Physical origin of the buckling in CuO2: Electron-phonon coupling and Raman spectra

It is shown theoretically that the buckling of the CuO2 planes in certain c uprate systems can be explained in terms of an electric held across the pla nes which originates from different valences of atoms above and below the p lane, This held results also in a strong coupling of the Raman-active out-o f-phase vibration of the oxygen atoms (B-1g mode) to the electronic charge transfer between the two oxygens in the CuO2 plane. Consequently, the elect ric held can be deduced from the Fano-type line shape of the B-1g phonon. U sing the electric field estimated from the electron-phonon coupling the amp litude of the buckling is calculated and found to be in good agreement with the structural data. Direct experimental support for the idea proposed is obtained in studies of YBa2Cu3O6+x and Bi2Sr2(CaxY1-x)Cu2O8 with different oxygen and yttrium doping, respectively, including antiferromagnetic sample s. In the latter compound, symmetry breaking by replacing Ca partially by Y leads to an enhancement of the electron-phonon coupling by an order of mag nitude.