1,852 research outputs found
Enhanced thermoelectric properties of NaxCoO2 whisker crystals
Single-crystalline whiskers of thermoelectric cobalt oxide, NaxCoO2, have
been grown by an unconventional method from potassium-containing compositions,
and their transport properties, and chemical compositions were determined. The
growth mechanism was analyzed and interpreted by means of thermogravimetric
analysis. At 300 K, electrical resistivity r and thermoelectric power S of the
whisker are 102 u ohm cm and 83 uV/K, respectively. The power-factor, S2/r,
thus is enhanced up to ~ 68 uW/K2 cm.Comment: 8 pages, 4 figures, to appear in Appl. Phys. Let
Quantum Dynamics of Atom-molecule BECs in a Double-Well Potential
We investigate the dynamics of two-component Bose-Josephson junction composed
of atom-molecule BECs. Within the semiclassical approximation, the multi-degree
of freedom of this system permits chaotic dynamics, which does not occur in
single-component Bose-Josephson junctions. By investigating the level
statistics of the energy spectra using the exact diagonalization method, we
evaluate whether the dynamics of the system is periodic or non-periodic within
the semiclassical approximation. Additionally, we compare the semiclassical and
full-quantum dynamics.Comment: to appear in JLTP - QFS 200
Optical evidence for the proximity to a spin-density-wave metallic state in NaCoO
We present the optical properties of \na single crystals, measured over a
broad spectral range as a function of temperature (). The capability to
cover the energy range from the far-infrared up to the ultraviolet allows us to
perform reliable Kramers-Kronig transformation, in order to obtain the
absorption spectrum (i.e., the complex optical conductivity). To the complex
optical conductivity we apply the generalized Drude model, extracting the
frequency dependence of the scattering rate () and effective mass
() of the itinerant charge carriers. We find that at low temperatures and for . This suggests that \na is at
the verge of a spin-density-wave metallic phase
Novel phase diagram of superconductor NaxCoO2-yH2O in a 75 % relative humidity
We succeeded in synthesizing the powder samples of bilayer-hydrate sodium
cobalt oxide superconductors NaxCoO2-yH2O with Tc = 0 ~ 4.6 K by systematically
changing the keeping duration in a 75 % relative humidity atmosphere after
intercalation of water molecules. From the magnetic measurements, we found that
the one-day duration sample does not show any superconductivity down to 1.8 K,
and that the samples kept for 2 ~ 7 days show superconductivity, in which Tc
increases up to 4.6 K with increasing the duration. Tc and the superconducting
volume fraction are almost invariant between 7 days and 1month duration. The
59Co NQR spectra indicate a systematic change in the local charge distribution
on the CoO2 plane with change in duration.Comment: 4 pages, 5 figures, submitted to Journal of the Physical Society of
Japa
59Co-NQR study on superconducting NaxCoO2.yH2O
Layered Co oxide NaxCoO2.yH2O with a superconducting transition temperature
Tc =4.5 K has been studied by 59Co NQR. The nuclear spin relaxation rate 1/59T1
is nearly proportional to temperature T in the normal state. In the
superconducting state, it exhibits the coherence peak and decreases with
decreasing T below ~0.8Tc. Detailed comparison of the 1/T1T values and the
magnetic susceptibilities between NaxCoO2.yH2O and NaxCoO2 implies that the
metallic state of the former system is closer to a ferromagnetic phase than
that of the latter. These experimental results impose a restriction on the
mechanism of the superconductivity.Comment: 7 pages, 5 figures. to be published in J. Phys. Soc. Jpn. 72 (2003)
No.
Superconductivity in CoO Layers and the Resonating Valence Bond Mean Field Theory of the Triangular Lattice t-J model
Motivated by the recent discovery of superconductivity in two dimensional
CoO layers, we present some possibly useful results of the RVB mean field
theory applied to the triangular lattice. Away from half filling, the order
parameter is found to be complex, and yields a fully gapped quasiparticle
spectrum. The sign of the hopping plays a crucial role in the analysis, and we
find that superconductivity is as fragile for one sign as it is robust for the
other. NaCoOHO is argued to belong to the robust case, by
comparing the LDA Fermi surface with an effective tight binding model. The high
frequency Hall constant in this system is potentially interesting, since it is
pointed out to increase linearly with temperature without saturation for T
T.Comment: Published in Physical Review B, total 1 tex + 9 eps files. Erratum
added as separate tex file on November 7, 2003, a numerical factor corrected
in the erratum on Dec 3, 200
Electronic States in Two-Dimensional Triangular Cobalt Oxides: Role of Electronic Correlation
We obtain the electronic states and structures of two-dimensional cobalt
oxides, NaCoO (x=0, 0.35, 0.5 and 0.75) by utilizing the
full-potential linear muffin-tin orbitals (FP-LMTO) methods, from which some
essential electronic interaction parameters are estimated: the bare on-site
Coulomb interaction of cobalt U=7.5 eV renormalizes to 5 eV for x=0.35,
the hybridizations t and t are -1.40 and 0.70 eV,
respectively. The density of states at E decreases from 6-7 states/eV in
the local density approximation (LDA) to about 1.0 states/eV in the LDA+U
scheme. The role of the intercalation of water molecules and the microscopic
mechanism of the superconductivity in NaCoOmHO is
discussed.Comment: minor errors correcte
"Pudding mold" band drives large thermopower in NaCoO
In the present study, we pin down the origin of the coexistence of the large
thermopower and the large conductivity in NaCoO. It is revealed that
not just the density of states (DOS), the effective mass, nor the band width,
but the peculiar {\it shape} of the band referred to as the "pudding
mold" type, which consists of a dispersive portion and a somewhat flat portion,
is playing an important role in this phenomenon. The present study provides a
new guiding principle for designing good thermoelectric materials.Comment: 5 page
Oscillations of the F(R) dark energy in the accelerating universe
Oscillations of the dark energy around the phantom divide line,
, both during the matter era and also in the de Sitter epoch
are investigated. The analysis during the de Sitter epoch is revisited by
expanding the modified equations of motion around the de Sitter solution. Then,
during the matter epoch, the time dependence of the dark energy perturbations
is discussed by using two different local expansions. For high values of the
red shift, the matter epoch is a stable point of the theory, giving the
possibility to expand the -functions in terms of the dark energy
perturbations. In the late-time matter era, the realistic case is considered
where dark energy tends to a constant. The results obtained are confirmed by
precise numerical computation on a specific model of exponential gravity. A
novel and very detailed discussion is provided on the critical points in the
matter era and on the relation of the oscillations with possible singularities.Comment: 23 pages, 11 figures, version to appear in EPJ
Interlayer c-axis transport in the normal state of cuprates
A theoretical model of c-axis transport properties in cuprates is proposed.
Inter-plane and in-plane charge fluctuations make hopping between planes
incoherent and diffusive (the in-plane momentum is not conserved after
tunneling). The non-Drude optical conductivity and the
power-law temperature dependence of the {\it dc} conductivity are generically
explained by the strong fluctuations excited in the process of tunneling.
Several microscopic models of the charge fluctuation spectrum are considered.Comment: 8 page
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