1,978 research outputs found
Specific-heat evidence for strong electron correlations in the thermoelectric material (Na,Ca)Co_{2}O_{4}
The specific heat of (Na,Ca)Co_{2}O_{4} is measured at low-temperatures to
determine the magnitude of the electronic specific-heat coefficient \gamma, in
an attempt to gain an insight into the origin of the unusually large
thermoelectric power of this compound. It is found that \gamma is as large as
48 mJ/molK^2, which is an order of magnitude larger than \gamma of simple
metals. This indicates that (Na,Ca)Co_{2}O_{4} is a strongly-correlated
electron system, where the strong correlation probably comes from the
low-dimensionality and the frustrated spin structure. We discuss how the large
thermopower and its dependence on Ca doping can be understood with the strong
electron correlations.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.
In-plane anisotropy on the transport properties in the modulated Bi_2O_2-based conductors Bi-2212 and Bi-Sr-Co-O
We investigated the in-plane anisotropy on the resistivity and thermopower of
the Bi_2Sr_2CaCu_2O_{8+\delta} (Bi-2212) and Bi-Sr-Co-O (BiCo) single crystals.
In Bi-2212, the b-axis resistivity is higher than the a-axis resistivity, and
is expressed as a sum of the a-axis resistivity and an additional residual
resistivity. A downward deviation due to pseudogap is observed below a
characteristic temperature T^*, which is isotropic in the form of conductivity.
These results suggest that the modulation structure along the b-axis works as
an anisotropic scattering center, but does not affect the pseudogap formation.
On the other hand, the anisotropy of the resistivity and the thermopower in
Pb-doped BiCo is substantial, probably owing to the misfit structure between
the hexagonal CoO_2 layer and the rock salt Bi_2O_2 layer. However, the
anisotropy in the resistivity in Pb-free BiCo is very small, suggesting that
the in-plane anisotropy is averaged by the modulation structure, whose
direction is tilted by 45 deg from the a- and b-axes.Comment: 4pages 5 figures, Proceedings of ISS2001, Physica C (in press
Out-of-plane thermopower of strongly correlated layered systems: an application to Bi_2(Sr,La)_2CaCu_2O_{8+\delta}
We calculate the out-of-plane thermopower in a quasi-two dimensional system,
and argue that this quantity is an effective probe of the asymmetry of the
single-particle spectral function. We find that the temperature and doping
dependence of the out-of-plane thermopower in Bi_2(Sr,La)_2CaCu_2O_{8+\delta}
single crystals is broadly consistent with the behavior of the spectral
function determined from ARPES and tunneling experiments. We also investigate
the relationship between out-of-plane thermopower and entropy in a quasi-two
dimensional material. We present experimental evidence that at moderate
temperatures, there is a qualitative correspondence between the out-of-plane
thermopower in Bi_2(Sr,La)_2CaCu_2O_{8+\delta}, and the entropy obtained from
specific heat measurements. Finally, we argue that the derivative of the
entropy with respect to particle number may be the more appropriate quantity to
compare with the thermopower, rather than the entropy per particle.Comment: 11 pages, 5 figures. v2: substantially rewritten, including a more
detailed analysis of the relationship between thermopower and entrop
Thermal conductivity of the thermoelectric layered cobalt oxides measured by the Harman method
In-plane thermal conductivity of the thermoelectric layered cobalt oxides has
been measured using the Harman method, in which thermal conductivity is
obtained from temperature gradient induced by applied current. We have found
that the charge reservoir block (the block other than the CoO block)
dominates the thermal conduction, where a nano-block integration concept is
effective for material design. We have further found that the thermal
conductivity shows a small but finite in-plane anisotropy between and
axes, which can be ascribed to the misfit structure.Comment: 4 pages, 4 figures, J. Appl. Phys. (scheduled on July 1, 2004
Spatial distribution of ions in a linear octopole radio-frequency ion trap in the space-charge limit
We have explored the spatial distribution of an ion cloud trapped in a linear
octopole radio-frequency (rf) ion trap. The two-dimensional distribution of the
column density of stored silver dimer cations was measured via
photofragment-ion yields as a function of the position of the incident laser
beam over the transverse cross section of the trap. The profile of the ion
distribution was found to be dependent on the number of loaded ions. Under high
ion-loading conditions with a significant space-charge effect, ions form a ring
profile with a maximum at the outer region of the trap, whereas they are
localized near the center axis region at low loading of the ions. These results
are explained quantitatively by a model calculation based on equilibrium
between the space-charge-induced potential and the effective potential of the
multipole rf field. The maximum adiabaticity parameter \eta_max is estimated to
be about 0.13 for the high ion-density condition in the present octopole ion
trap, which is lower than typical values reported for low ion densities; this
is probably due to additional instability caused by the space charge.Comment: 8 pages, 5 figure
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FGF2 is expressed in human and murine embryonic choroid plexus and affects choroid plexus cell behaviour
<p>Abstract</p> <p>Background</p> <p>Although fibroblast growth factor (Fgf) signalling plays crucial roles in several developing and mature tissues, little information is currently available on expression of Fgf2 during early choroid plexus development and whether Fgf2 directly affects the behaviour of the choroid plexus epithelium (CPe). The purpose of this study was to investigate expression of Fgf2 in rodent and human developing CPe and possible function of Fgf2, using <it>in vitro </it>models. The application of Fgf2 to brain <it>in vivo </it>can affect the whole tissue, making it difficult to assess specific responses of the CPe.</p> <p>Methods</p> <p>Expression of Fgf2 was studied by immunohistochemistry in rodent and human embryonic choroid plexus. Effects of Fgf2 on growth, secretion, aggregation and gene expression was investigated using rodent CPe vesicles, a three-dimensional polarized culture model that closely mimics CPe properties <it>in vivo</it>, and rodent CPe monolayer cultures.</p> <p>Results</p> <p>Fgf2 was present early in development of the choroid plexus both in mouse and human, suggesting the importance of this ligand in Fgf signalling in the developing choroid plexus. Parallel analysis of Fgf2 expression and cell proliferation during CP development suggests that Fgf2 is not involved in CPe proliferation <it>in vivo</it>. Consistent with this observation is the failure of Fgf2 to increase proliferation in the tri-dimensional vesicle culture model. The CPe however, can respond to Fgf2 treatment, as the diameter of CPe vesicles is significantly increased by treatment with this growth factor. We show that this is due to an increase in cell aggregation during vesicle formation rather than increased secretion into the vesicle lumen. Finally, Fgf2 regulates expression of the CPe-associated transcription factors, <it>Foxj1 </it>and <it>E2f5</it>, whereas transthyretin, a marker of secretory activity, is not affected by Fgf2 treatment.</p> <p>Conclusion</p> <p>Fgf2 expression early in the development of both human and rodent choroid plexus, and its ability to modulate behaviour and gene expression in CPe, supports the view that Fgf signalling plays a role in the maintenance of integrity and function of this specialized epithelium, and that this role is conserved between rodents and humans.</p
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