Electron–Phonon Interaction and Phonon Renormalization in the Lamellar Cobaltate Na x CoO2

Abstract

We study theoretically the electron–phonon interaction in Na x CoO2. For the A 1g and E 1g phonon modes found in Raman experiments, we calculate the matrix elements of the electron–phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency ω, we investigate the doping dependence of these two phonon modes. Due to the momentum dependence of the electron–phonon interaction, we find the strongest renormalization of the E 1g mode around the Brillouin zone boundary which should be observed in the neutron scattering. At the same time, the A 1g mode shows the strongest coupling to the conducting electrons around the Γ point and reveals its doping dependence in the Raman experiments. Our results shed light on the possible importance of the electron–phonon interaction in the lamellar sodium cobaltates.