Dynamics of Localized Photoexcitations in Condensed Matter Systems

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Technical abstract:Localization of electronic states plays a critical role in determining the properties of a wide range of materials: polaron formation has a profound impact on the charge transport properties of electronic materials, and formation of self-trapped excitons, or exciton-polarons, dramatically changes optical properties and energy transport mechanisms. While the equilibrium properties of quasiparticles are well-established in many systems, a full understanding of the dynamics of the process of quasiparticle formation has yet to be achieved. In addition to its fundamental significance, such knowledge promises a means to understand, and thereby exploit, the fast electronic and optical response of materials. This project consists of studies of the coupled electronic and vibrational dynamics inherent to localization, carried out in quasi-one-dimensional materials in which the strength of the electron-phonon interactions that drive the localization dynamics can be systematically tuned. Studies of these systems will be accomplished using femtosecond time-resolved techniques that are sensitive to electronic, vibrational, and structural dynamics. The synergistic combination of time-resolved spectroscopic and x-ray techniques, together with theoretical modeling, promises to enhance the understanding of photoinduced structural changes, both in these systems and more generally. Non-technical abstract:Localization of electronic states plays a critical role in determining the properties of a wide range of materials. The formation of localized charge carriers, or polarons, has a profound impact on the charge transport properties of electronic materials, and the formation of localized electronic excitations (self-trapped excitons or exciton-polarons) dramatically changes optical properties and energy transport mechanisms. In addition to its fundamental significance, knowledge of the underlying physics of localization processes promises a means to understand, and thereby exploit, the fast electronic and optical response of materials. This project studies the coupled electronic and vibrational dynamics inherent to localization, carried out in materials in which the strength of the electron-phonon interactions that drive the localization dynamics can be systematically tuned. Studies of these materials will be accomplished using ultrafast time-resolved techniques that are sensitive to electronic, vibrational, and structural dynamics. The synergistic combination of time-resolved spectroscopic and x-ray techniques, together with theoretical modeling, promises to enhance the understanding of photoinduced structural changes, both in these systems and more generally. This project will support the work of 2 Ph.D. students. The combination of basic research involving fundamental issues in condensed matter physics with practical issues in experiment development provides an excellent training ground; students working in ultrafast spectroscopy of electronic materials are well-prepared to enter positions in academia, government laboratories, and hightech industry, given their background in both basic and applied work.