Photo-induced Phase Transition in Praseodymium Calcium Manganites: Time-resolved X-ray Spectroscopy and Scattering Studie

One of the most important scientific challenges in condensed matter physics is to understand and ultimately control the emergent properties of complex materials, i.e. crystalline solids that are not well-described by single-electron band structure models. In such materials, unique and technologically relevant properties often emerge from strong interaction among electrons, and coupling between charge, spin, orbital, and vibrational (lattice) degrees of freedom. The focus of this thesis is on a model complex material, Pr 1-xCaxMnO3, and on the application of time-resolved x-ray techniques to advance our understanding of the physics that underlie the remarkable properties of this material.;As an important member of transition metal oxides family, manganites have attracted huge research interest since the discovery of Colossal Magneto-Resistance (magnetic-field induced insulator-to-metal phase transition) phenomenon in 1950's. Pr1-xCaxMnO3 is among the most studied manganite compounds since it exhibits the highest magneto-resistance ratio. Recently, it was found that the insulator-to-metal phase transition in this material can be induced through other external stimuli (in addition to magnetic field), such as pressure, electric field and even ultrafast laser excitation.;This transient photo-induced phase transition in Pr1-xCa xMnO3 (x=0.3) has been a hot research topic, not only because it has potential applications for future ultrafast data storage technology, but also because it provides a model system to study the physics of a strongly correlated system far away from its equilibrium state. Time-resolved optical spectroscopy and transport experiments have been previously used to study the photo-induced phase transition in Pr1-xCaxMnO 3 (x=0.3); however, the mechanism behind the phase transition is still an open question because the dynamics of the coupled lattice and electronic structure changes associated with the phase transition cannot be directly probed through traditional experimental methods.;X-ray techniques have been indispensable experimental tools in studying the lattice and electronic structure of solid state materials. With the development of ultrafast time-resolved x-ray beamlines at synchrotron light sources and at x-ray free electron lasers, ultrafast time-resolved x-ray techniques have become more and more widely used to study structural dynamics in condensed matter systems. This thesis reports a series of experimental studies on the photo-induced phase transition in Pr1-xCaxMnO3 (x=0.3, 0.5) by ultrafast time resolved x-ray spectroscopy and scattering techniques. New insight regarding the mechanism of the photo-induced phase transition in Pr1-xCaxMnO3 (x=0.3, 0.5) is provided through these studies.;Important background information is introduced in chapter 1, including the physics of manganites, the photo-induced phase transition, and the ultrafast x-ray experimental techniques used in this thesis.;In chapters 2 and 3, x-ray absorption near edge spectroscopy is used to measure the electronic structure changes in Pr0.7Ca0.3 MnO3 while it is undergoing an insulator to metal phase transition. The photo-induced electronic structure change is introduced in chapter 2, and the magnetic field induced electronic structure change is shown in chapter 3. The direct comparison between the two x-ray absorption spectra indicates that the photo-induced metallic phase in Pr0.7Ca0.3MnO 3 is highly similar to the magnetic field induced metallic phase.;Electronic excitation is the first step in photo-induced phase transitions in solid state materials. In order to elucidate the electronic excitation corresponding to the 1.5eV photon used to induce the phase transition, resonant inelastic x-ray scattering (RIXS) is used to measure the elementary excitations in Pr0.5Ca0.5MnO3. In chapter 4, temperature dependent RIXS studies at the Mn L-edge is introduced and a d-d transition is found around 1.5eV whose intensity is associated with the charge/orbital ordering melting process in Pr0.5Ca0.5MnO3.;Chapters 5 and 6 present static and ultrafast time resolved resonant soft x-ray scattering studies of the electronic ordering in Pr1-xCa xMnO3. Spin ordering is found to be dominant in the low temperature ground state of Pr0.7Ca0.3MnO3. An ultrafast "two step" melting process and glassy recovery dynamics of the spin ordering are found in Pr0.7Ca0.3MnO3. Based on the findings from all the experiments, a microscopic picture of spin-order melting from photo-induced inter-site d-d transitions is proposed to explain the photo-induced phase transition Pr0.7Ca0.3MnO3.;Finally, chapter 7 summarizes the thesis, and discusses future directions on the research of strongly correlated materials via ultrafast x-ray techniques.