4,723 research outputs found
Quantum transport through molecular wires
We explore electron transport properties in molecular wires made of
heterocyclic molecules (pyrrole, furan and thiophene) by using the Green's
function technique. Parametric calculations are given based on the
tight-binding model to describe the electron transport in these wires. It is
observed that the transport properties are significantly influenced by (a) the
heteroatoms in the heterocyclic molecules and (b) the molecule-to-electrodes
coupling strength. Conductance () shows sharp resonance peaks associated
with the molecular energy levels in the limit of weak molecular coupling, while
they get broadened in the strong molecular coupling limit. These resonances get
shifted with the change of the heteroatoms in these heterocyclic molecules. All
the essential features of the electron transfer through these molecular wires
become much more clearly visible from the study of our current-voltage
(-) characteristics, and they provide several key informations in the
study of molecular transport.Comment: 8 pages, 4 figure
Multi-Pulse Laser Wakefield Acceleration: A New Route to Efficient, High-Repetition-Rate Plasma Accelerators and High Flux Radiation Sources
Laser-driven plasma accelerators can generate accelerating gradients three
orders of magnitude larger than radio-frequency accelerators and have achieved
beam energies above 1 GeV in centimetre long stages. However, the pulse
repetition rate and wall-plug efficiency of plasma accelerators is limited by
the driving laser to less than approximately 1 Hz and 0.1% respectively. Here
we investigate the prospects for exciting the plasma wave with trains of
low-energy laser pulses rather than a single high-energy pulse. Resonantly
exciting the wakefield in this way would enable the use of different
technologies, such as fibre or thin-disc lasers, which are able to operate at
multi-kilohertz pulse repetition rates and with wall-plug efficiencies two
orders of magnitude higher than current laser systems. We outline the
parameters of efficient, GeV-scale, 10-kHz plasma accelerators and show that
they could drive compact X-ray sources with average photon fluxes comparable to
those of third-generation light source but with significantly improved temporal
resolution. Likewise FEL operation could be driven with comparable peak power
but with significantly larger repetition rates than extant FELs
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Genetic Diversity and the Structure of Genealogies in Rapidly Adapting Populations
Positive selection distorts the structure of genealogies and hence alters patterns of genetic variation within a population. Most analyses of these distortions focus on the signatures of hitchhiking due to hard or soft selective sweeps at a single genetic locus. However, in linked regions of rapidly adapting genomes, multiple beneficial mutations at different loci can segregate simultaneously within the population, an effect known as clonal interference. This leads to a subtle interplay between hitchhiking and interference effects, which leads to a unique signature of rapid adaptation on genetic variation both at the selected sites and at linked neutral loci. Here, we introduce an effective coalescent theory (a “fitness-class coalescent”) that describes how positive selection at many perfectly linked sites alters the structure of genealogies. We use this theory to calculate several simple statistics describing genetic variation within a rapidly adapting population and to implement efficient backward-time coalescent simulations, which can be used to predict how clonal interference alters the expected patterns of molecular evolution.Organismic and Evolutionary BiologyPhysic
A mesoscopic ring as a XNOR gate: An exact result
We describe XNOR gate response in a mesoscopic ring threaded by a magnetic
flux . The ring is attached symmetrically to two semi-infinite
one-dimensional metallic electrodes and two gate voltages, viz, and
, are applied in one arm of the ring which are treated as the inputs of
the XNOR gate. The calculations are based on the tight-binding model and the
Green's function method, which numerically compute the conductance-energy and
current-voltage characteristics as functions of the ring-to-electrode coupling
strength, magnetic flux and gate voltages. Our theoretical study shows that,
for a particular value of () (, the elementary
flux-quantum), a high output current (1) (in the logical sense) appears if both
the two inputs to the gate are the same, while if one but not both inputs are
high (1), a low output current (0) results. It clearly exhibits the XNOR gate
behavior and this aspect may be utilized in designing an electronic logic gate.Comment: 8 pages, 5 figure
Reconstructing nonlinear plasma wakefields using a generalized temporally encoded spectral shifting analysis
We generalize the temporally encoded spectral shifting (TESS) analysis for measuring plasma wakefields using spectral interferometry to dissimilar probe pulses of arbitrary spectral profile and to measuring nonlinear wakefields. We demonstrate that the Gaussian approximation used up until now results in a substantial miscalculation of the wakefield amplitude, by a factor of up to two. A method to accurately measure higher amplitude quasilinear and nonlinear wakefields is suggested, using an extension to the TESS procedure, and we place some limits on its accuracy in these regimes. These extensions and improvements to the analysis demonstrate its potential for rapid and accurate on-shot diagnosis of plasma wakefields, even at low plasma densities
Optimizing information flow in small genetic networks. I
In order to survive, reproduce and (in multicellular organisms)
differentiate, cells must control the concentrations of the myriad different
proteins that are encoded in the genome. The precision of this control is
limited by the inevitable randomness of individual molecular events. Here we
explore how cells can maximize their control power in the presence of these
physical limits; formally, we solve the theoretical problem of maximizing the
information transferred from inputs to outputs when the number of available
molecules is held fixed. We start with the simplest version of the problem, in
which a single transcription factor protein controls the readout of one or more
genes by binding to DNA. We further simplify by assuming that this regulatory
network operates in steady state, that the noise is small relative to the
available dynamic range, and that the target genes do not interact. Even in
this simple limit, we find a surprisingly rich set of optimal solutions.
Importantly, for each locally optimal regulatory network, all parameters are
determined once the physical constraints on the number of available molecules
are specified. Although we are solving an over--simplified version of the
problem facing real cells, we see parallels between the structure of these
optimal solutions and the behavior of actual genetic regulatory networks.
Subsequent papers will discuss more complete versions of the problem
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