429 research outputs found
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Seismic Velocities and Attenuation in a Heated Underground Granitic Repository Volume II
Recommended from our members
Seismic Velocities and Attenuation in a Heated Underground Granitic Repository Volume I
FeCoCp3 Molecular Magnets as Spin Filters
Metallorganic molecules have been proposed as excellent spin filters in
molecular spintronics because of the large spin-polarization of their
electronic structure. However, most of the studies involving spin transport,
have disregarded fundamental aspects such as the magnetic anisotropy of the
molecule and the excitation of spin-flip processes during electron transport.
Here, we study a molecule containing a Co and an Fe atoms stacked between three
cyclopentadienyl rings that presents a large magnetic anisotropy and a S=1.
These figures are superior to other molecules with the same transition metal,
and improves the spin-filtering capacities of the molecule. Non-equilibrium
Green's functions calculations based on density functional theory predict
excellent spin-filtering properties both in tunnel and contact transport
regimes. However, exciting the first magnetic state drastically reduces the
current's spin polarization. Furthermore, a difference of temperature between
electrodes leads to strong thermoelectric effects that also suppress spin
polarization. Our study shows that in-principle good molecular candidates for
spintronics need to be confronted with inelastic and thermoelectric effects
Sociobiological Control of Plasmid copy number
Background:
All known mechanisms and genes responsible for the regulation of plasmid replication lie with the plasmid rather than the chromosome. It is possible therefore that there can be copy-up mutants. Copy-up mutants will have within host selective advantage. This would eventually result into instability of bacteria-plasmid association. In spite of this possibility low copy number plasmids appear to exist stably in host populations. We examined this paradox using a computer simulation model.

Model:
Our multilevel selection model assumes a wild type with tightly regulated replication to ensure low copy number. A mutant with slightly relaxed replication regulation can act as a “cheater” or “selfish” plasmid and can enjoy a greater within-host-fitness. However the host of a cheater plasmid has to pay a greater cost. As a result, in host level competition, host cell with low copy number plasmid has a greater fitness. Furthermore, another mutant that has lost the genes required for conjugation was introduced in the model. The non-conjugal mutant was assumed to undergo conjugal transfer in the presence of another conjugal plasmid in the host cell.

Results:
The simulatons showed that if the cost of carrying a plasmid was low, the copy-up mutant could drive the wild type to extinction or very low frequencies. Consequently, another mutant with a higher copy number could invade the first invader. This process could result into an increasing copy number. However above a certain copy number within-host selection was overcompensated by host level selection leading to a rock-paper-scissor (RPS) like situation. The RPS situation allowed the coexistence of high and low copy number plasmids. The non-conjugal “hypercheaters” could further arrest the copy numbers to a substantially lower level.

Conclusions:
These sociobiological interactions might explain the stability of copy numbers better than molecular mechanisms of replication regulation alone
Efficiency of Energy Conversion in Thermoelectric Nanojunctions
Using first-principles approaches, this study investigated the efficiency of
energy conversion in nanojunctions, described by the thermoelectric figure of
merit . We obtained the qualitative and quantitative descriptions for the
dependence of on temperatures and lengths. A characteristic temperature:
was observed. When , . When , tends to a saturation value. The dependence of
on the wire length for the metallic atomic chains is opposite to that for
the insulating molecules: for aluminum atomic (conducting) wires, the
saturation value of increases as the length increases; while for
alkanethiol (insulating) chains, the saturation value of decreases as the
length increases. can also be enhanced by choosing low-elasticity bridging
materials or creating poor thermal contacts in nanojunctions. The results of
this study may be of interest to research attempting to increase the efficiency
of energy conversion in nano thermoelectric devices.Comment: 2 figure
The number of transmission channels through a single-molecule junction
We calculate transmission eigenvalue distributions for Pt-benzene-Pt and
Pt-butadiene-Pt junctions using realistic state-of-the-art many-body
techniques. An effective field theory of interacting -electrons is used to
include screening and van der Waals interactions with the metal electrodes. We
find that the number of dominant transmission channels in a molecular junction
is equal to the degeneracy of the molecular orbital closest to the metal Fermi
level.Comment: 9 pages, 8 figure
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
Effect of Thermoelectric Cooling in Nanoscale Junctions
We propose a thermoelectric cooling device based on an atomic-sized junction.
Using first-principles approaches, we investigate the working conditions and
the coefficient of performance (COP) of an atomic-scale electronic refrigerator
where the effects of phonon's thermal current and local heating are included.
It is observed that the functioning of the thermoelectric nano-refrigerator is
restricted to a narrow range of driving voltages. Compared with the bulk
thermoelectric system with the overwhelmingly irreversible Joule heating, the
4-Al atomic refrigerator has a higher efficiency than a bulk thermoelectric
refrigerator with the same due to suppressed local heating via the
quasi-ballistic electron transport and small driving voltages. Quantum nature
due to the size minimization offered by atomic-level control of properties
facilitates electron cooling beyond the expectation of the conventional
thermoelectric device theory.Comment: 8 figure
The Influence of Molecular Adsorption on Elongating Gold Nanowires
Using molecular dynamics simulations, we study the impact of physisorbing
adsorbates on the structural and mechanical evolution of gold nanowires (AuNWs)
undergoing elongation. We used various adsorbate models in our simulations,
with each model giving rise to a different surface coverage and mobility of the
adsorbed phase. We find that the local structure and mobility of the adsorbed
phase remains relatively uniform across all segments of an elongating AuNW,
except for the thinning region of the wire where the high mobility of Au atoms
disrupts the monolayer structure, giving rise to higher solvent mobility. We
analyzed the AuNW trajectories by measuring the ductile elongation of the wires
and detecting the presence of characteristic structural motifs that appeared
during elongation. Our findings indicate that adsorbates facilitate the
formation of high-energy structural motifs and lead to significantly higher
ductile elongations. In particular, our simulations result in a large number of
monatomic chains and helical structures possessing mechanical stability in
excess of what we observe in vacuum. Conversely, we find that a molecular
species that interacts weakly (i.e., does not adsorb) with AuNWs worsens the
mechanical stability of monatomic chains.Comment: To appear in Journal of Physical Chemistry
Electrical resistance: an atomistic view
This tutorial article presents a "bottom-up" view of electrical resistance
starting from something really small, like a molecule, and then discussing the
issues that arises as we move to bigger conductors. Remarkably, no serious
quantum mechanics is needed to understand electrical conduction through
something really small, except for unusual things like the Kondo effect that
are seen only for a special range of parameters. This article starts with
energy level diagrams (section 2), shows that the broadening that accompanies
coupling limits the conductance to a maximum of q^2/h per level (sections 3,
4), describes how a change in the shape of the self-consistent potential
profile can turn a symmetric current-voltage characteristic into a rectifying
one (sections 5, 6), shows that many interesting effects in molecular
electronics can be understood in terms of a simple model (section 7),
introduces the non-equilibrium Green function (NEGF) formalism as a
sophisticated version of this simple model with ordinary numbers replaced by
appropriate matrices (section 8) and ends with a personal view of unsolved
problems in the field of nanoscale electron transport (section 9). Appendix A
discusses the Coulomb blockade regime of transport, while appendix B presents a
formal derivation of the NEGF equations. MATLAB codes for numerical examples
are listed in the appendix C
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