LATTICE-DYNAMICS OF YBA2CU3O6+X (X = 0,1)

We report shell model based calculations of the lattice dynamics of YBa2Cu3O6 and YBa2Cu3O7. Two models are proposed for Yba2cu3o6, the model parameters being obtained by a least squares fit to recent neutron scattering results. The parameters obtained are then used, with modification, to model the lattice dynamics of YBa2Cu3O7. Since this material has significant carrier concentration, even in the normal state we have used both screened and unscreened versions of the shell model. The screened model gives a better overall description of the lattice dynamics of YBa2Cu3O7. The phonon dispersion curves along the symmetry directions and the weighted phonon density of states for both YBa2Cu3O6 and YBa2Cu3O7 are presented.