189 research outputs found
Metal-nonmetal transition in LixCoO2 thin film and thermopower enhancement at high Li concentration
We investigate the transport properties of LixCoO2 thin films whose
resistivities are nearly an order of magnitude lower than those of the bulk
polycrystals. A metal-nonmetal transition occurs at ~0.8 in a biphasic domain,
and the Seebeck coefficient (S) is drastically increased at ~140 K (= T*) with
increasing the Li concentration to show a peak of magnitude ~120 \muV/K in the
S-T curve of x = 0.87. We show that T* corresponds to a crossover temperature
in the conduction, most likely reflecting the correlation-induced temperature
dependence in the low-energy excitations
Magnetoresistance scaling in the layered cobaltate Ca3Co4O9
We investigate the low temperature magnetic field dependences of both the
resistivity and the magnetization in the misfit cobaltate Ca3Co4O9 from 60 K
down to 2 K. The measured negative magnetoresistance reveals a scaling behavior
with the magnetization which demonstrates a spin dependent diffusion mechanism.
This scaling is also found to be consistent with a shadowed metalliclike
conduction over the whole temperature range. By explaining the observed
transport crossover, this result shed a new light on the nature of the
elementary excitations relevant to the transport
Strongly correlated properties of the thermoelectric cobalt oxide Ca3Co4O9
We have performed both in-plane resistivity, Hall effect and specific heat
measurements on the thermoelectric cobalt oxide CaCoO. Four
distinct transport regimes are found as a function of temperature,
corresponding to a low temperature insulating one up to 63 K,
a strongly correlated Fermi liquid up to 140 K, with
and , followed
by an incoherent metal with and a high temperature insulator above
T510 K . Specific heat Sommerfeld coefficient
mJ/(mol.K) confirms a rather large value of the electronic effective mass
and fulfils the Kadowaki-Woods ratio 10 . Resistivity measurements under pressure reveal a
decrease of the Fermi liquid transport coefficient A with an increase of
as a function of pressure while the product remains constant and
of order . Both thermodynamic and transport properties suggest a strong
renormalization of the quasiparticles coherence scale of order that seems
to govern also thermopower.Comment: 5 pages, 6 figures, accepted for publication in Physical Review
Dual electronic states in thermoelectric cobalt oxide
We investigate the low temperature magnetic field dependence of the
resistivity in the thermoelectric misfit cobalt oxide [Bi1.7Ca2O4]0.59CoO2 from
60 K down to 3 K. The scaling of the negative magnetoresistance demonstrates a
spin dependent transport mechanism due to a strong Hund's coupling. The
inferred microscopic description implies dual electronic states which explain
the coexistence between localized and itinerant electrons both contributing to
the thermopower. By shedding a new light on the electronic states which lead to
a high thermopower, this result likely provides a new potential way to optimize
the thermoelectric properties
Microscopic mechanism of low thermal conductivity in lead-telluride
The microscopic physics behind low lattice thermal conductivity of single
crystal rocksalt lead telluride (PbTe) is investigated. Mode-dependent phonon
(normal and umklapp) scattering rates and their impact on thermal conductivity
were quantified by the first-principles-based anharmonic lattice dynamics
calculations that accurately reproduce thermal conductivity in a wide
temperature range. The low thermal conductivity of PbTe is attributed to the
scattering of longitudinal acoustic phonons by transverse optical phonons with
large anharmonicity, and small group velocity of the soft transverse acoustic
phonons. This results in enhancing the relative contribution of optical
phonons, which are usually minor heat carrier in bulk materials.Comment: 18 pages, 4 figures, accepted for publication in Phys. Rev.
Thermoelectric power factor under strain-induced band-alignment in the half-Heuslers NbCoSn and TiCoSb
Band convergence is an effective strategy to improve the thermoelectric
performance of complex bandstructure thermoelectric materials. Half-Heuslers
are good candidates for band convergence studies because they have multiple
bands near the valence bad edge that can be converged through various band
engineering approaches providing power factor improvement opportunities.
Theoretical calculations to identify the outcome of band convergence employ
various approximations for the carrier scattering relaxation times (the most
common being the constant relaxation time approximation) due to the high
computational complexity involved in extracting them accurately. Here, we
compare the outcome of strain-induced band convergence under two such
scattering scenarios: i) the most commonly used constant relaxation time
approximation and ii) energy dependent inter- and intra-valley scattering
considerations for the half-Heuslers NbCoSn and TiCoSb. We show that the
outcome of band convergence on the power factor depends on the carrier
scattering assumptions, as well as the temperature. For both materials
examined, band convergence improves the power factor. For NbCoSn, however, band
convergence becomes more beneficial as temperature increases, under both
scattering relaxation time assumptions. In the case of TiCoSb, on the other
hand, constant relaxation time considerations also indicate that the relative
power factor improvement increases with temperature, but under the energy
dependent scattering time considerations, the relative improvement weakens with
temperature. This indicates that the scattering details need to be accurately
considered in band convergence studies to predict more accurate trends.Comment: 21 pages, 8 figures. arXiv admin note: text overlap with
arXiv:1905.0795
Large-scale Synthesis of β-SiC Nanochains and Their Raman/Photoluminescence Properties
Although the SiC/SiO2 nanochain heterojunction has been synthesized, the chained homogeneous nanostructure of SiC has not been reported before. Herein, the novel β-SiC nanochains are synthesized assisted by the AAO template. The characterized results demonstrate that the nanostructures are constructed by spheres of 25–30 nm and conjoint wires of 15–20 nm in diameters. Raman and photoluminescence measurements are used to explore the unique optical properties. A speed-alternating vapor–solid (SA-VS) growth mechanism is proposed to interpret the formation of this typical nanochains. The achieved nanochains enrich the species of one-dimensional (1D) nanostructures and may hold great potential applications in nanotechnology
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