UC Davis

Mesoscale phenomena plays a critical role across phase transitions in functional materials. Fluctuation-mediated heterogeneous processes can drive atomic-scale phenomena such as nucleation, domain-wall formation and motion, phase separation and can play a significant role in determining the dynamical material properties under external stimuli such as changes in temperature, electric field etc. Not only from a fundamental perspective, but from an applications perspective, it is imperative to understand fluctuations as they can greatly effect stability and functional properties of a material. However, it is challenging to measure nanoscale fluctuations as it requires both temporal and spatial sensitivity. In this dissertation, I utilized synchrotron based coherent x-ray scattering technique, x-ray photon correlation spectroscopy (XPCS) to capture fluctuation in different material systems. I investigated domain fluctuations in ferroelectric BaTiO3 heterostructures, and molecular fluctuations in chalcogenide AsSe4, Se and Ge3As52S45 glasses. Comparison of these two different systems provides unique insights into critical scaling behavior. For BaTiO3 heterostructures, XPCS studies provide a direct comparison of the role of domain fluctuations in first- and second-order phase transformations. Domain fluctuations were observed up to 25°C above the domain transformation temperature. After a small window of stability, as the Curie temperature was approached, slower domain fluctuations were observed, potentially due to the structural transformation associated with the ferroelectric to paraelectric transformation. The observed time evolution and reconfiguration of domain patterns in two different heterostructures highlight the role played by phase coexistence and elastic boundary conditions in altering fluctuation timescales in ferroelectric thin films. XPCS was also utilized to investigate molecular fluctuations and crystallization in chalcogenide glasses near the glass transition temperature. These studies showed that local composition at the interface plays an important role in determining the relaxation timescales. and also show the fundamental role of nanoscale fluctuations in both amorphous and crystalline materials.