X-ray-absorption studies ofYBa2Cu3O7−δandGdBa2Cu3O7−δsuperconductors

Abstract

X-ray-absorption measurements of the high-${T}_{c}$ superconductors $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ show that their near-edge and extended fine structures (EXAFS) are essentially identical over the temperature range of 4.2 to 688 K. The results indicate that not only are $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ very similar to each other in local bonding and electronic structure, but also remain virtually unchanged, electronically and structurally, over this wide range in temperature, from far below to well above ${T}_{c}$. The positions of the Cu near-edge spectra for these superconductors suggest a Cu valence between that of Cu in CuO, which has a formal valence of II, and that in KCu${\mathrm{O}}_{2}$, with a formal valence of III. Our EXAFS structural results agree well with those from neutron diffraction data. Recently proposed alternate structures are shown to be inconsistent with our data. The characteristic vibration frequencies of the near-neighbor atoms show that the two-dimensional Cu---O planes and the one-dimensional Cu---O chains form the most rigid parts of the structure, and these layers are bound more weakly along the $c$ axis via the Ba and Y planes. These oscillator frequencies are identified with specific near-neighbor pairs, and the values are consistent with those for some of the modes seen by inelastic neutron scattering as well as by infrared and Raman spectroscopy. No evidence of phonon softening is apparent in any of the phonon modes studied, providing evidence against a strong electron-phonon coupling mechanism.