Microscopic calculation of the phonon dynamics of Sr 2 RuO 4 compared with La 2 CuO 4

The phonon dynamics of the low-temperature superconductor Sr2RuO4 is calculated quantitatively in linear response theory and compared with the structurally isomorphic high-temperature superconductor La2CuO4. Our calculation corrects for a typical deficit of LDA-based calculations which always predict a too large electronic kz-dispersion insufficient to describe the c-axis response in the real materials. With a more realistic computation of the electronic band structure the frequency and wavevector dependent irreducible polarization part of the density response function is determined and used for adiabatic and nonadiabatic phonon calculations. Our analysis for Sr2RuO4 reveals important differences from the lattice dynamics of pand n-doped cuprates. Consistent with experimental evidence from inelastic neutron scattering the anomalous doping related softening of the strongly coupling highfrequency oxygen bond-stretching modes (OBSM) which is generic for the cuprate superconductors is largely suppressed or completely absent, respectively, depending on the actual value of the on-site Coulomb repulsion of the Ru4d orbitals. Also the presence of a characteristic Λ1-mode with a very steep dispersion coupling strongly with the electrons is missing in Sr2RuO4. Moreover, we evaluate the possibility of a phonon-plasmon scenario for Sr2RuO4 which has been shown recently to be realistic for La2CuO4. In contrast to La2CuO4 in Sr2RuO4 the very low lying plasmons are overdamped along the c-axis. PACS numbers: 74.25Kc, 63.20.Dj, 74.70Pq, 74.72.Dn Microscopic calculation of the phonon dynamics of Sr2RuO4 compared with La2CuO4 2