426 research outputs found
Increasing thermoelectric performance using coherent transport
We show that coherent electron transport through zero-dimensional systems can
be used to tailor the shape of the system's transmission function. This
quantum-engineering approach can be used to enhance the performance of quantum
dots or molecules in thermal-to-electric power conversion. Specifically, we
show that electron interference in a two-level system can substantially improve
the maximum thermoelectric power and the efficiency at maximum power by
suppressing parasitic charge flow near the Fermi energy, and by reducing
electronic heat conduction. We discuss possible realizations of this approach
in molecular junctions or quantum dots.Comment: 4+ pages, 4 figure
Correlation-induced conductance suppression at level degeneracy in a quantum dot
The large, level-dependent g-factors in an InSb nanowire quantum dot allow
for the occurrence of a variety of level crossings in the dot. While we observe
the standard conductance enhancement in the Coulomb blockade region for aligned
levels with different spins due to the Kondo effect, a vanishing of the
conductance is found at the alignment of levels with equal spins. This
conductance suppression appears as a canyon cutting through the web of direct
tunneling lines and an enclosed Coulomb blockade region. In the center of the
Coulomb blockade region, we observe the predicted correlation-induced
resonance, which now turns out to be part of a larger scenario. Our findings
are supported by numerical and analytical calculations.Comment: 5 pages, 4 figure
Multi-Orbital Molecular Compound (TTM-TTP)I_3: Effective Model and Fragment Decomposition
The electronic structure of the molecular compound (TTM-TTP)I_3, which
exhibits a peculiar intra-molecular charge ordering, has been studied using
multi-configuration ab initio calculations. First we derive an effective
Hubbard-type model based on the molecular orbitals (MOs) of TTM-TTP; we set up
a two-orbital Hamiltonian for the two MOs near the Fermi energy and determine
its full parameters: the transfer integrals, the Coulomb and exchange
interactions. The tight-binding band structure obtained from these transfer
integrals is consistent with the result of the direct band calculation based on
density functional theory. Then, by decomposing the frontier MOs into two
parts, i.e., fragments, we find that the stacked TTM-TTP molecules can be
described by a two-leg ladder model, while the inter-fragment Coulomb energies
are scaled to the inverse of their distances. This result indicates that the
fragment picture that we proposed earlier [M.-L. Bonnet et al.: J. Chem. Phys.
132 (2010) 214705] successfully describes the low-energy properties of this
compound.Comment: 5 pages, 4 figures, published versio
Yb2+-doped SrCl2: electronic structure of impurity states and impurity-trapped excitons
First-principles electronic structure calculations of the excited states of Yb2+-doped Sr Cl2 crystals up to 65 000 cm-1 reveal the existence of unexpected excited states with double-well potential energy surfaces and dual electronic structure lying above and very close in energy to the 4f 135d manifold, with which they interact strongly through spin-orbit coupling. The double-well energy curves result from avoided crossings between Yb-trapped exciton states (more stable at short Yb–Cl distances) and 4f 136s impurity states (more stable at long Yb–Cl distances); the former are found to be preionization states in which the impurity holds the excited electron in close lying empty interstitials located outside the YbCl8 moiety. Spin-orbit coupling between the double-well states and the lower lying 4f 135d impurity states spreads the dual electronic structure character to lower energies and, hence, the instability of the divalent oxidation state is also spread. To some extent, the dual electronic structure (impurity-trapped exciton–impurity state) of some excited states expresses and gives support to hypotheses of interaction between Yb2+ and Yb3+ pairs proposed to understand the complex spectroscopy of the material and conciliates these hypotheses with interpretations in terms of the existence of only one type of Yb2+ defect. The results presented confirm the presence of impurity states of the 4f 136s configuration among the 4f 135d manifolds, as proposed in literature, but their energies are very different from those assumed. The Yb-trapped excitons found in this chloride host can be seen as precursors of the luminescent Yb-trapped excitons characterized experimentally in the isomorphous SrF2 crystals
γ-Secretase modulators show selectivity for γ-secretase–mediated amyloid precursor protein intramembrane processing
The aggregation of β-amyloid peptide 42 results in the formation of toxic oligomers and plaques, which plays a pivotal role in Alzheimer's disease pathogenesis. Aβ42 is one of several Aβ peptides, all of Aβ30 to Aβ43 that are produced as a result of γ-secretase–mediated regulated intramembrane proteolysis of the amyloid precursor protein. γ-Secretase modulators (GSMs) represent a promising class of Aβ42-lowering anti-amyloidogenic compounds for the treatment of AD. Gamma-secretase modulators change the relative proportion of secreted Aβ peptides, while sparing the γ-secretase–mediated processing event resulting in the release of the cytoplasmic APP intracellular domain. In this study, we have characterized how GSMs affect the γ-secretase cleavage of three γ-secretase substrates, E-cadherin, ephrin type A receptor 4 (EphA4) and ephrin type B receptor 2 (EphB2), which all are implicated in important contexts of cell signalling. By using a reporter gene assay, we demonstrate that the γ-secretase–dependent generation of EphA4 and EphB2 intracellular domains is unaffected by GSMs. We also show that γ-secretase processing of EphA4 and EphB2 results in the release of several Aβ-like peptides, but that only the production of Aβ-like proteins from EphA4 is modulated by GSMs, but with an order of magnitude lower potency as compared to Aβ modulation. Collectively, these results suggest that GSMs are selective for γ-secretase–mediated Aβ production
On ordinal utility, cardinal utility, and random utility
Though the Random Utility Model (RUM) was conceived
entirely in terms of ordinal utility, the apparatus throughwhich it is widely practised exhibits properties of
cardinal utility. The adoption of cardinal utility as a
working operation of ordinal is perfectly valid, provided
interpretations drawn from that operation remain faithful
to ordinal utility. The paper considers whether the latterrequirement holds true for several measurements commonly
derived from RUM. In particular it is found that
measurements of consumer surplus change may depart from
ordinal utility, and exploit the cardinality inherent in
the practical apparatus.
Heat dissipation in atomic-scale junctions
Atomic and single-molecule junctions represent the ultimate limit to the
miniaturization of electrical circuits. They are also ideal platforms to test
quantum transport theories that are required to describe charge and energy
transfer in novel functional nanodevices. Recent work has successfully probed
electric and thermoelectric phenomena in atomic-scale junctions. However, heat
dissipation and transport in atomic-scale devices remain poorly characterized
due to experimental challenges. Here, using custom-fabricated scanning probes
with integrated nanoscale thermocouples, we show that heat dissipation in the
electrodes of molecular junctions, whose transmission characteristics are
strongly dependent on energy, is asymmetric, i.e. unequal and dependent on both
the bias polarity and the identity of majority charge carriers (electrons vs.
holes). In contrast, atomic junctions whose transmission characteristics show
weak energy dependence do not exhibit appreciable asymmetry. Our results
unambiguously relate the electronic transmission characteristics of
atomic-scale junctions to their heat dissipation properties establishing a
framework for understanding heat dissipation in a range of mesoscopic systems
where transport is elastic. We anticipate that the techniques established here
will enable the study of Peltier effects at the atomic scale, a field that has
been barely explored experimentally despite interesting theoretical
predictions. Furthermore, the experimental advances described here are also
expected to enable the study of heat transport in atomic and molecular
junctions, which is an important and challenging scientific and technological
goal that has remained elusive.Comment: supporting information available in the journal web site or upon
reques
A linear nonequilibrium thermodynamics approach to optimization of thermoelectric devices
Improvement of thermoelectric systems in terms of performance and range of
applications relies on progress in materials science and optimization of device
operation. In this chapter, we focuse on optimization by taking into account
the interaction of the system with its environment. For this purpose, we
consider the illustrative case of a thermoelectric generator coupled to two
temperature baths via heat exchangers characterized by a thermal resistance,
and we analyze its working conditions. Our main message is that both electrical
and thermal impedance matching conditions must be met for optimal device
performance. Our analysis is fundamentally based on linear nonequilibrium
thermodynamics using the force-flux formalism. An outlook on mesoscopic systems
is also given.Comment: Chapter 14 in "Thermoelectric Nanomaterials", Editors Kunihito
Koumoto and Takao Mori, Springer Series in Materials Science Volume 182
(2013
On the possibility of magneto-structural correlations: detailed studies of di-nickel carboxylate complexes
A series of water-bridged dinickel complexes of the general formula [Ni<sub>2</sub>(μ<sub>2</sub>-OH<sub>2</sub>)(μ2-
O<sub>2</sub>C<sup>t</sup>Bu)<sub>2</sub>(O<sub>2</sub>C<sup>t</sup>Bu)2(L)(L0)] (L = HO<sub>2</sub>C<sup>t</sup>Bu, L0 = HO<sub>2</sub>C<sup>t</sup>Bu (1), pyridine (2),
3-methylpyridine (4); L = L0 = pyridine (3), 3-methylpyridine (5)) has been synthesized
and structurally characterized by X-ray crystallography. The magnetic properties
have been probed by magnetometry and EPR spectroscopy, and detailed measurements
show that the axial zero-field splitting, D, of the nickel(ii) ions is on the same order as
the isotropic exchange interaction, J, between the nickel sites. The isotropic exchange
interaction can be related to the angle between the nickel centers and the bridging
water molecule, while the magnitude of D can be related to the coordination sphere at
the nickel sites
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