1,782 research outputs found
Free electron lasers for transmission of energy in space
A one-dimensional resonant-particle model of a free electron laser (FEL) is used to calculate laser gain and conversion efficiency of electron energy to photon energy. The optical beam profile for a resonant optical cavity is included in the model as an axial variation of laser intensity. The electron beam profile is matched to the optical beam profile and modeled as an axial variation of current density. Effective energy spread due to beam emittance is included. Accelerators appropriate for a space-based FEL oscillator are reviewed. Constraints on the concentric optical resonator and on systems required for space operation are described. An example is given of a space-based FEL that would produce 1.7 MW of average output power at 0.5 micrometer wavelength with over 50% conversion efficiency of electrical energy to laser energy. It would utilize a 10 m-long amplifier centered in a 200 m-long optical cavity. A 3-amp, 65 meV electrostatic accelerator would provide the electron beam and recover the beam after it passes through the amplifier. Three to five shuttle flights would be needed to place the laser in orbit
Compensation in epitaxial cubic SiC films
Hall measurements on four n-type cubic SiC films epitaxially grown by chemical vapor deposition on SiC substrates are reported. The temperature dependent carrier concentrations indicate that the samples are highly compensated. Donor ionization energies, E sub D, are less than one half the values previously reported. The values for E sub D and the donor concentration N sub D, combined with results for small bulk platelets with nitrogen donors, suggest the relation E sub D (N sub D) = E sub D(O) - alpha N sub N sup 1/3 for cubic SiC. A curve fit gives alpha is approx 2.6x10/5 meV cm and E sub D (O) approx 48 meV, which is the generally accepted value of E sub D(O) for nitrogen donors in cubic SiC
Raman frequency shift in oxygen functionalized carbon nanotubes
In terms of lattice dynamics theory, we study the vibrational properties of
the oxygen-functionalized single wall carbon nanotubes (O-SWCNs). Due to the
C-O and O-O interactions, many degenerate phonon modes are split and even some
new phonon modes are obtained, different from the bare SWCNs. A distinct Raman
shift is found in both the radial breathing mode and G modes, depending not
only on the tube diameter and chirality but also on oxygen coverage and
adsorption configurations. With the oxygen coverage increasing, interesting, a
nonmonotonic up- and down-shift is observed in G modes, which is contributed to
the competition between the bond expansion and contraction, there coexisting in
the functionalized carbon nanotube.Comment: 4 pages, 3 figures, 1 tabl
Spin-dependent resonant tunneling through semimetallic ErAs quantum wells
Resonant tunneling through semimetallic ErAs quantum wells embedded in GaAs
structures with AlAs barriers was recently found to exhibit an intriguing
behavior in magnetic fields which is explained in terms of tunneling selection
rules and the spin-polarized band structure including spin-orbit coupling.Comment: 4 pages, figures supplied as self-unpacking figures.uu, uses
epsfig.sty to incorporate figures in preprin
Diffraction in low-energy electron scattering from DNA: bridging gas phase and solid state theory
Using high-quality gas phase electron scattering calculations and multiple
scattering theory, we attempt to gain insights on the radiation damage to DNA
induced by secondary low-energy electrons in the condensed phase, and to bridge
the existing gap with the gas phase theory and experiments. The origin of
different resonant features (arising from single molecules or diffraction) is
discussed and the calculations are compared to existing experiments in thin
films.Comment: 40 pages preprint, 12 figures, submitted to J. Chem. Phy
Dynamic receptor team formation can explain the high signal transduction gain in E. coli
Evolution has provided many organisms with sophisticated sensory systems that
enable them to respond to signals in their environment. The response frequently
involves alteration in the pattern of movement, such as the chemokinesis of the
bacterium Escherichia coli, which swims by rotating its flagella. When rotated
counterclockwise (CCW) the flagella coalesce into a propulsive bundle,
producing a relatively straight ``run'', and when rotated clockwise (CW) they
fly apart, resulting in a ``tumble'' which reorients the cell with little
translocation. A stochastic process generates the runs and tumbles, and in a
chemoeffector gradient runs that carry the cell in a favorable direction are
extended. The overall structure of the signal transduction pathways is
well-characterized in E. coli, but important details are still not understood.
Only recently has a source of gain in the signal transduction network been
identified experimentally, and here we present a mathematical model based on
dynamic assembly of receptor teams that can explain this observation.Comment: Accepted for publication in the Biophysical Journa
Increased accuracy of ligand sensing by receptor internalization
Many types of cells can sense external ligand concentrations with
cell-surface receptors at extremely high accuracy. Interestingly, ligand-bound
receptors are often internalized, a process also known as receptor-mediated
endocytosis. While internalization is involved in a vast number of important
functions for the life of a cell, it was recently also suggested to increase
the accuracy of sensing ligand as the overcounting of the same ligand molecules
is reduced. Here we show, by extending simple ligand-receptor models to
out-of-equilibrium thermodynamics, that internalization increases the accuracy
with which cells can measure ligand concentrations in the external environment.
Comparison with experimental rates of real receptors demonstrates that our
model has indeed biological significance.Comment: 9 pages, 4 figures, accepted for publication in Physical Review
Breather decay into a vortex/anti-vortex pair in a Josephson Ladder
We present experimental evidence for a new behavior which involves discrete
breathers and vortices in a Josephson Ladder. Breathers can be visualized as
the creation and subsequent annihilation of vortex/anti-vortex pairs. An
externally applied magnetic field breaks the vortex/anti-vortex symmetry and
causes the breather to split apart. The motion of the vortex or anti-vortex
creates multi-site breathers, which are always to one side or the other of the
original breather depending on the sign of the applied field. This asymmetry in
applied field is experimentally observed.Comment: 10 pages, 5 figure
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