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X-Ray Photon Correlation Spectroscopy as a Probe of Non-Equilibrium Phase Dynamics in Ferroelectric Superlattices [electronic resource].

[S.l.] : Stanford University. 2024 Ann Arbor : ProQuest Dissertations & Theses , 2024

1 online resource(94 p.)

Source: Dissertations Abstracts International, Volume: 86-05, Section: B.
Includes supplementary digital materials.
Advisor: Lindenberg, Aaron.

Thesis (Ph.D.)--Stanford University, 2024.

A central paradigm of non-equilibrium physics concerns the dynamics of heterogeneity and disorder, impacting processes ranging from the behavior of glasses to the emergent functionality of active matter. Understanding these complex mesoscopic systems requires probing the microscopic trajectories associated with irreversible processes, the role of fluctuations and entropy growth, and the timescales on which non-equilibrium responses are ultimately maintained. Approaches that illuminate these processes in model systems may enable a more general understanding of other heterogeneous non-equilibrium phenomena, and potentially define ultimate speed and energy cost limits for information processing technologies. The high brilliance, high degree of coherence, and short pulse duration of X-ray free electron lasers (XFELs) open new possibilities for probing these heterogenous materials dynamics at the angstrom, nanometer, and mesoscopic length scale and femtosecond to microsecond time scale. In this work, we describe new efforts to use pump-probe X-ray diffraction (XRD) and X-ray photon correlation spectroscopy (XPCS) to visualize the non-equilibrium and irreversible phase dynamics of materials. We analyze the phase transformation of a superlattice of PbTiO3/SrTiO3 transforming from a phase mixture of vortex and ferroelectric domains into a polar supercrystal phase using XRD and XPCS in a pump-probe, single-shot geometry. These experiments together with multiple modeling approaches uncover a non-equilibrium correlation response spanning greater than 10 orders of magnitude in timescales, with multistep behavior similar to the plateaus observed in supercooled liquids and glasses. We show that the long time dynamics can be understood in terms of with stochastic domain wall dynamics that arise during this phase transition. This experiment demonstrated a method enabled by XFELs to perform novel in situ studies of the mesoscale dynamics of systems undergoing irreversible processes. Many structural changes in materials caused by a variety of pumping mechanisms can be interrogated by this technique, enabling a new way to gather information about the mesoscale ultrafast dynamics of heterogeneous materials. Additional experiments have revealed the existence of other metastable phases in PbTiO3/SrTiO3 that have shown ultrafast changes in speckle pattern as a result of optical excitation.

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