Evidence for non-conventional pairing in Na$\_x$CoO$\_2$$\cdot$yH$\_2$O

TL;DR

This study investigates lattice dynamics in Na$_x$CoO$_2$·yH$_2$O, revealing phonon softening with doping and suggesting a non-conventional superconducting mechanism based on electron-phonon interactions.

Contribution

First inelastic x-ray scattering analysis of phonon behavior in Na$_x$CoO$_2$·yH$_2$O, linking phonon softening to non-conventional superconductivity.

Findings

Phonon dispersion shows significant softening near zone boundary with doping.

First-principle calculations match experimental phonon spectra.

Electron-phonon coupling estimates support non-conventional pairing.

Abstract

We report on first investigation of the lattice dynamics in the novel superconducting material Na$\_{0.35}$CoO$\_2$$\cdot$1.3H$\_2$O and the non-hydrated parent compound Na$\_{0.7}$CoO$\_2$ by inelastic x-ray scattering. The measured phonon dispersion along the $\Gamma-M$ 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.