J. Phys. IV France
Volume 131, December 2005
Page(s) 27 - 32
Publié en ligne 18 janvier 2006
International Workshop on Electronic Crystals
S. Brazovskii, P. Monceau and N. Kirova
J. Phys. IV France 131 (2005) 27-32

DOI: 10.1051/jp4:2005131006

Superconductivity, charge order and anomalous magnetism in sodium cobaltates

H. Alloul1, I.R. Mukhamedshin1, 2, N. Blanchard1 and G. Collin3

1  Laboratoire de Physique des Solides, UMR 8502, Université Paris-Sud, 91405 Orsay, France
2  Physics Department, Kazan State University, 420008 Kazan, Russia
3  Laboratoire Léon Brillouin, CE Saclay, CEA-CNRS, 91191 Gif-sur-Yvette, France

Oriented powder samples of NaxCoO2, with quasi single phase composition have been synthesized and studied by x rays, SQUID magnetometry, 23Na and 59Co NMR. We evidence that the dry and superconducting (SC) samples with $x\approx 0.35$ differ from those with $0.50\leq x<1$ inasmuch as the Co sites display an uniform charge state in the former while they differentiate markedly in the latter. For $0.50\leq x<1,$ the local Co charge and magnetic properties appear correlated with definite orderings of the Na+ ions. For $x\simeq 0.70$ we could distinguish non magnetic Co3+ sites with no spin susceptibility from more magnetic Co sites, among which Co4+ sites can be excluded. These sites form a single strongly correlated electronic band which exhibits a quasi localized T dependent magnetism which might only become Fermi liquid like below 2 K. This contrasts with the Pauli like magnetism found for $x\approx 0.35$ in the dry and SC compounds. Those display quasi identical metallic states, with a weak spin susceptibility which only contributes to a small shift of the 59Co NMR. Its variation below Tc reveals a strong suppression of the spin susceptibility, compatible with singlet superconducting pairing.

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