116 research outputs found
59Co NMR evidence for charge and orbital order in the kagome like structure of Na2/3CoO2
We report a complete set of Co NMR data taken on the phase of
sodium cobaltates NaCoO, for which we have formerly established the
in plane Na ordering and its three dimensional stacking from a combination of
symmetry arguments taken from Na and Co NQR/NMR data. Here we resolve all the
parameters of the Zeeman and quadrupolar Hamiltonians for all cobalt sites in
the unit cell and report the temperature dependencies of the NMR shift and spin
lattice relaxation data for these sites. We confirm that three
non-magnetic Co (Co1) are in axially symmetric positions and that the
doped holes are delocalized on the nine complementary magnetic cobalt sites
(Co2) of the atomic unit cell. The moderately complicated atomic structure
resumes then in a very simple electronic structure in which the electrons
delocalize on the Co2 kagom\'e sublattice of the triangular lattice of Co
sites. The observation of a single temperature dependence of the spin
susceptibilities indicates that a single band picture applies, and that the
magnetic properties are dominated by the static and dynamic electronic
properties at the Co2 sites. We evidence that they display a strong in plane
electronic anisotropy initially unexpected but which accords perfectly with an
orbital ordering along the kagom\'e sublattice organization. These detailed
data should now permit realistic calculations of the electronic properties of
this compound in order to determine the incidence of electronic correlations.Comment: 20 pages, 15 figures, 3 tables, published in Phys. Rev. B, added new
Appendix
Na atomic order, Co charge disproportionation and magnetism in NaCoO for large Na contents
We have synthesized and characterized four different stable phases of Na
ordered NaCoO, for . Above 100 K they display similar
Curie-Weiss susceptibilities as well as ferromagnetic spin fluctuations
in the CoO planes revealed by Na NMR data. In all phases from
Co NMR data we display evidences that the Co disproportionate already
above 300 K into non magnetic Co and magnetic Co
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 at
22 K. We suggest that the charge order also governs the low energy
scales and transverse couplings
59Co NMR study of the Co states in superconducting and anhydrous cobaltates
Co NMR spectra in oriented powders of NaCoO and in its
hydrated superconducting phase (HSC) NaCoO,1.3HO reveal a
single electronic Co state with identical independent NMR shift tensor.
These phases differ markedly from NaCoO, in which we resolve 3
types of Co sites. The large T variation of their spin susceptibilities and the anisotropy of the orbital susceptibility allow us
to conclude that charge disproportionation occurs, in a non magnetic Co
and two magnetic sites with about 0.3 and 0.7 holes in the multiplet.
The data are consistent with those for the single Co site in the anhydrous and
HSC phase assuming the expected Co charge.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
Complex magnetic differentiation of cobalts in NaCoO with 22K N\'eel temperature
Single crystals of sodium cobaltates NaCoO with
were grown by the floating zone technique. Using electrochemical Na
de-intercalation method we reduced the sodium content in the as-grown crystals
down to pure phase with 22 K N\'eel temperature and . The
Co NMR study in the paramagnetic state of the K phase
permitted us to evidence that at least 6 Co sites are differentiated. They
could be separated by their magnetic behaviour into three types: a single site
with cobalt close to non-magnetic Co, two sites with the most magnetic
cobalts in the system, and the remaining three sites displaying an intermediate
behaviour. This unusual magnetic differentiation calls for more detailed NMR
experiments on our well characterized samples.Comment: 5 pages, 4 figures, submitted to JETP Let
Synthesis of sodium cobaltate NaCoO single crystals with controlled Na ordering
In this study, we synthesized single crystals of NaCoO with
using the optical floating zone technique. A thorough
electrochemical treatment of the samples permitted us to control the
de-intercalation of Na to obtain single crystal samples of stable Na ordered
phases with . Comparisons of the bulk magnetic properties with those
observed in the Na ordered powder samples confirmed the high quality of these
single crystal phases. The ab plane resistivity was measured for the Na ordered
samples and it was quite reproducible for different sample batches. The data
were analogous to those found in previous initial experimental studies on
single crystals, but the lower residual resistivity and sharper
anti-ferromagnetic transitions determined for our samples confirmed their
higher quality.Comment: 15 pages, 7 figure
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