One type of order that has been observed to compete with superconductivity in cuprates involves alternating charge and antiferromagnetic stripes. Recent neutron scattering studies indicate that the magnetic excitation spectrum of a stripe-ordered sample is very similar to that observed in superconducting samples. In fact, it now appears that there may be a universal magnetic spectrum for the cuprates. One likely implication of this universal spectrum is that stripes of a dynamic form are present in the superconducting samples. On cooling through the superconducting transition temperature, a gap opens in the magnetic spectrum, and the weight lost at low energy piles up above the gap; the transition temperature is correlated with the size of the spin gap. Depending on the magnitude of the spin gap with respect to the magnetic spectrum, the enhanced magnetic scattering at low temperature can be either commensurate or incommensurate. Connections between stripe correlations and superconductivity are discussed.
For anisotropic structures such as those found for the high Tc superconductors, the utility of x-ray absorption measurements can be enhanced by taking advantage of the inherent polarization of synchrotron radiation. Polarization dependent measurements have been made on all four classes of high Tc materials (La, Y, Bi, and Tl based Cu oxides) after orienting them magnetically. All show similar polarization dependence for their near edge structure which is characteristic of planar Cu compounds. The paper concentrates on results for YBa2Cu3O7-x materials. The two types of Cu sites in this material have different polarization dependence, and measurements with the x-ray polarization parallel and perpendicular to the c-axis can be used to distinguish their contributions to the near edge structure. We can clearly resolve the 4p(pi) contributions from the two sites. The extended fine structure also shows distinct polarization dependence which allows the separation of contributions from in-plane and out-of-plane bonds. Measurements on oxygen deficient materials show contributions from Cu1+ which is determined to be on the Cu(1) (linear chain) sites from the polarization dependence, and to vary linearly with the O concentration.
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