ANISOTROPY DEPENDENCE OF THE C-AXIS PHONON-DISPERSION IN THE HIGH-TEMPERATURE SUPERCONDUCTORS

A recent calculation for La2CuO4 using a realistic electronic band str ucture based on the local-density approximation (LDA) for the,descript ion of the electronic density response has predicted a low-lying plasm on branch along the c-axis coupling with the phonons of appropriate sy mmetry. However, the calculated frequency of the plasmon is still high enough to screen the optical phonons more perfectly as seen in the op tical c-axis spectra which display the typical features of an ionic in sulator, namely optical A(2u) modes almost unchanged from the insulato r upon doping. On the other hand, this means that corresponding A(2u) discontinuities must show up in the phonon spectrum. However, this is in contrast with the current interpretation of the measured c-axis neu tron data for a La1.9Sr0.1CuO4 crystal looking ras is typical for a th ree-dimensional anisotropic metal in the adiabatic approximation, very similar to what we obtained in our LDA-based calculation. A possible solution for this ''inconsistency'' between optical and neutron result s is presented by investigating systematically the dependence of the p honon dispersion along the c axis on the anisotropy of the system. Mor eover, calculating the phonon-induced changes of the self-consistent p otential an electron feels as a function of anisotropy we find that LD A-based calculations, which underestimate the anisotropy of optimally doped LaCuO, lead to a very strong coupling (and thus to a favorable s ituation for pairing) in the plasmonlike channel. On the other hand, i n a more strongly anisotropic scenario the phononlike modes provide a favorable situation for pairing.