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 Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. 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 ${\ensuremath{\sigma}}^{2}(T)$ for Cu-O and $\mathrm{Y}\ensuremath{-}X$ ($X=\mathrm{O}, \mathrm{Cu}, \mathrm{Ba}, \mathrm{and} \mathrm{Y}$) 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 Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. 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.