Effect of Na content and hydration on the excitation spectrum of the cobaltite Na_xCoO_2 yH_2O

TL;DR

This study uses Raman scattering to explore how sodium content and hydration affect the lattice dynamics and electronic structure of superconducting Na_xCoO_2·yH_2O, revealing links between disorder, electronic levels, and superconductivity.

Contribution

It provides new insights into how Na content and hydration influence lattice strain, electronic levels, and superconductivity in Na_xCoO_2·yH_2O through Raman spectroscopy analysis.

Findings

Hydration reduces lattice strain caused by structural misfit.

Disorder on Na sites correlates with the splitting of the a1g electronic level.

Electronic structure changes are linked to superconductivity.

Abstract

We report on a Raman scattering study on the superconducting cobaltite $Na_xCoO_2\cdot yH_2O$ as function of Na content and hydration (x$\approx$1/3, 3/4 and y$\approx$0, 2/3, 4/3). The observed phonon scattering and scattering continua are analyzed in terms of lattice strain due to the structural misfit and disorder. Hydration, due to the intercalation of one or two $H_2O$ layers, releases a part of this strain. Our Raman data suggest a connection between disorder on the partly occupied Na sites, the split off of the $a_{1g}$ level from the other $t_{2g}$ states of $Co^{4+}$ and superconductivity.