Topics in the Dynamics of Disordered Systems

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Non-Technical Abstract: This grant supports theoretical research on the properties of disordered materials. Studies will be made of the motion of charged particles (ions) in glasses and on the motion of hydrogen in amorphous silicon. The first material phenomena could find application as a computer memory device. The second material has many applications including solar cells. Students at all levels will participate in the projects and international collaborations will be continued in Spain, Britain and Japan.Project SummaryThis research proposal aims to address outstanding scientific issues in disordered materials. The motivation arises from the need to understand the remarkably fast dynamics of transition metal ions in glasses and the bonding and motion of hydrogen in hydrogenated amorphous silicon. In both cases, there exist applications of great importance that will benefit from a fundamental understanding of the materials and especially the atomic dynamics. We work closely with leading experimental research groups and a successful small company in Tempe, Arizona.Intellectual MeritThe dynamics of ions in glasses is an area of sustained interest and importance. We studyamorphous phases of GeSe glasses heavily doped with Ag. The motion of the Ag ions in these materials is remarkable for rapidity of transport. We will elucidate the processes of hopping and trapping, of basic interest to the theory of relaxation in these systems. These glasses are prototypical solid electrolytes, and have been employed in the exceptionally promising "Programmable Metallization Cell" low power, non-volatile computer memory device pioneered by Axon Technologies of Arizona. We propose to work with the inventors of the device to develop an atomistic understanding of its operation. Previous work has shown that suitable density functional methods can faithfully model the structure and dynamics of these materials.Hydrogenated amorphous silicon is an important electronic material heavily used in thin film transistor applications (as for example in laptop displays), as a bolometric material for night vision devices, and is also one of the most important solar photovoltaic materials. On experimental grounds, the motion of H is known to determine key attributes of the material, and is associated with light-induced metastability (Staebler-Wronski effect). We have been able to accurately track the motion of hydrogen with suitably precise first principles simulations and realistic structural models. In collaboration with leading experimental groups, we will determine the dynamics of H and the atomistic origins of the light-induced metastability using state of the art first principles methods, both in the electronic ground state and in a simulated light-excited state. We will clarify current models of metastability with accurate results from atomisticsimulation.Broader ImpactThe broader impact of the fundamental research described above is to provide a microscopic understanding of an emerging computer memory device, and light-induced degradation of an important solar photovoltaic material. We will ensure the involvement of under-represented groups in the United States (for example, two current students are African, and our most recent graduate is African). Education of graduate students, undergraduates and post-docs will continue to be a top priority for this research program.International collaboration with researchers from Britain, Spain and Japan is anticipated.