The influence of phonon symmetry and electronic structure on the electron-phonon coupling momentum dependence in cuprates

The experimental determination of the magnitude and momentum dependence of electron-phonon coupling (EPC) is an outstanding problem in condensed matter physics. The intensity of phonon peaks in Resonant Inelastic X-ray Scattering (RIXS) spectra can be related to the underlying EPC strength under significant approximations whose validity deserves careful verification. We measured the Cu L3 RIXS phonon intensity as a function of incident photon energy and momentum transfer in several layered cuprates. For CaCuO2, La2−xSrxCuO4+δ, and YBa2Cu3O6, using a generally accepted theoretical model, we quantitatively estimate the EPC for the bond-stretching mode along the high-symmetry directions (ζ,0) and (ζ,ζ), and as a function of the azimuthal angle φ at fixed q∥. We compare our results with theoretical predictions and find that the q∥-dependence of the phonon RIXS intensity can be largely ascribed to the phonon symmetry. However, a more satisfactory prediction of the experimental results requires an accurate description of the electronic structure close to the Fermi level. Our extensive investigation indicates that Cu L3 RIXS can reliably determine the momentum dependence of EPC for the bond-stretching modes of cuprates. Moreover, the large experimental basis provided here constitutes a stringent test for advanced theoretical predictions on the EPC.