Raman scattering and X-ray diffraction study in layered cuprates

Abstract

Raman scattering and powder X-ray diffraction in the parent compounds of T,T'-phase and (Ca 0.86Sr 0.14)CuO 2 were measured to examine the two-magnon (ω 2m) and highest phonon energy (ω 2p) dependences on Cu-O bond length ( d), and to determine the structural parameters of the T'-phase. We found that both ω 2m and ω 2p exhibit strong power-law dependence on d, ω 2m∝ d -4.6, and ω 2p∝ d -6.3, in the T'-phase except for Gd 2CuO 4, and both energies for Gd 2CuO 4 deviate from the power-law relation. The strong d dependence of ω 2p suggests that this phonon is strongly coupled with charge-transfer electronic state. The observed phonon is assigned to two-phonon scattering of the bond-stretching mode of Cu and O atoms in the basal plane. The depression of ω 2p for Gd 2CuO 4 implies a structural change, such as oxygen distortion in the CuO 2 plane, which influences the charge-transfer energy gap Δ and ω 2m. The Δ values were estimated from the Madelung site potentials V i , which were calculated based on the crystal structural parameters determined by Rietveld analysis. Cu 3d-O 2p transfer energy t pd, and on-site Coulomb repulsive potentials, U d and U p, were determined using a one-dimensional Cu 2O cluster model by fitting ω 2m as a function of d. Finally, we discuss the relationship between crystal structure and carrier doping. The result of ΔV: ( V R - V Cu ) dependence on d suggests that the carrier concentration is not constant in T'-phase and change in the carrier concentration influences T c in these materials.