Untwinned single crystals of YBa2Cu3O7- delta : An optical investigation of the a-b anisotropy.

Raman and IR-measurements on the high- T c material series La 2 CuO 4 and YBa 2 Cu 3 O 7 are reviewed. This summary focuses on lattice vibrations in these superconducting systems, on impurity phases, the influence of oxygen stoichiometry and element substitution. The determination of the superconducting gap, the phonon renormalization in the superconducting state, and the isotope effect with its implications on the theory of superconductivity in these materials are discussed. In addition, the measured lattice vibrations are compared with the results of lattice dynamics calculations.

Lattice vibrations in high-Tc superconductors: Optical spectroscopy and lattice dynamics☆

The infrared reflectivity of sintered ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, prepared under carefully controlled oxidation conditions, has been measured between 20 and 10 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at temperatures between 10 and 300 K. In this paper we analyze in detail the temperature- and frequency-dependent far-infrared part of the spectrum. A Kramers-Kronig analysis allows us to compare the phonon part of the spectra with lattice-dynamical calculations. There is fairly good agreement concerning the assigned eigenfrequencies. The oscillator strengths of the experimentally observed phonons, however, are all stronger than those from a lattice-dynamics model which assumes ionic charges close to the formal ones. This is especially so for the lowest phonon found at 155 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which shows a strength S=60\ifmmode\pm\else\textpm\fi{}5 being about 15 times higher than theoretically expected. A possible origin of this effect is discussed qualitatively in terms of a Cu(1)-O(IV) charge shift during the vibration induced by the polarization field. The electronic background of the spectra has been analyzed by a Mattis-Bardeen type of dielectric response below ${T}_{c}$. The best fit yielded, for low T, a gap distribution in the range from about 160 to 370 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, corresponding to 2\ensuremath{\Delta}(0)/${k}_{B}$${T}_{c}$ values between 2.6 and 6.0.

Phonon anomalies and range of superconducting energy gaps from infrared studies of YBa2Cu3O7- delta.

Temperature-dependent positron-electron momentum densities have been studied by two-dimensional angular correlation of annihilation radiation from 10 to 320 K in YBa/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub x/ samples. The positron ground-state charge density, computed by the linearized augmented plane-wave method, indicates that in YBa/sub 2/Cu/sub 3/O/sub 7/ delocalized positrons sample preferentially the linear copper-oxygen chains. Positron localization due to disorder in these chains is invoked to explain the striking differences observed between superconducting (xapprox. =0.02) and nonsuperconducting (xapprox. =0.70) samples

High sensitivity of positrons to oxygen vacancies and to copper-oxygen chain disorder in YBa2Cu

By means of inelastic neutron scattering we have studied the phonon densities of states (PDOS) ofY based 123 superconductors with oxygen concentrations varying between O7 and O6. We find drastic changes in the PDOS above 40 meV which develop in a systematic manner if we switch from superconducting to semiconducting samples. Model calculations clearly show that this cannot be explained solely by structural changes but is likely to reflect strong differences in the electron-phonon coupling.

Strong changes in the phonon spectra of 123 superconductors by varying oxygen concentration

Infrared and Raman spectra of the new superconducting cuprate perovskites MBa2Cu3O7, M =Nd, Dy, Er, Tm

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Infrared and Raman spectra of the new superconducting cuprate perovskites MBa2Cu3O7, M =Nd, Dy, Er, Tm