J. Mustre de Leon

Axial oxygen-centered lattice instabilities in YBa sub 2 Cu sub 3 O sub 7 : An application of the analysis of extended x-ray-absorption fine structure in anharmonic systems

Analysis of polarized copper {ital K}-edge extended x-ray-absorption fine-structure (EXAFS) measurements on YBa{sub 2}Cu{sub 3}O{sub 7} for temperatures 10{le}{ital T}{sub nom}{le}105 K indicates that the axial oxygen moves in a double-well potential which softens within a fluctuation region associated with the onset of superconductivity in this material. This fluctuation follows from the coupling between the phonons derived from this double-well potential and the superconducting order parameter. The advantages of EXAFS compared to crystallographic measurements in discerning these aspects of the local structure are discussed. Metrical parameters and the characteristics of the potential are determined by curve fits of the EXAFS. This method is based on the calculation of radial distribution functions from selected model potentials, the forms of which are determined along with the absorber-scatterer distance, scatterer type, and number, by optimization of fits of the experimental data with the EXAFS calculated from these parameters. Unlike perturbative treatments, this approach is ideally suited for highly anharmonic systems because the putative potential can be a very close approximation to the real one and it also offers the advantage of providing dynamical information not available through perturbative treatments.

Relative importance of recent improvements in the modelling of substrate X-ray photoelectron diffraction - Ni 2p3/2 emission from Ni(001)

In an attempt to assess the relative importance of recent improvements in the theoretical description of X-ray photoelectron diffraction (XPD) data we consider an extensive set of Ni 2p3/2 azimuthal measurements from a clean Ni(001) surface, taken at a very high angular resolution of approximately ±1.0°. In particular, we discuss the sensitivity of the theory-experiment agreement to the variation of the following parameters: cluster size, electron inelastic attenuation length, substrate inner potential, photon polarization, angular broadening due to the finite acceptance of the electron analyzer, and the absolute accuracy in the experimental emission directions relative to the crystal axes. It is then demonstrated how different levels of scattering cluster theory, including single scattering (SS) of plane p-waves, SS of spherical p-waves, SS of spherical waves with proper angular momentum final states, and finally multiple scattering (MS) of spherical waves, affect the calculated XPD curves. Experiment and theory are compared by means of R-factors, and we are thus able to assess the reliability of structural information that one can expect at different levels of theory, thereby minimizing computational complexity.

An assessment of multiple-scattering effects in Auger electron and photoelectron diffraction

Abstract Multiple-scattering effects in Auger electron and photoelectron spectroscopies are assessed using a new spherrical-wave multiple-scattering formalism which is based on a separable approximation to the scattering Greens functions.