The phonon dynamics of Sr2RuO4: microscopic calculation and comparison with that of La2CuO4

The phonon dynamics of the low-temperature superconductor Sr2RuO4 is calculated quantitatively in linear response theory and compared with that of the structurally isomorphic high-temperature superconductor La2CuO4. Our calculation corrects for a typical deficiency of local density approximation-based calculations, which always predict too large an electronic k(z)-dispersion insufficient for describing the c-axis response of 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 p- and n-doped cuprates. Consistently with experimental evidence from inelastic neutron scattering, the anomalous doping related softening of the strongly coupling high-frequency oxygen bond-stretching modes 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 Ru 4d orbitals. Also the presence of a characteristic Lambda 1 mode in La2CuO4 with a very steep dispersion coupled strongly to the electrons is not found for 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 the case for La2CuO4, in Sr2RuO4 the plasmons that are very low lying are overdamped along the c-axis.