9316803 Lee Technical abstract: Many materials that are barely on the insulating side of a metal-insulator transition conduct electricity by variable range hopping (VRH), for which the conductivity depends on temperature as exp(T0/T)1/n, for integer n > 1. While VRH by non-interacting electrons is well understood, recent experiments have shown that VRH at low temperatures in the presence of magnetic perturbations leads to new and unexpected phenomena, including the possibility of a new spin-charge coupled quasiparticle. It is planned to investigate the many-body density-of-states and elementary excitation spectrum in VRH conductors using electron tunneling and magneto transport experiments at low temperatures and high magnetic fields. The goal is to directly measure the proposed energy gap structure inferred from conductivity data and to elucidate the effects of magnetic scattering on hopping conduction. Non-technical abstract: A fundamental problem in condensed matter physics is concerned with the mechanism of electrical conduction in a solid. While the understanding of common metals and semiconductors is highly developed, many materials exhibit unusual, often poorly understood electrical behavior. A commonly observed form of unconventional electrical conduction is known as variable- range hopping (VRH). Since many diverse technologically important solids are VRH conductors, it is of practical importance to develop a more profound physical understanding of this process. Recent experiments have shown that VRH conduction can yield complex and completely unexpected behavior. In order to elucidate the fundamental principles which underlie this electrical conductiom it is proposed to study directly rhe microscopic properties of the charge carriers in different VRH conductors. ***
