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Nanoscale Studies of Strongly Correlated Electron Materials with Coherent X-Rays


Emergent behaviors arising from strongly correlated electron materials continue to puzzle physicists as there are still no clear explanations behind the underpinning mechanisms. The key hides in the collective behavior originating from the interactions among the electronic degrees of freedom, and the objective is to use various experimental probes to investigate the spin, charge, orbital and lattice contributions and their correlated behavior. This dissertation presents a study into how several advanced coherent x-ray techniques can be used to investigate the nanoscale landscape, both spatially and temporally, of strongly correlated electron materials to understand both the underlying physics and how to incorporate these materials into technological devices. In particular, resonant elastic x-ray scattering and x-ray photon correlation spectroscopy are used to study the charge and orbital order tied to the metal-insulator transition in magnetite (Fe3O4), first observed by Verwey back in 1939. The results presented in Chapter 3 suggest that trimeron fluctuations due to thermal electron hopping among Fe 3d orbital sites play a vital role in the metal-insulator transition. Chapter 4 is an investigation into electron-phonon coupling times in elemental chromium using ultrafast x-ray pump-probe. Chapters 5 and 6 focus on the use of x-ray nanodiffraction to spatially resolve the strain in square LSMO nanoislands and the electrical-induced strain propagation in a ferroelectric/ferromagnetic (PZT/LSMO) heterostructure. Understanding the spatial extent of the correlation between strain to both the electric polarization and magnetic domains in these materials is essential for designs in sensory and computing architectures. Finally, Chapter 7 presents a simulation of a double probe x-ray speckle visibility spectroscopy experiment, a technique that relies on the change of contrast on an area detector to resolve dynamic timescales in the femto- to nanosecond range. Based on the simulated results, an analysis method that relies on selective binning of the detector image pixels is proposed to overcome the obstacles presented by an experimental imperfections such as an angular misalignment between the two probes.

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