****Technical Abstract**** This research project seeks to understand the structure-property/function correlation in colossal magnetoresistive (CMR) materials and semiconductor quantum dots by directly examining their structural dynamics in real time. The structural dynamics will be probed at the atomic spatiotemporal resolution using a novel technique of femtosecond electron diffraction (FED) developed in PI's laboratory. In conjunction with FED, femtosecond time-resolved optical spectroscopy measurements in the relevant degrees of freedom will also be conducted to thoroughly characterize the dynamical behavior in these materials. Issues and questions to be addressed include: (1) Dynamics of photoinduced phase transition in CMR materials: What are the steps involved and what are the associated timescales? What is the role of Jahn-Teller distortion in these photoinduced phase transition? (2) Dynamics of electron-phonon coupling in semiconductor quantum dots: How fast is the photo-deposited energy transferred from charge carrier to lattice? How does this time scale depend on the size, the type and structure of quantum dots, and the surface conditions? What is the role of phonon bottleneck? These research will provide a real-time and atomic-level view of these dynamics and hold the potential to fully reveal the related physical mechanisms and the structure-function correlations that would otherwise be impossible to extract. This project integrates research and education to train students in cutting-edge techniques and modern methods in the forefront areas of condensed matter physics and ultrafast science.
The property of matter is largely determined by its microscopic structure, that is, how the constituent atoms are arranged at the atomic level. A well-known example is the carbon and its allotropes, such as graphite, diamond and buckyball C60. Likewise, structural changes and transformations involving the rearrangement of constituent atoms and/or molecules dictate many important processes in nature, ranging from seemingly simple processes such as melting to complex biological reactions. Probing structural dynamics at the atomic spatiotemporal scales has attracted considerable interest. In our previous research funded by NSF, we developed a unique tool of femtosecond electron diffraction and demonstrated its capability of probing structural dynamics at the time and length scales of atomic motions. Here, we will use this new technique to study several condensed matter physics problems of both fundamental physical importance and interesting technological implications. They include the dynamics of photo-induced structural transformation in colossal magnetoresistive materials and the relaxation of photo-generated carrier in semiconductor nano-crystals. The former study will reveal the elementary steps involved, thus gaining new insight to the transition pathways. The latter research will contribute to the understanding of the energy redistribution in nano-crystals and help improving the energy conversion efficiency in nano-crystal based solar cells. Participating undergraduate and graduate students will acquire training and knowledge in the forefronts of contemporary condensed matter physics and ultrafast science, and will be prepared for careers in academia, industry, and national laboratories.
