16,544 research outputs found
Heat current through an artificial Kondo impurity beyond linear response
We investigate the heat current of a strongly interacting quantum dot in the
presence of a voltage bias in the Kondo regime. Using the slave- boson
mean-field theory, we discuss the behavior of the energy flow and the Joule
heating. We find that both contributions to the heat current dis- play
interesting symmetry properties under reversal of the applied dc bias. We show
that the symmetries arise from the behavior of the dot trans- mission function.
Importantly, the transmission probability is a function of both energy and
voltage. This allows us to analyze the heat current in the nonlinear regime of
transport. We observe that nonlinearities ap- pear already for voltages smaller
than the Kondo temperature. Finally, we suggest to use the contact and electric
symmetry coefficients as a way to measure pure energy currents.Comment: 9 pages, 2 figures, proceeding of the Low Temperature Physics
Conferenc
Strongly nonlinear thermovoltage and heat dissipation in interacting quantum dots
We investigate the nonlinear regime of charge and energy transport through
Coulomb-blockaded quantum dots. We discuss crossed effects that arise when
electrons move in response to thermal gradients (Seebeck effect) or energy
flows in reaction to voltage differences (Peltier effect). We find that the
differential thermoelectric conductance shows a characteristic Coulomb
butterfly structure due to charging effects. Importantly, we show that
experimentally observed thermovoltage zeros are caused by the activation of
Coulomb resonances at large thermal shifts. Furthermore, the power dissipation
asymmetry between the two attached electrodes can be manipulated with the
applied voltage, which has implications for the efficient design of nanoscale
coolers.Comment: 6 pages, 4 figure
Fate of the spin-\frac{1}{2} Kondo effect in the presence of temperature gradients
We consider a strongly interacting quantum dot connected to two leads held at
quite different temperatures. Our aim is to study the behavior of the Kondo
effect in the presence of large thermal biases. We use three different
approaches, namely, a perturbation formalism based on the Kondo Hamiltonian, a
slave-boson mean-field theory for the Anderson model at large charging energies
and a truncated equation-of-motion approach beyond the Hartree-Fock
approximation. The two former formalisms yield a suppression of the Kondo peak
for thermal gradients above the Kondo temperature, showing a remarkably good
agreement despite their different ranges of validity. The third technique
allows us to analyze the full density of states within a wide range of
energies. Additionally, we have investigated the quantum transport properties
(electric current and thermocurrent) beyond linear response. In the
voltage-driven case, we reproduce the split differential conductance due to the
presence of different electrochemical potentials. In the temperature-driven
case, we observe a strongly nonlinear thermocurrent as a function of the
applied thermal gradient. Depending on the parameters, we can find nontrivial
zeros in the electric current for finite values of the temperature bias.
Importantly, these thermocurrent zeros yield direct access to the system's
characteristic energy scales (Kondo temperature and charging energy).Comment: 14 pages, 11 figures, revised versio
The Effects of Carbon Coating on the Electrochemical Performance of Metal-Oxide Short Fiber Anodes for Lithium-Ion Batteries
This thesis focuses on the process of oxidizing centrifugally spun precursor fibers and the subsequent process of carbon coating via chemical vapor deposition (CVD) for use as anode material in lithium-ion batteries (LIBs). Metal oxides have been studied as a potential replacement for graphite as they have been shown to have high theoretical capacities, good electronic conductivity, and can be synthesized using low-cost, scalable methods. However, metal oxides with high theoretical capacities also have low cycle life. To avoid this, metal oxides have been integrated with carbon to expand their life cycle. The work in this thesis shows the synthesis of SnO2/TiO2 composite short fibers with different ratios, followed by the deposition of carbon to be used as active material for LIB anodes. When tested, SnO2/TiO2 (3:1) CVD with a deposition time of 60-min demonstrated a specific capacity of 499 mAh g-1 and capacity retention of 111% after 100 cycles. In comparison, the 30-min had a specific capacity of 653 mAh g-1 after 61 cycles and is projected to have a capacity retention of 93.2% after 100 cycles. The results were individually compared to the non-coated and parent materials of SnO2 and TiO2
The responses of small and large firms to tight credit shocks : the case of 2008 through the lens of Gertler and Gilchrist (1994)
Do large firms and small firms behave differently when credit becomes more costly or harder to obtain? Past research has found that small firms are more likely to be credit-constrained and thus tend to be affected more negatively than large firms during such times. Recent findings from the 2007-2009 recession, however, raise questions about the roles of small and large firms during periods of tight creditBusiness cycles ; Recessions
Interactions and thermoelectric effects in a parallel-coupled double quantum dot
We investigate the nonequilibrium transport properties of a double quantum
dot system connected in parallel to two leads, including intradot
electron-electron interaction. In the absence of interactions the system
supports a bound state in the continuum. This state is revealed as a Fano
antiresonance in the transmission when the energy levels of the dots are
detuned. Using the Keldysh nonequilibrium Green's function formalism, we find
that the occurrence of the Fano antiresonance survives in the presence of
Coulomb repulsion. We give precise predictions for the experimental detection
of bound states in the continuum. First, we calculate the differential
conductance as a function of the applied voltage and the dot level detuning and
find that crossing points in the diamond structure are revealed as minima due
to the transmission antiresonances. Second, we determine the thermoelectric
current in response to an applied temperature bias. In the linear regime,
quantum interference gives rise to sharp peaks in the thermoelectric
conductance. Remarkably, we find interaction induced strong current
nonlinearities for large thermal gradients that may lead to several nontrivial
zeros in the thermocurrent. The latter property is especially attractive for
thermoelectric applications.Comment: 9 pages, 8 figure
CATTLE/BEEF SUBSECTOR'S STRUCTURE AND COMPETITION UNDER FREE TRADE
Industrial Organization, International Relations/Trade,
Amino acid metabolism conflicts with protein diversity
The twenty protein coding amino acids are found in proteomes with different
relative abundances. The most abundant amino acid, leucine, is nearly an order
of magnitude more prevalent than the least abundant amino acid, cysteine. Amino
acid metabolic costs differ similarly, constraining their incorporation into
proteins. On the other hand, sequence diversity is necessary for protein
folding, function and evolution. Here we present a simple model for a
cost-diversity trade-off postulating that natural proteomes minimize amino acid
metabolic flux while maximizing sequence entropy. The model explains the
relative abundances of amino acids across a diverse set of proteomes. We found
that the data is remarkably well explained when the cost function accounts for
amino acid chemical decay. More than one hundred proteomes reach comparable
solutions to the trade-off by different combinations of cost and diversity.
Quantifying the interplay between proteome size and entropy shows that
proteomes can get optimally large and diverse
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