Raman scattering study of Sr_14-xCa_xCu₂₄O₄₁

Polarized Raman scattering spectra of Sr_14-xCa_xCu₂₄O₄₁ have been measured for x = 0 and 11.5 at ambient pressure. In the spectra, the intra-molecular vibrations due to the unit structure of (Sr/Ca)₂Cu₂O₃ and CuO₂ have been clearly observed below 700 cm^-1. A comparison of the number of the observed peaks with the results of a factor group analysis indicates that the crystallographic symmetry of the ladder is F₂₂₂ for x = 0 and 11.5, instead of centro-symmetric F_mmm, while that of the chain is A_mma for both compositions. However the symmetry of the chain for x = 11.5 is close to F₂₂₂ or F_mmm. That is, the structure of the chains changes from an incomplete staggered structure to almost complete staggered one with increasing x. In the (c, c) and (a, a) spectra, the broad two-magnon peaks have been observed at around 3000cm^-1. From the analysis of the recent theory of the magnetic scattering in the 2-leg spin ladder system done by Natsume et al., the following magnetic parameters for x = 0 are obtained: J_leg = 151 meV, J_rung = 91 meV, and Δ_ladder = 44 meV. The values agree with those reported by neutron scattering.