Superconductivity, charge order and anomalous magnetism in sodium cobaltates

Abstract
Oriented powder samples of Na_xCoO₂, 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 differ from those with inasmuch as the Co sites display an uniform charge state in the former while they differentiate markedly in the latter. For the local Co charge and magnetic properties appear correlated with definite orderings of the Na⁺ ions. For 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 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 T_c reveals a strong suppression of the spin susceptibility, compatible with singlet superconducting pairing.