The phonon dynamics of Sr(2)RuO(4): microscopic calculation and comparison with that of La(2)CuO(4).

The phonon dynamics of the low-temperature superconductor Sr(2)RuO(4) is calculated quantitatively in linear response theory and compared with that of the structurally isomorphic high-temperature superconductor La(2)CuO(4). 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 Sr(2)RuO(4) 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 Λ(1) mode in La(2)CuO(4) with a very steep dispersion coupled strongly to the electrons is not found for Sr(2)RuO(4). Moreover, we evaluate the possibility of a phonon-plasmon scenario for Sr(2)RuO(4), which has been shown recently to be realistic for La(2)CuO(4). In contrast to the case for La(2)CuO(4), in Sr(2)RuO(4) the plasmons that are very low lying are overdamped along the c-axis.