59 Co NMR spectra in oriented powders of the superconducting (HSC) Na0.35CoO2,1.3H2O and Na0.35CoO2 compounds reveal a single electronic Co state with identical T independent NMR shift tensor. These phases differ markedly from Na0.7CoO2, in which we resolve 3 types of Co sites. The large T variation of their spin susceptibilities χs and the anisotropy of the orbital susceptibility χ 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 t2g 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.

Oriented powder samples of NaxCoO2 are studied by 23Na NMR and SQUID magnetometry. In nominal 0.50

We have synthesized and characterized four different stable phases of Na ordered NaxCoO2, for 0.65 < x < 0.8. Above 100 K they display similar Curie-Weiss susceptibilities as well as ferromagnetic q = 0 spin fluctuations in the CoO2 planes revealed by 23 Na NMR data. In all phases from 59 Co NMR data we display evidences that the Co disproportionate already above 300 K into non-magnetic Co 3+ and magnetic ≈ Co 3.5+ sites on which holes delocalize. This allows us to understand that metallic magnetism is favored for these large Na contents. Below 100 K the phases differentiate, and a magnetic order sets in only for x 0.75 at TN = 22 K. We suggest that the charge order also governs the low-T energy scales and transverse couplings.

We report 23 Na and 59 Co nuclear magnetic (NMR) and quadrupolar resonance (NQR) studies for the x = 2/3 phase of the lamellar oxide NaxCoO2, which allowed us to establish reliably the atomic order of the Na layers and their stacking between the CoO2 slabs. We evidence that the Na + order stabilizes filled non-magnetic Co 3+ ions on 25% of the cobalt sites arranged in a triangular sublattice. The transferred holes are delocalized on the 75% complementary cobalt sites which unexpectedly display a planar cobalt kagomé structure. These experimental results resolve a puzzling issue by precluding localized moments pictures for the magnetic properties. They establish that the quasi-ferromagnetic properties result from a narrow band connecting a frustrated arrangement of atomic orbitals, and open the route to unravel through similar studies the electronic properties of the diverse ordered phases of sodium cobaltates.

We have synthesized various samples of the x =2/ 3 phase of sodium cobaltate Na x CoO 2 and performed x-ray powder diffractions spectra to compare the diffraction with the structure proposed previously from NMR and nuclear quadrupole resonance ͑NQR͒ experiments ͓H. Alloul, I. R. Mukhamedshin, T. A. Platova, and A. V. Dooglav, EPL 85, 47006 ͑2009͔͒. Rietveld analyses of the data are found in perfect agreement with those and confirm the concentration x =2/ 3 obtained in the synthesis procedure. They even give indications on the atomic displacements of Na inside the unit cell. The detailed NQR data allow us to identify the NQR transitions and electric field gradient parameters for four cobalt sites and three Na sites. The spin-lattice and spin-spin relaxation rates are found much smaller for the nonmagnetic Co 3+ sites than for the magnetic sites on which the holes are delocalized. The atomic ordering of the Na layers is therefore at the source of this ordered distribution of cobalt charges. The method used here to resolve the Na ordering and the subsequent Co charge order can be used valuably for similar structural determinations for various phases with x Ͼ 0.45 for which Na ordering has been established.

Using muon spin rotation, well-defined bulk approximately 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x=0.85 with the highest transition temperature ever observed for x>or=0.75. This stresses the diversity of x>or=0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x=0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.

We report a systematic study of the c lattice parameter in the NaxCoO2 phases versus Na content x > 0.5, in which sodium always displays ordered arrangements. This allows us to single out the first phase which exhibits an antiferromagnetic order at a Néel temperature TN =22 K which is found to occur for x ≈ 0.77(1). Pure samples of this phase have been studied both as aligned powders and single crystals. They exhibit identical 23 Na NMR spectra in which three sets of Na sites could be fully resolved, and are found to display T dependencies of their NMR shifts which scale with each other. This allows us to establish that the T variation of the shifts is due to the paramagnetism of the Co sites with formal charge state larger than 3 + . The existence of a sodium site with axial charge symmetry and the intensity ratio between the sets of 23 Na lines permits us to reveal that the two-dimensional structure of the Na order corresponds to 10 Na sites on top of a 13 Co sites unit cell, that is with x = 10/13 ≈ 0.77. This structure fits with that determined from local density calculations and involves triangles of 3 Na sites located on top of Co sites (so called Na1 sites). The associated ordering of the Na vacancies is quite distinct from that found for x < 0.75.

R.R. Gainov).Nuclear quadrupole resonance (NQR) on copper nuclei has been applied for studies of the electronic properties of quasi-two-dimensional low-temperature superconductor CuS (covellite) in the temperature region between 1.47 and 290 K. Two NQR signals corresponding to two nonequivalent sites of copper in the structure, Cu(1) and Cu(2), has been found. The temperature dependences of copper quadrupole frequencies, line-widths and spin-lattice relaxation rates, which so far had never been investigated so precisely for this material, altogether demonstrate the structural phase transition near 55 K, which accompanies transformations of electronic spectrum not typical for simple metals. The analysis of NQR results and their comparison with literature data show that the valence of copper ions at both sites is intermediate in character between monovalent and divalent states with the dominant of the former. It has been found that there is a strong hybridization of Cu(1) and Cu(2) conduction bands at low temperatures, indicating that the charge delocalization between these ions takes place even in 2D regime. Based on our data, the occurrence of energy gap, charge fluctuations and charge-density waves, as well as the nature of phase transition in CuS are discussed. It is concluded that some physical properties of CuS are similar to those of high-temperature superconductors (HTSC) in normal state.PACS number(s): 76.60.Gv; 74.70.Ad; 71.28.+d; 91.60.Pn.