3 9803969 Castner This is a condensed matter physics project that will investigate frequency dependent transport properties of the heavily doped semiconductors Si:P, Si:As, and other systems, close to the metal- insulator transition (MIT) using a helical resonator. The helical resonator allows measurements in the frequency range 100 MHz to 3 GHz. Measurements of both sample-induced frequency shifts and quality factor changes will yield information about the dielectric response and conductivity of semiconductor samples as a function of dopant levels near the MIT. The primary objectives of the project are (1) to clarify the nature of localization and interaction corrections the complex dielectric response of barely metallic samples in order to compare with theory, (2) to search for an unusual frequency dependence of conductivity at the critical concentration, as temperature is lowered, and (3) to extend the microwave measurements to frequencies well above 3 GHz using conventional network analyzers and resonant microwave cavities. Addition research will involve a study of the electron spin resonance (ESR) linewidths of antimony doped Si. All of these measurements bear on the general problem of conductivity in highly correlated electron systems. The results will be relevant to a number of related scientific areas including the high-Tc superconductivity problem. The project exposes graduate students to fundamental physics and provides experience with a number of electrical measurement techniques. Students associated with the project are prepared for a range of career opportunities. %%% Silicon, as the most important electronic material today, has been extensively studied for more than four decades. Heavily doped silicon exhibits an insulator-to-metal transition as the doping of donors or acceptors is increased above a critical density. Although static transport and thermal properties of this phase transition have been extensively studied there is still no generally accepted understanding of this unusual phase transition which features disorder, electron-electron interactions, and strong electron correlation. Although some infrared data exists there has been insufficient study of the dynamical properties of metallic samples as a function of frequency, doping and temperature. Microwave studies will be undertaken with a novel slow wave device (helical resonator), along with conventional microwave cavities. The scientific objectives are to understand the dynamical conductivity and dielectric response functions of both the localized and itinerant electrons present in barely metallic samples. These microwave studies afford an excellent area for the training of graduate students. This work may also impact our understanding of high temperature superconductivity. ***
