Coupled Charge and Spin Transport in Topological Insulators

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Topological insulators constitute a new class of quantum materials with bulk insulating energy gaps and gapless Dirac-cone edge or surface states. The surface states are protected against time-reversal-invariant perturbations such as non-magnetic impurities, defects, and reconstruction. The charge is uniquely coupled to the spin, and charge current creates spin polarization. Since the surface states are topologically protected, and the momentum states are coupled to spin states, scattering is reduced and noise is suppressed. In thin topological insulators, a Rashba-type spin splitting occurs which can be controlled by a gate voltage. The thermoelectric figure of merit, ZT, increases as film thickness is reduced. In summary, topological insulators have shown exceptional properties for thermoelectric, charge, and spin transport. These materials and properties will be investigated from an engineering electronics point of view. Devices that exploit these properties will be built, modeled and characterized, and the performance metrics and fundamental limits of such devices will be determined. Intellectual Merit: This investigation will be simultaneously carried out both experimentally and theoretically. The project will (i) add to the fundamental knowledge of the material properties and physical processes in highly-scaled topological insulator materials; (ii) build, model, and characterize devices that exploit topological insulating properties for computation, signal processing, and sensing; (iii) determine the performance metrics and the fundamental limitations of such devices, (iv) explore the use of topological insulators for low-dissipation, low-noise interconnects; and (v) develop the electrochemical atomic layer deposition technique to grow few-atomic-layer films of topological insulators. All materials will be extensively characterized using a wide range of methods including atomic force, scanning electron, and transmission electron microscopy, low energy electron diffraction, X-ray spectroscopy, Auger spectroscopy, electron probe micro-analysis, micro-Raman spectroscopy, electrical, and thermal measurements. Experimental measurements will be compared to device models and ab initio, density functional theory calculations of the electronic structure and vibrational modes of the thin film and nanowire materials. Transformative concepts include the use of low-dissipation, low noise topologically protected states of topological insulators for electronic / spintronic devices and low-noise, low-power interconnects.Broader Impact: The successful project has the potential to lead to new technologies that exploit the low-dissipation, low-noise states of topological insulators for computation, communications, and sensors. The broader impacts of this project affect all 5 example areas described within the grant proposal guide, and they are particularly strong in the areas of (i) broadening participation of underrepresented groups and (ii) promoting teaching and training through undergraduate research. The University of California Riverside is a Hispanic serving institution with the largest Hispanic student population among all of the University of California campuses. The principal investigator and co-principal investigator have a long history of successful supervision of underrepresented minorities, they served as principal investigator and co-principal investigator of the National Science Foundation Research Experience for Undergraduates Site for Nano Materials and Devices that focused on minority undergraduate student participation in research, and they plan to hire minority graduate and undergraduate students as research assistants for this project.