Quantum Diffusion in Fluctuating Media

Michigan State University, East Lansing MI

Investigators

Abstract

The main focus of this project is the analysis of wave motion in a disordered environment. In a broader context, the research is aimed at answering a basic scientific question: "What are the effects of disorder?" This is a fundamental question relevant to any mathematical model, even one in which disorder is not explicitly included. After all, any real world system is subject to a small amount of noise, and experience shows that even weak disorder may have a profound effect on the behavior of the system. The equations studied in this project arise in the theory of electrical conduction in disordered materials, but are of general interest because of the fundamental nature of both wave motion and disorder. Progress in understanding the solutions to these equations will improve basic understanding of models of theoretical physics and applied mathematics. In addition, a central goal of the research is pedagogical: to introduce undergraduate and graduate students to a fundamental subject and convey to them that mathematics is a vibrant, growing field. The project will proceed through a program of research on the effects of disorder in physical models. A key goal is to analyze the diffusion of waves in a weakly disordered medium over arbitrarily long times. There is a rich non-rigorous theory of the weakly disordered regime in the physics literature based on heuristic analyses and uncontrolled, renormalized perturbation theory which suggests that waves propagate diffusively, characterized by spreading of wave packets over a distance proportional to the square root of t in time t. However, we are far from having a rigorous analysis of the mathematics involved. A major challenge is that diffusive propagation does not occur for waves in a non-random medium. Thus, a naive approach in which the disorder is incorporated perturbatively has not worked, indeed in the physics literature the problem is attacked with renormalized perturbation theory. In recent years the PI and various post-doc and student collaborators have considered the problem of wave diffusion in time-dependent random media, with the time dependence generated by a Markov process. For such models the diffusive propagation, e.g., for the tight binding Schrödinger equation, can be established by spectral analysis. One aim of the present project is to approach the time independent equation as a perturbation of these time dependent equations, which have the virtue of sharing the expected qualitative behavior.