We report on first investigation of the lattice dynamics in the novel superconducting material Na_0.35CoO_2⋅1.3H_2O and the non-hydrated parent compound Na_0.7CoO_2 by inelastic x-ray scattering. The measured phonon dispersion along the Γ−M direction show a marked softening with hole doping of two optical phonon branches close to the Brillouin zone boundary. The phonon spectra, dispersion, and softening are well reproduced by first-principle calculations. The calculations indicates that the soft branches are mainly composed of Co-vibration modes. The estimation of the critical temperature based on electron-phonon coupling mechanism undisputedly points to a non-conventional superconducting state in this material.
@article{arxiv.cond-mat/0601288,
title = {Evidence for non-conventional pairing in Na$\_x$CoO$\_2$$\cdot$yH$\_2$O},
author = {Jean-Pascal Rueff and M. Calandra and M. D'astuto and Ph. Leininger and A. Shukla and A. Bossak and M. Krisch and H. Ishii and Y. Cai and P. Badica and T. Sasaki and K. Yamada and K. Togano},
journal= {arXiv preprint arXiv:cond-mat/0601288},
year = {2007}
}