Phonon softening in Nax Co O2·y H2 O: Implications for the Fermi surface topology and the superconducting state

In this work we investigate the relevance of phonons for the superconductivity in Na0.35 Co O2·1.3 H2 O. We measure phonon dispersion in this material as well as in the nonhydrated parent compound Na0.7 Co O2 by inelastic x-ray scattering. The measured phonon dispersion along the Γ-M direction shows a marked softening of Co-related optical phonon branches close to the Brillouin zone boundary, with hole doping. The phonon spectra, dispersion, and softening of Co-vibration modes are well reproduced by first-principles calculations. The critical temperature estimated from a phonon-mediated mechanism is much lower than the measured one, suggesting a nonconventional superconducting state in this material.