43 research outputs found
Efficient thermoelectric energy conversion on quasi-localized electron states in diameter modulated nanowires
It is known that the thermoelectric efficiency of nanowires increases when their diameter decreases. Recently, we proposed that increase of the thermoelectric efficiency could be achieved by modulating the diameter of the nanowires. We showed that the electron thermoelectric properties depend strongly on the geometry of the diameter modulation. Moreover, it has been shown by another group that the phonon conductivity decreases in nanowires when they are modulated by dots. Here, the thermoelectric efficiency of diameter modulated nanowires is estimated, in the ballistic regime, by taking into account the electron and phonon transmission properties. It is demonstrated that quasi-localized states can be formed that are prosperous for efficient thermoelectric energy conversion
Weak localization effect on thermomagnetic phenomena
The quantum transport equation (QTE) is extended to study weak localization
(WL) effects on galvanomagnetic and thermomagnetic phenomena. QTE has many
advantages over the linear response method (LRM): (i) particle-hole asymmetry
which is necessary for the Hall effect is taken into account by the
nonequilibrium distribution function, while LRM requires expansion near the
Fermi surface, (ii) when calculating response to the temperature gradient, the
problem of WL correction to the heat current operator is avoided, (iii)
magnetic field is directly introduced to QTE, while the LRM deals with the
vector potential and and special attention should be paid to maintain gauge
invariance, e.g. when calculating the Nernst effect the heat current operator
should be modified to include the external magnetic field. We reproduce in a
very compact form known results for the conductivity, the Hall and the
thermoelectric effects and then we study our main problem, WL correction to the
Nernst coefficient (transverse thermopower).Comment: 20 pages 2 figure
Phonon-drag effects on thermoelectric power
We carry out a calculation of the phonon-drag contribution to the
thermoelectric power of bulk semiconductors and quantum well structures for the
first time using the balance equation transport theory extended to the weakly
nonuniform systems. Introducing wavevector and phonon-mode dependent relaxation
times due to phonon-phonon interactions, the formula obtained can be used not
only at low temperatures where the phonon mean free path is determined by
boundary scattering, but also at high temperatures. In the linear transport
limit, is equivalent to the result obtained from the Boltzmann equation
with a relaxation time approximation. The theory is applied to experiments and
agreement is found between the theoretical predictions and experimental
results. The role of hot-electron effects in is discussed. The importance
of the contribution of to thermoelectric power in the hot-electron
transport condition is emphasized.Comment: 8 pages, REVTEX 3.0, 7 figures avilable upon reques
Finite-Size and surface effects in maghemite nanoparticles: Monte Carlo simulations
Finite-size and surface effects in fine particle systems are investigated by
Monte Carlo simulation of a model of a -FeO (maghemite) single
particle. Periodic boundary conditions have been used to simulate the bulk
properties and the results compared with those for a spherical shaped particle
with free boundaries to evidence the role played by the surface on the
anomalous magnetic properties displayed by these systems at low temperatures.
Several outcomes of the model are in qualitative agreement with the
experimental findings. A reduction of the magnetic ordering temperature,
spontaneous magnetization, and coercive field is observed as the particle size
is decreased. Moreover, the hysteresis loops become elongated with high values
of the differential susceptibility, resembling those from frustrated or
disordered systems. These facts are consequence of the formation of a surface
layer with higher degree of magnetic disorder than the core, which, for small
sizes, dominates the magnetization processes of the particle. However, in
contradiction with the assumptions of some authors, our model does not predict
the freezing of the surface layer into a spin-glass-like state. The results
indicate that magnetic disorder at the surface simply facilitates the thermal
demagnetization of the particle at zero field, while the magnetization is
increased at moderate fields, since surface disorder diminishes ferrimagnetic
correlations within the particle. The change in shape of the hysteresis loops
with the particle size demonstrates that the reversal mode is strongly
influenced by the reduced atomic coordination and disorder at the surface.Comment: Twocolumn RevTex format. 19 pages, 15 Figures included. Submitted to
Phys. Rev.
Thermoelectric power of nondegenerate Kane semiconductors under the conditions of mutual electron-phonon drag in a high electric field
The thermoelectric power of nondegenerate Kane semiconductors with due regard
for the electron and phonon heating, and their thermal and mutual drags is
investigated. The electron spectrum is taken in the Kane two-band form. It is
shown that the nonparabolicity of electron spectrum significantly influences
the magnitude of the thermoelectric power and leads to a change of its sign and
dependence on the heating electric field. The field dependence of the
thermoelectric power is determined analytically under various drag conditions.Comment: 25 pages, RevTex formatted, 3 table
Phonon drag thermopower and weak localization
Previous experimental work on a two-dimensional (2D) electron gas in a
Si-on-sapphire device led to the conclusion that both conductivity and phonon
drag thermopower are affected to the same relative extent by weak
localization. The present paper presents further experimental and theoretical
results on these transport coefficients for two very low mobility 2D electron
gases in doped GaAs/GaAlAs quantum wells. The experiments
were carried out in the temperature range 3-7K where phonon drag dominates the
thermopower and, contrary to the previous work, the changes observed in the
thermopower due to weak localization were found to be an order of magnitude
less than those in the conductivity. A theoretical framework for phonon drag
thermopower in 2D and 3D semiconductors is presented which accounts for this
insensitivity of to weak localization. It also provides transparent
physical explanations of many previous experimental and theoretical results.Comment: 19 page Revtex file, 3 Postscript figur
Development of a new marker system for identifying the complex members of the low-molecular-weight glutenin subunit gene family in bread wheat (Triticum aestivum L.)
Low-molecular-weight glutenin subunits (LMW-GSs) play an important role in determining the bread-making quality of bread wheat. However, LMW-GSs display high polymorphic protein complexes encoded by multiple genes, and elucidating the complex LMW-GS gene family in bread wheat remains challenging. In the present study, using conventional polymerase chain reaction (PCR) with conserved primers and high-resolution capillary electrophoresis, we developed a new molecular marker system for identifying LMW-GS gene family members. Based on sequence alignment of 13 LMW-GS genes previously identified in the Chinese bread wheat variety Xiaoyan 54 and other genes available in GenBank, PCR primers were developed and assigned to conserved sequences spanning the length polymorphism regions of LMW-GS genes. After PCR amplification, 17 DNA fragments in Xiaoyan 54 were detected using capillary electrophoresis. In total, 13 fragments were identical to previously identified LMW-GS genes, and the other 4 were derived from unique LMW-GS genes by sequencing. This marker system was also used to identify LMW-GS genes in Chinese Spring and its group 1 nulliātetrasomic lines. Among the 17 detected DNA fragments, 4 were located on chromosome 1A, 5 on 1B, and 8 on 1D. The results suggest that this marker system is useful for large-scale identification of LMW-GS genes in bread wheat varieties, and for the selection of desirable LMW-GS genes to improve the bread-making quality in wheat molecular breeding programmes
The annealed-nanograin phase: A route to simultaneous increase of the conductivity and the Seebeck coefficient and high thermoelectric performance
We introduce the annealed-nanograin (a-NG) phase effect and propose it
as a new route to high thermoelectric performance. We support that in
granular materials with small nanograins, the core of the grains
(G-phase) and the grain boundaries (GB-phase) can be electrostatically
coupled so that transport is dominated by a single phase, the a-NG
phase. We show that concurrent increase in the mobility and the Seebeck
coefficient can take place when originally defective nanograins are
thermally annealed, because defect repair reduces scatterers in the core
of the nanograins and concurrently stimulates more ionized impurities
and higher energy barriers at the grain boundaries to fulfill charge
neutrality. We compare the a-NG phase with the two phases of a composite
grain (the G-phase and the GB-phase) and show that a transition takes
place from dominant ionized impurity scattering to dominant phonon
scattering. This transition is the signature of the formation of the
a-NG phase and the thermoelectric power factor enhancement. Our model
has been validated by interpretation of experimental observations in
highly B-doped nanocrytalline films. Our findings can be used to
engineer nanostructured materials with high thermoelectric performance.
Published under license by AIP Publishing