Phonon dispersion and anomalies in one-layer high-temperature superconductors

Abstract

Phonon dynamics, charge response and the phonon density ofstates are calculated for the high-temperature superconductorsHgBa2CuO4 andBi2Sr2CuO6 within amicroscopic model for the electronic density response. The results are compared with previous calculationsfor La2CuO4 and Nd2CuO4. Our main focus is on the phonon anomalies which are connected with the high-frequencyoxygen bond-stretching modes (OBSM) found before in our calculations for p-dopedLa2CuO4 andn-doped Nd2CuO4. We investigate the question if the characteristic softening of the OBSMand the related strong coupling to the electrons is also present inHgBa2CuO4 andBi2Sr2CuO6. In particular, the importance of the contribution of the more delocalized Cu 4s statebesides the localized Cu 3d state on the softening is investigated and the differentanticrossing behavior due to the presence of several phonon modes with the same symmetryas the OBSM is studied. This makes the identification of the anomalies quitecomplicated. Furthermore, the influence of electronic polarization processes at ionsout of the CuO plane in the ionic layers on the phonon dynamics is calculated.In this context the qualitative type of charge response, i.e. the presence or notof possible metallic charge fluctuations at the Hg or Bi ion, respectively, linkedvia the apex oxygen to the CuO plane, proves to be very sensitive for certainphonon modes. All the calculations are compared to the experimental resultsavailable so far. The latter, however, are rather incomplete for the Hg and Bicompounds.