CALCULATED EFFECTS OF CHARGE FLUCTUATIONS ON THE PHONON-DISPERSION OFYBA2CU3O6 AND YBA2CU3O7 .2. RESULTS

Using the model presented in the preceding paper we investigate the ef fects of charge fluctuations (CF) on the phonon dispersion of YBa2Cu3O 6 (O6) and YBa2Cu3O7 (O7). Starting from an ab-initio rigid-ion model as a reference system, CF are allowed for at the copper- and oxygen io ns. The CF are treated as adiabatic electronic degrees of freedom. Wit hin the rigid-ion model (RIM) the structural parameters are calculated by minimization of the energy. The results agree reasonably well with the experiment, indicating the suitability of the ionic model as a st arting point and the importance of ionic forces for the properties of the high-temperature superconductors (HTSC) in general. Next, the phon on dispersion is calculated in the RIM as well as including CF additio nally and the renormalization of the individual modes is discussed. By restricting the CF optionally to the planes, effects arising specific ally from CF in the planes on the one hand and from CF in the chain as well as at the axial bridging oxygens (O4) on the other hand can be s eparated. We find the oxygen axial modes at the GAMMA- and Z point (A1 g/A21u in O6, Ag/B1u in O7) particularly interesting. Most of these mo des show considerable renormalizations. Moreover, the GAMMA/Z-axial mo des are characterized by the possibility of having CF of the same sign in the whole CuO planes what distinguishes them from the modes at oth er symmetry points. In particular, the Z-point axial modes are singula r in having CF of alternating sign in consecutive structural units in c direction. Such a ''c-direction-charge-transfer'' has been shown pre viously to be an effective screening mechanism in La2CuO4. Indeed, we find a drastic renormalization of the plane-oxygen A(g) mode at the Z point (A(g)(O23;Z)) in O7 (oxygen ions in neighboring planes vibrating in-phase), at least in the adiabatic approximation used here. In the insulating phase this mode exhibits, on the other hand, very large cha nges of the potential at the ion sites, whereas its renormalization is moderate only. The reason for this behaviour is that in the insulatin g phase in case of a two-dimensional electronic structure the charge t ransfer (screening) is restricted locally in the structural unit and l ong-range charge transfer is not possible as in the metal. However, a strong suppression of screening for this mode can also be expected for the metallic phase in O7 in case non-adiabatic electron-phonon coupli ng would be important. The A(g)(O23;Z)-mode thus seems to be by far th e most interesting mode in O7. These features are directly related to the layered structure of the HTSC compounds considered here. The O4-ax ial-breathing modes show significant renormalizations too, and are cha racterized by plane-chain charge transfer. Moreover, besides the O23- and O4-modes, the yttrium modes appear to be important too. In additio n to the phonon-dispersion curves, we present values for the CF amplit udes and screened site-potential changes at the copper- and oxygen ion s. Finally, we give transverse effective charges and dielectric consta nts for the insulating phase (O6) as calculated within our formalism.