Electronic structure and ordered phases in epitaxially-stabilized n-type cuprates

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A well-known electron-hole asymmetry exists in the doping phase diagram of the cuprates, and understanding the mechanisms for this asymmetry could lead to a better understanding of the physics of the cuprates and the origin of high temperature superconductivity. In the hole-doped cuprates, experiments on a wide variety of compounds with differing structure and composition enable the disentangling of material-specific properties from universal ones. In the electron-doped cuprates, due to a limited availability of thermodynamically stable compounds, much of the experimental work has been focused on the so-called T’ cuprates, with most of those efforts focused on Nd2−xCexCuO4, for which the highest quality single crystals can be grown. Epitaxial growth can be used to stabilize metastable compounds in thin film form and expand the range of materials available for study. In this dissertation, we use a combination of in situ angle-resolved photoemission spectroscopy (ARPES) and ex situ probes to study thin film samples of the electron-doped cuprates T’-La2CuO4 (T’-LCO) and infinite-layer Sr1−xLaxCuO4 (SLCO), both metastable in bulk. In T’-LCO, we have used ARPES to directly measure its band structure and show that it is electron-doped without cation doping, the first such report in an electron-doped cuprate. We also show that its carrier concentration can be controlled by compensating electron carriers with doped holes by substituting Sr2+ for La3+, driving the system back into a Mott insulating state. We propose that intrinsic oxygen vacancy defects are responsible for electron doping in this compound, suggesting a new route towards electron doping in the cuprates and highlighting the importance of oxygen stoichiometry in the properties of electron-doped cuprates. In SLCO, we have used resonant soft x-ray scattering (RSXS) at the Cu L3 and O K edge to study charge ordering in the Cu-O plane, finding a unique form of charge ordering with a long and nearly doping-independent periodicity of eight unit cells and with possible correlations along both the Cu-O bond directions as well as diagonal to them. This behavior is totally unlike what has been seen in the hole-doped cuprates as well as electron-doped Nd2−xCexCuO4, nearly all of which show stripe ordering along the Cu-O bond direction with 3 to 5 unit cell periodicity. Our results show that charge ordering in electron-doped cuprates has a dramatically different phenomenology to that in the hole-doped cuprates, a fact which may be related to the differing orbital characters of electron and hole carriers in the cuprates as well as the higher strength of antiferromagnetic correlations in the electron-doped cuprates..
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Condensed matter physics; thin films; Angle-resolved photoemisison spectroscopy; epitaxy; Charge order; Cuprates; Resonant soft x-ray scattering
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Shen, Kyle M.
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Davis, James C.
Kim, Eun-Ah
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Ph. D., Physics
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Doctor of Philosophy
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Attribution-NonCommercial-ShareAlike 4.0 International
dissertation or thesis
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