Local structural studies of oriented high temperature superconducting cuprates by polarized XAFS spectroscopy

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Local structural studies of oriented high temperature superconducting cuprates by polarized XAFS spectroscopy

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Title: Local structural studies of oriented high temperature superconducting cuprates by polarized XAFS spectroscopy
Author: Haskel, Daniel, 1966-
Abstract: Doping (Sr,Ba) in La$\sb{2-x}$(Sr,Ba)$\sb{x}$CuO$\sb4$ induces high T$\sb{c}$ superconductivity in addition to profound changes in structural, magnetic and normal state electronic properties. The purpose of this thesis is to investigate the structural characteristics accompanying this doping by performing orientation dependent x-ray absorption fine structure (XAFS) measurements on magnetically aligned powders. This type of measurements allowed obtaining critical information at the La/(Sr,Ba) site previously unavailable, as detailed below.The measurements show that hole carriers introduced with Sr are polaronic in nature as evident from the two site configuration found for the O(2) apical neighboring Sr and the lack of temperature dependence in the O(2) distribution, which indicates that the hole states associated with each site are not discrete but rather broader than $k\sb{B}$T up to T = 300K. There is a good theoretical argument suggesting each O(2) site is associated with holes being doped into O(1) 2p$\sb{x,y}$-Cu 3d$\sb{x\sp2-y\sp2}$ in-plane and O(2) 2p$\sb{z}$-Cu 3d$\sb{3z\sp2-r\sp2}$ out-of-plane electronic bands resulting in two different Jahn-Teller distortions of the CuO$\sb6$ octahedra neighboring Sr, where the doped holes are peaked. Based on this argument, the predominance of out-of-plane character for the doped holes, as evidenced from the concentration dependence of the relative population of O(2) sites, would imply that theories of high T$\sb{c}$ relying only on in-plane character of the doped holes are not complete in describing the properties of these cuprates.Our measurements showed that all structural phase transitions in La$\sb{2-x}$(Sr,Ba)$\sb{x}$CuO$\sb4$ have a significant order-disorder component, as opposed to the purely displacive models found in crystallographic studies. The CuO$\sb6$ octahedra are locally tilted in the high-doping, high-temperature phases but fail to order over long range resulting in the average structures of the crystallographic studies. A critical parameter in assessing the plausibility of a density of states (DOS) effect causing T$\sb{c}$ suppression in $\rm La\sb{1.875}Ba\sb{0.125}CuO\sb4,$ the tilt angle of the CuO$\sb6$ octahedra in the LTT phase, was directly measured and found to be locally larger than the average value measured by diffraction techniques. This result strengthens the theoretical case for a DOS effect at the basis of T$\sb{c}$ suppression at $x=0.125.$
Description: Thesis (Ph. D.)--University of Washington, 1998
URI: http://hdl.handle.net/1773/9712

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