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Metal-insulator transition, orbital symmetries and gaps in correlated oxides : an impurity control approach Hossain, M. A.


The primary objective of this research is to develop and study newer ways to control electronic properties of correlated oxide systems using impurities. This goal has been achieved by introducing dilute localized 3d Mn impurities in place of a delocalized 4d Ru sites in a 2-dimensional Ru-O matrix and doping an electronically reconstructed polar surface of YBCO via surface impurities. The first part of the work concentrates on X-ray Absorption Spectroscopy (XAS) and Resonant Soft X-ray Scattering (RSXS) studies on lightly Mn doped Sr₃Ru₂O₇ . Our goal is to understand the electronic structure of the material both at room and low temperature and ultimately to understand the mechanism behind the low temperature metal-insulator transition in this compound. With XAS, we zoom into the local electronic structure of the impurities themselves and discovered unusual valence and crystal field level inversion in the Mn impurities. With the help of density functional theory and cluster multiplet calculations, we developed a model to describe the hierarchy of the crystal eld levels of the Mn impurities. Long range magnetic properties of the compound has been probed with Resonant Soft X-ray Scattering (RSXS) on the Mn and Ru L-edges. We have found and analyzed the (¼,¼,0) structurally forbidden diffraction peak and connected it to an antiferromagnetic instability in the parent compound. Doping dependent scattering studies revealed the magnetic structure of the low temperature insulating phase and ultimately the mechanism behind the metal-insulator transition itself. Angle Resolved Photoemission Spectroscopy (ARPES) has been used to investigate the low energy electronic structure of underdoped high-Tc superconductor YBCO. It has been revealed that due to the presence of polar surfaces there is electronic reconstruction on the surface of a cleaved YBCO sample. A novel technique has been devised that allows one to control the surface doping level in-situ by evaporating Potassium (K) on the YBCO surface. We showed that K tunes the surface doping level by donating electrons and thereby makes it possible to continuously tune the surface doping level throughout the phase diagram.

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