RAMAN-SCATTERING STUDY OF R2CUO4 (R=PR, ND, SM, AND GD)

For Gd2CuO4, a symmetry-forbidden phonon for the tetragonal T' structu re is observed. This shows that oxygen in the CuO2 plane is locally di storted along the plane. Such distortion has not been observed for oth er T'-type superconductors. The normal-mode calculation shows that the correlation between the CuO2 plane and R-O layers increases with decr easing ionic radius. The increased three-dimensional character and the appearance of the distortion of O in the CuO2 plane along the a axis are useful experimental results that help clarify the mechanism that c auses the disappearance of superconductivity for Gd.