Susan M. Mini

A new approach to determining the charge distribution in copper compounds

An analysis of the energy moments of X-ray absorption spectra at the copper K edge has been used to assign characteristic energies for a series of copper compounds with formal valences of +1, +2, and +3. The results indicate that there is a good correlation between the characteristic energies defined in this way for near-edge X-ray absorption spectra and the charge distribution in these copper compounds.

The relationship between the CuO bond length and Tc in the superconducting series La1.85 − xSr0.15NdxCuO4

Abstract We have examined the relationship between variations in the CuO bond length and the superconducting critical temperature in the series (La 1.85 − x Nd x )Sr 0.15 CuO 4 . Comparing lattice constants determined from X-ray diffraction, and resistivity determined by four-probe resistance, we see that the maximum in T c does not correspond to the minimum CuO bond length. We conclude that the CuO bond distance is not the only variable which affects T c in this system.

General-purpose high-resolution double-channel-cut monochromator for use on the ChemMatCARS insertion-device beamline at the Advanced Photon Source

The high-brilliance radiation provided by third-generation synchrotron x-ray facilities such as the Advanced Photon Source has greatly enhanced the power of such chemical and materials research techniques as x-ray-absorption spectroscopy, surface scattering, small-angle x-ray scattering, and chemical crystallography. In order to take full advantage of these source properties, we propose the development of a general-purpose high-energy-resolution double-channel-cut monochromator based on a similar device developed for nuclear-resonant-scattering techniques by the Experimental Facilities Division at the APS. The instrumentation will be installed at the APS on the ChemMatCARS insertion-device beamline at sector 15. It will be used primarily for valence-specific static and time-resolved chemical-crystallography experiments as well as anomalous- scattering experiments planned for surface diffraction and small-angle/wide-angle x-ray scattering. Two channel-cut crystals placed in a dispersive (-n, +n, +n, -n) geometry provide a fixed-offset beam with an energy resolution (Delta) E/E approximately 10-5. Several sets of channel-cut-crystal pairs, designed to cover an energy range from 6 to 35 keV with a matching angular acceptance to the source, are used in conjunction with the beamline high- heat-load double-crystal monochromator.

Polarized x-ray absorption spectroscopy for the study of superconductors and magnetic materials

Synchrotron radiation is a good source of polarized radiation in the x-ray regime. The radiation obtained from a bending magnet source is linearly polarized in the bending plane and has a varying degree of circular polarization away from the bending plane. This feature of synchrotron radiation can be taken advantage of with proper optics to selectively use the type of polarized radiation required for the experiment in question. Linear polarized radiation is used to study the anisotropic nature of electronic and atomic structure by x-ray absorption techniques from single crystal and oriented powder samples. We will give a specific example of the use of linearly polarized x-ray absorption spectroscopy measurements for the study of the magnetically oriented layered copper oxide superconductors. While such linear dichroism measurements help identify the symmetry of the empty electronic states, circular dichroism measurements in magnetic systems help in determining the spin contribution to the absorption process. We will discuss magnetic circular dichroism measurements of the ordered-disordered invar alloy Fe3Pt.

X-ray absorption spectroscopy with polarized synchrotron radiation

X-ray absorption spectroscopy based on the polarized nature of synchrotron radiation is considered. It is concluded that linearly polarized radiation can be used for studying the anisotropic nature of electronic and atomic structure by measuring X-ray absorption near edge structure and extended X-ray absorption fine structure spectra of single crystals or oriented powders. Circularly polarized radiation can be used for studying the spin density of final electronic states in a magnetic system. Particular attention is given to experimental results on absorption of linearly polarized radiation through a highly anisotropic medium of oriented copper oxides at the Cu K-edge.

Polarization-dependent EXAFS studies in layered copper oxide superconductors

Linearly polarized synchrotron radiation is used to study the structure of magnetically aligned layered copper oxide high temperature superconductors. Polarized extended X-ray absorption fine structure (EXAFS) measurements of oriented powders of YBa2Cu3O6.9 are reported at temperatures ranging from 20-300 K. Measurements made at the Cu K-edge with the X-ray polarization vector oriented perpendicular and parallel to the c axis can be used to effectively probe the structure around the Cu-atoms in the ab (horizontal layer) plane as well as in the c (vertical layer) plane.

Probing Charge on Cu in Oxide Superconductors

The non-destructive measurement of charge on an ion in a solid has always been a source of great controversy. In this chapter, we will review how x-ray absorption spectroscopy addresses this issue. We will describe the underlying principles of the technique, list some recent experiments, and try to explain the controversial aspects. A new method of evaluation of the position of the absorption edge will be introduced, and its relation to the Mulliken populations determined by discrete variational method for calculating orbital states of a molecular cluster will be discussed. We will then show how x-ray absorption spectroscopy can probe variations in effective charge on atoms as a result of chemical doping. Finally, we will point out some newer developments as a result of using polarized x-rays and oriented crystals.