Lattice vibrational studies of superconducting YBa2Cu3O7 by polarized extended x-ray-absorption fine-structure measurements

Abstract

We report polarization-dependent extended x-ray-absorption fine-structure (EXAFS) measurements of the superconducting YBa2Cu3O7. Measurements made at the Cu and Y K edges with the x-ray polarization vector oriented perpendicular and parallel to the c axis have enabled us to probe scattering atoms in the horizontal layer plane as well as in the vertical layer plane. Three different Cu-O vibrational modes which are associated with the Cu(1)-O pair and the Cu(2)-O pair along the c axis and the planar Cu-O pair are distinguished. The lowest vibrational frequency of the Cu(2)-O pair along the c axis indicates that its bond strength is much weaker than those of other Cu-O pairs. No anomalies are observed in the temperature dependence of the mean-square relative displacements σ2(T) for Cu-O and Y−X (X=O, Cu, Ba, and Y) pairs. There appears to be no significant softening of any phonons which modulate Cu-O bond lengths due to coupling with electrons, thus casting some doubt on the importance of phonons for superconductivity in YBa2Cu3O7. The relation between EXAFS results and other vibrational investigations is presented. The discrepancy between current data and other EXAFS reports is also discussed. Furthermore, a careful examination of orientation-dependent Y spectra has ruled out the possibility of extensive Cu and Y antisite disorder.

Source-reported materials — not catalogue approval

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