Infrared- and Raman-active modes of Bi-based cuprate superconductors calculated in a microscopic model

Abstract

We present phonon frequencies and eigenvectors for the c-axis-polarized infrared- and Raman-active modes of A2u and A1g symmetry in the single-layer Bi2Sr2CuO6 (Bi-2201) and the bilayer Bi2Sr2CaCu2O8 (Bi-2212) cuprate superconductors within our recently proposed microscopic formulation of the electronic density response and lattice dynamics in high-temperature superconductors (HTSCs). The model includes localized electronic charge-fluctuations as well as anisotropic dipole-fluctuations. The former have been shown to be the dominating polarization process in the CuO planes while the latter are particularly important for the ions in the ionic layers of the HTSC. The effect of both types of screening contribution on the c-axis lattice dynamics of the Bi-based cuprates is investigated. A characteristic and strong renormalization of certain A2u and A1g modes is shown to be related to the metallicity of the BiO layers. We obtain a good agreement between the calculated A2u frequencies and new experimental results obtained by ellipsometric measurements on single crystals and propose an assignment of the modes. Moreover, the calculated A1g Raman modes and eigenvectors are compared with measured frequencies and the contradictory assignments of the modes found in the literature.