1,988 research outputs found
Electrochemical synthesis and properties of CoO2, the x = 0 phase of the AxCoO2 systems (A = Li, Na)
Single-phase bulk samples of the "exotic" CoO2, the x = 0 phase of the AxCoO2
systems (A = Li, Na), were successfully synthesized through electrochemical
de-intercalation of Li from pristine LiCoO2 samples. The samples of pure CoO2
were found to be essentially oxygen stoichiometric and possess a hexagonal
structure consisting of stacked triangular-lattice CoO2 layers only. The
magnetism of CoO2 is featured with a temperature-independent susceptibility of
the magnitude of 10-3 emu/mol Oe, being essentially identical to that of a
Li-doped phase, Li0.12CoO2. It is most likely that the CoO2 phase is a
Pauli-paramagnetic metal with itinerant electrons.Comment: 12 pages, 3 figure
Electronic phase diagram of the layered cobalt oxide system, LixCoO2 (0.0 <= x <= 1.0)
Here we report the magnetic properties of the layered cobalt oxide system,
LixCoO2, in the whole range of Li composition, 0 <= x <= 1. Based on
dc-magnetic susceptibility data, combined with results of 59Co-NMR/NQR
observations, the electronic phase diagram of LixCoO2 has been established. As
in the related material NaxCoO2, a magnetic critical point is found to exist
between x = 0.35 and 0.40, which separates a Pauli-paramagnetic and a
Curie-Weiss metals. In the Pauli-paramagnetic regime (x <= 0.35), the
antiferromagnetic spin correlations systematically increase with decreasing x.
Nevertheless, CoO2, the x = 0 end member is a non-correlated metal in the whole
temperature range studied. In the Curie-Weiss regime (x >= 0.40), on the other
hand, various phase transitions are observed. For x = 0.40, a susceptibility
hump is seen at 30 K, suggesting the onset of static AF order. A magnetic jump,
which is likely to be triggered by charge ordering, is clearly observed at Tt =
175 K in samples with x = 0.50 (= 1/2) and 0.67 (= 2/3), while only a tiny kink
appears at T = 210 K in the sample with an intermediate Li composition, x =
0.60. Thus, the phase diagram of the LixCoO2 system is complex, and the
electronic properties are sensitively influenced by the Li content (x).Comment: 29 pages, 1 table, 9 figure
Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2
Thermoelectric properties of the layered cobalt oxide system LixCoO2 were
investigated in a wide range of Li composition, 0.98 >= x >= 0.35. Single-phase
bulk samples of LixCoO2 were successfully obtained through electrochemical
deintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x >=
0.94 is semiconductive, the highly Li-deficient phase (0.75 >= x >= 0.35)
exhibits metallic conductivity. The magnitude of Seebeck coefficient at 293 K
(S293K) significantly depends on the Li content (x). The S293K value is as
large as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K
to +10 uV/K as x is lowered within a Li composition range of 0.75 >= x >= 0.50.
This behavior is in sharp contrast to the results of x <= 0.40 for which the
S293K value is small and independent of x (+10 uV/K), indicating that a
discontinuous change in the thermoelectric characteristics takes place at x =
0.40 ~ 0.50. The unusually large Seebeck coefficient and metallic conductivity
are found to coexist in a narrow range of Li composition at about x = 0.75. The
coexistence, which leads to an enhanced thermoelectric power factor, may be
attributed to unusual electronic structure of the two-dimensional CoO2 block.Comment: 29 pages, 1 table, 8 figure
New Josephson Plasma Modes in Underdoped YBa2Cu3O6.6 Induced by Parallel Magnetic Field
The c-axis reflectivity spectrum of underdoped YBa2Cu3O6.6 (YBCO) is measured
below Tc=59K in parallel magnetic fields H//CuO2 up to 7T. Upon application of
a parallel field, a new peak appears at finite frequency in the optical
conductivity at the expense of suppression of c-axis condensate weight. We
conclude that the dramatic change originates from different Josephson coupling
strengths between bilayers with and without Josephson vortices. We find that
the 400cm^-1 broad conductivity peak in YBCO gains the spectral weight under
parallel magnetic field; this indicates that the condensate weight at \omega =0
is distributed to the intra-bilayer mode as well as to the new optical
Josephson mode.Comment: 4 pages, 3 figure
Reheating after f(R) inflation
The reheating dynamics after the inflation induced by -corrected
model is considered. To avoid the complexity of solving the fourth order
equations, we analyze the inflationary and reheating dynamics in the Einstein
frame and its analytical solutions are derived. We also perform numerical
calculation including the backreaction from the particle creation and compare
the results with the analytical solutions. Based on the results, observational
constraints on the model are discussed.Comment: 16 pages, 11 figure
A novel route to phase formation of cobalt oxyhydrates using KMnO4 as an oxidizing agent
We have first succeefully synthesized the sodium cobalt oxyhydrate
superconductors using KMnO4 as a de-intercalating and oxidizing agent. It is a
novel route to form the superconductive phase of NaxCoO2.yH2O without resorting
to the commonly used Br2/CH3CN solution. The role of the KMnO4 is to
de-intercalate the Na+ from the parent compound Na0.7CoO2 and oxidize the Co
ion as a result. The higher molar ratio of KMnO4 relative to the sodium content
tends to remove more Na+ from the parent compound and results in a slight
expansion of the c-axis in the unit cell. The superconducting transition
temperature is 4.6-3.8 K for samples treated by the aqueous KMnO4 solution with
the molar ratio of KMnO4 relative to the sodium content in the range of 0.3 and
2.29.Comment: 10 pages, 3 figure
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