PHONON-DISPERSION OF LA2CUO4 FROM A TIGHT-BINDING ANALYSIS OF THE LAPW BAND-STRUCTURE

The phonon dispersion curves for LaCuO are calculated for the main sym metry directions. This is achieved by decomposing the electronic densi ty response and correspondingly the electron-phonon interaction into l ocal and nonlocal contributions. The local part is approximated by an ab initio rigid ion model taking ion softening into account as calcula ted from a tight-binding analysis of the LAPW bandstructure. Moreover, covalence effects are treated approximatively. Such a model serves as a reference system for the insulating phase. For the description of t he metallic phase the electronic polarizability Pi is calculated in th e tight-binding representation from the LAPW results and nonlocal scre ening effects of the charge-fluctuation type in the ionic shells are c onsidered. The calculated phonon frequencies are compared with those a s obtained by earlier investigations of Pi using a less complete repre sentation of the electronic bandstructure and also with new, specific experimental results from inelastic neutron scattering exploring impor tant phonon anomalies. From our calculations the growing importance of the itinerant component of the charge carriers in the CuO plane to ac hieve high T-c-values can be extracted.