistent framework.

NEGF provides a versatile approach to quantum device physics and non-equilibrium systems.

Abstract

The Non-Equilibrium Green Function (NEGF) method was established in the 1960's through the classic work of Schwinger, Kadanoff, Baym, Keldysh and others using many-body perturbation theory (MBPT) and the diagrammatic theory for non-equilibrium processes. Much of the literature is based on the original MBPT-based approach and this makes it inaccessible to those unfamiliar with advanced quantum statistical mechanics. We obtain the NEGF equations directly from a one-electron Schr\"odinger equation using relatively elementary arguments. These equations have been used to discuss many problems of great interest such as quantized conductance, (integer) quantum Hall effect, Anderson localization, resonant tunneling and spin transport without a systematic treatment of many-body effects. But it goes beyond purely coherent transport allowing us to include phase-breaking interactions (both momentum-relaxing and momentum-conserving as well as spin-conserving and spin-relaxing) within a self-consistent Born approximation. We believe that the scope and utility of the NEGF equations transcend the MBPT-based approach originally used to derive it. NEGF teaches us how to combine quantum dynamics with "contacts" much as Boltzmann taught us how to combine classical dynamics with "contacts", using the word contacts in a broad, figurative sense to denote all kinds of entropy-driven processes. We believe that this approach to "contact-ing" the Schr\"odinger equation should be of broad interest to anyone working on device physics or non-equilibrium statistical mechanics in general.