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

I. R. Mukhamedshin; H. Alloul; G. Collin; N. Blanchard

doi:10.1103/PhysRevLett.94.247602

paper

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

arXiv:cond-mat/0505380 · doi:10.1103/PhysRevLett.94.247602

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 $I ¨ ^{s}$ and the anisotropy of the orbital susceptibility $I ¨ ^{or b}$ 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 $t_{2 g}$ 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.

5 pages, 3 figures, accepted for publication in Phys. Rev. Lett