24 research outputs found
First principles lattice dynamics of NaCoO
We report first principles linear response calculations on NaCoO. Phonon
frequencies and eigenvectors are obtained throughout the Brillouin zone for two
geometries with different Na site occupancies. While most of the phonon modes
are found to be unsensitive to the Na site occupancy, there are two modes
dominated by out-of-plane vibrations of Na giving very different frequencies
for different geometries. One of these two modes, the A mode, is
infrared-active, and can be used as a suitable sensor of Na
distribution/ordering. The longitudinal-transverse splitting of the zone-center
optical-mode frequencies, Born effective charges and the dielectric constants
are also reported, showing considerable anisotropy. The calculated frequencies
of Raman-active modes generally agree with the experimental values of
corresponding Na de-intercalated and/or hydrated compounds, while it requires
better experimental data to clarify the infrared-active mode frequencies.Comment: 12 pages, 2 figure
Stoichiometric oxygen content in NaxCoO2
The crystal structure and oxygen stoichiometry in two-layer Na0.74CoO2 and
Na0.38CoO2 at room temperature are analyzed by powder neutron diffraction. Two
sets of diffraction data for each sample, taken at different incident neutron
wavelengths, lambda=1.1968 Angstroms and lambda=1.5403 Angstroms, are analyzed
simultaneously by the Rietveld method, allowing for the independent refinement
of all structural parameters. The fractional oxygen site occupancies are found
to be 1.01(1) for Na0.74CoO2 and 0.99(2) for Na0.38CoO2 respectively. These
results indicate that the oxygen content of these phases is stoichiometric to a
precision of 1 to 2%, and therefore the formal cobalt oxidation state is
determined solely by the sodium content. The analysis also reveals that both
types of sodium ions in the structure are in off-center distorted trigonal
prismatic geometry.Comment: 15 pages, 2 tables, 3 figure
Localized versus itinerant magnetic moments in Na0.72CoO2
Based on experimental 59Co-NMR data in the temperature range between 0.1 and
300 K, we address the problem of the character of the Co 3d-electron based
magnetism in Na0.7CoO2. Temperature dependent 59Co-NMR spectra reveal different
Co environments below 300 K and their differentiation increases with decreasing
temperature. We show that the 23Na- and 59Co-NMR data may consistently be
interpreted by assuming that below room temperature the Co 3d-electrons are
itinerant. Their magnetic interaction appears to favor an antiferromagnetic
coupling, and we identify a substantial orbital contribution corb to the
d-electron susceptibility. At low temperatures corb seems to acquire some
temperature dependence, suggesting an increasing influence of spin-orbit
coupling. The temperature dependence of the spin-lattice relaxation rate
T1-1(T) confirms significant variations in the dynamics of this electronic
subsystem between 200 and 300K, as previously suggested. Below 200 K, Na0.7CoO2
may be viewed as a weak antiferromagnet with TN below 1 K but this scenario
still leaves a number of open questions.Comment: 8.7 pages, 6 Figures, submitted to Phys. Rev.
Coupling between electronic and structural degrees of freedom in the triangular lattice conductor NaxCoO2
The determination by powder neutron diffraction of the ambient temperature
crystal structures of compounds in the NaxCoO2 family, for 0.3 < x <= 1.0, is
reported. The structures consist of triangular CoO2 layers with Na ions
distributed in intervening charge reservoir layers. The shapes of the CoO6
octahedra that make up the CoO2 layers are found to be critically dependent on
the electron count and on the distribution of the Na ions in the intervening
layers, where two types of Na sites are available. Correlation of the shapes of
cobalt-oxygen octahedra, the Na ion positions, and the electronic phase diagram
in NaxCoO2 is made, showing how structural and electronic degrees of freedom
can be coupled in electrically conducting triangular lattice systems.Comment: 15 pages, 1 tables, 6 figures Submitted to Physical Review
Mesoscopic phase separation in NaCoO ()
NMR, EPR and magnetization measurements in NaCoO for are presented. While the EPR signal arises from Co magnetic
moments ordering at K, Co NMR signal originates from
cobalt nuclei in metallic regions with no long range magnetic order and
characterized by a generalized susceptibility typical of strongly correlated
metallic systems. This phase separation in metallic and magnetic insulating
regions is argued to occur below ( K). Above an
anomalous decrease in the intensity of the EPR signal is observed and
associated with the delocalization of the electrons which for were
localized on Co orbitals. It is pointed out that the in-plane
antiferromagnetic coupling cannot be the driving force for the phase
separation.Comment: 14 figure
Structure and Dynamics of Superconducting NaxCoO(2) Hydrate and Its Unhydrated Analog
Neutron scattering has been used to investigate the crystal structure and
lattice dynamics of superconducting Na0.3CoO2 1.4(H/D)2O, and the parent
Na0.3CoO2 material. The structure of Na0.3CoO2 consists of alternate layers of
CoO2 and Na and is the same as the structure at higher Na concentrations. For
the superconductor, the water forms two additional layers between the Na and
CoO2, increasing the c-axis lattice parameter of the hexagonal P63/mmc space
group from 11.16 A to 19.5 A. The Na ions are found to occupy a different
configuration from the parent compound, while the water forms a structure that
replicates the structure of ice. Both types of sites are only partially
occupied. The CoO2 layer in these structures is robust, on the other hand, and
we find a strong inverse correlation between the CoO2 layer thickness and the
superconducting transition temperature (TC increases with decreasing
thickness). The phonon density-of-states for Na0.3CoO2 exhibits distinct
acoustic and optic bands, with a high-energy cutoff of ~100 meV. The lattice
dynamical scattering for the superconductor is dominated by the hydrogen modes,
with librational and bending modes that are quite similar to ice, supporting
the structural model that the water intercalates and forms ice-like layers in
the superconductor.Comment: 14 pages, 7 figures, Phys. Rev. B (in press). Minor changes + two
figures removed as requested by refere
Unconventional Charge Ordering in Na0.70CoO2 below 300 K
We present the results of measurements of the dc-magnetic susceptibility
chi(T) and the 23Na-NMR response of Na_{0.70}CoO_{2} at temperatures between 50
and 340 K. The chi(T) data suggest that for T > 75 K, the Co ions adopt an
effective configuration of Co^{3.4+}. The 23Na-NMR response reveals pronounced
anomalies near 250 and 295 K, but no evidence for magnetic phase transitions is
found in chi(T). Our data suggest the onset of a dramatic change in the Co
3d-electron spin dynamics at 295 K. This process is completed at 230 K. Our
results maybe interpreted as evidence for either a tendency to electron
localization or an unconventional charge-density wave phenomenon within the
cobalt oxide layer, CoO_2, 3d electron system near room temperature.Comment: 4 pages, 4 figures, re-submitted to Physical Review Letters. The
manuscript has been revised following the recommendations of the referees.
The discussion section contains substantial change
Raman spectroscopy study of NaxCoO2 and superconducting NaxCoO2.yH2O
The Raman spectra of the parent compound NaxCoO2 (x=0.75) and the
superconducting oxyhydrates NaxCoO2.yH2O with different superconducting
temperatures (Tc) have been measured. Five Raman active phonons around 195 cm-1
(E1g), 482 cm-1, 522 cm-1, 616 cm-1 (3E2g), 663 cm-1 (A1g) appear in all
spectra. These spectra change systematically along with the intercalation of
H2O and superconducting properties. In particular, the Raman active phonons
(A1g and E1g) involving the oxygen motions within the Co-O layers show up
monotonous decrease in frequency along with superconducting temperature Tc. The
fundamental properties and alternations of other active Raman phonons in the
superconducting materials have also been discussed.Comment: 16 pages, 4 figures, 2 table
Crystal Structure of the Sodium Cobaltate Deuterate Superconductor NaxCoO2o4xD2O (x=1/3)
Neutron and x-ray powder diffraction have been used to investigate the
crystal structures of a sample of the newly-discovered superconducting sodium
cobaltate deuterate compound with composition Na0.31(3)CoO2o1.25(2)D2O and its
anhydrous parent compound Na0.61(1)CoO2. The deuterate superconducting compound
is formed by coordinating four D2O molecules (two above and two below) to each
Na ion in a way that gives Na-O distances nearly equal to those in the parent
compound. One deuteron of the D2O molecule is hydrogen bonded to an oxygen atom
in the CoO2 plane and the oxygen atom and the second deuteron of each D2O
molecule lie approximately in a plane between the Na layer and the CoO2 layers.
This coordination of Na by four D2O molecules leads to ordering of the Na ions
and D2O molecules. The sample studied here, which has Tc=4.5 K, has a refined
composition of Na0.31(3)CoO2o1.25(2)D2O, in agreement with the expected 1:4
ratio of Na to D2O. These results show that the optimal superconducting
composition should be viewed as a specific hydrated compound, not a solid
solution of Na and D2O (H2O) in NaxCoO2oyD2O. Studies of physical properties
vs. Na or D2O composition should be viewed with caution until it is verified
that the compound remains in the same phase over the composition range of the
study.Comment: 22 pages, 8 figure
Bulk antiferromagnetism in single crystals
Susceptibility, specific heat, and muon spin rotation measurements on
high-quality single crystals of have revealed bulk
antiferromagnetism with N\'{e}el temperature K and an
ordered moment perpendicular to the layers. The magnetic order
encompasses nearly 100% of the crystal volume. The susceptibility exhibits a
broad peak around 30 K, characteristic of two-dimensional antiferromagnetic
fluctuations. The in-plane resistivity is metallic at high temperatures and
exhibits a minimum at .Comment: published versio