Resonant Raman scattering in the superconducting cuprates: Frozen-phonon versus perturbational approach

We consider in detail Raman scattering by vibration of the apical oxygen io ns in the RBa2Cu3O7 superconducting cuprates. The scattering intensity is v ery sensitive to the ratio of diagonal and off-diagonal matrix elements of electron-phonon coupling, bandstructure, and carrier concentration. Our res ults show a large quantitative difference between the results of frozen-pho non and perturbational approach to the Raman process. The discrepancy becom es especially large when interband transitions to the states near the Fermi level are close to resonance with the incident light. The calculation of p honon-induced ion charge fluctuations shows an analogous discrepancy. The r eason for these effects is the possibility of carrier redistribution betwee n different parts of the Fermi surface arising in the frozen-phonon approxi mation. Our results show that Raman scattering in superconducting superlatt ices is very sensitive to the properties of the states near the Fermi level . For this reason experiments performed on the superlattices can help to re solve the discrepancy.