Raman scattering from high-Tc superconductors.

Abstract

Raman scattering experiments from high-1; superconductors of the RBa2Cu307-~ type, with R Y, Pr, Nd, Eu, Gd, Dy, and Yb, are reported. Because of the great difBculty in obtaining single-phase samples, spectra from all starting compounds and other known phases of Y-Ba-Cu are presented. This systematic approach has allowed the identi6cation of the lines observed in Raman spectra of the superconducting compounds as well as those of impurity phases. The lines originating from the superconducting yttrium compound were investigated as a function of quench temperature, and oxygen de5ciency. Polarization studies on single crystals are also presented. L IM acODUi-nON An important experiment to test the conventional electron-phonon-mediated Bardeen-Cooper-Schrieffer (BCS) mechanism for superconductivity in the recently discovered metallic copper oxides'2 is the isotope effect, s s in which ' 0 is replaced by ' O. On substitution no changes were observed in the superconducting properties in Refs. 3 and 4; while in Ref. 5 a small isotope effect is claimed. Nevertheless, a study of the phonon properties is crucial for an understanding of these intriguing materials even if the superconducting mechanism is proven not to be phonon mediated. During the short time since the discovery of these materials, there have already been numerous reports dealin~ with phonon properties, including infrared studies, 7 ' Raman studies, ' 's and inelastic neutron scattering studies. 's Even among the eleven Raman-scattering references substantial discrepancies are present. In agreement with Ref. 15, we find that many of these discrepancies are due to impurity phases, which, because of the usual preparation techniques, are almost always present in these compounds. Many of these phases are not metallic and consequently have much larger cross sections for Raman scattering so, that even though the impurities may be a small volume fraction, they can dominate the Raman spectra. Here we report the results of a systematic study of the YBa2Cus07 s system. We present spectra obtained from the usual starting compounds as well as from the known impurities which are likely to be present in typical samples. A comparison of these spectra with the ones obtained from nominally pure YBa2Cu307 — s has allowed the identification of many impurity lines, and more importantly, those hnes belonging to the superconducting compound. In addition to the identification of the lines originating from the superconducting compound, the behavior of a number of them as a function of quench temperature, oxygen stoichiometry, substitution of Y by a number of rareearth metals, and polarization studies on single crystals, are presented. Section II is a detailed factual report of our results. In Sec. III these are discussed and compared with other reports in the literature and our conclusions are given in Sec. IV.