59Co NMR study of the Co states in superconducting and anhydrous cobaltates

TL;DR

This study uses $^{59}$Co NMR to analyze cobalt electronic states in different cobaltates, revealing charge disproportionation and differences between superconducting and non-superconducting phases.

Contribution

It provides detailed NMR characterization of Co states, showing charge disproportionation and the uniform Co state in superconducting phases, which was not previously established.

Findings

Single Co electronic state in hydrated superconducting phase

Charge disproportionation into non-magnetic Co$^{3+}$ and magnetic sites in non-superconducting phase

Correlation of NMR data with Co$^{3.65+}$ charge state

Abstract

$^{59}$Co NMR spectra in oriented powders of Na$_{0.35}$CoO$_{2}$ and in its hydrated superconducting phase (HSC) Na$_{0.35}$CoO$_{2}$,1.3H$_{2}$O reveal a single electronic Co state with identical $T$ independent NMR shift tensor. These phases differ markedly from Na$_{0.7}$CoO$_{2}$, in which we resolve 3 types of Co sites. The large T variation of their spin susceptibilities $\chi ^{s}$ and the anisotropy of the orbital susceptibility $\chi ^{orb}$ allow us to conclude that charge disproportionation occurs, in a non magnetic Co$^{3+}$ and two magnetic sites with about 0.3 and 0.7 holes in the $t_{2g}$ multiplet. The data are consistent with those for the single Co site in the anhydrous and HSC phase assuming the expected Co$^{3.65+}$ charge.