FIRST PRINCIPLE CALCULATION OF THE RAMAN-SPECTRUM RELATED TO THE APICAL OXYGEN VIBRATION IN YBA2CU3O7

The lattice dynamical potential of the apical oxygen in YBa2Cu3O7 has been determined by the full potential linearized augmented plane wave (LAPW) total energy calculation. The vibronic potential has a single m inimum but anharmonic contributions of the third and fourth order. The vibrational Ramanspectrum for such a potential has been computed usin g the derivatives of the complex dielectric function with respect to t he apical oxygen. Good agreement has been obtained with experimental ( zz) polarized Ramanspectra of YBa2Cu3O6.9 single crystals not only in its fundamental mode but also in the overtones and their relative inte nsities. For comparison other potentials as e.g. double well structure s have been tested.