13,495 research outputs found
Effective Dielectric Response of Metamaterials
We use a homogenization procedure for Maxwell's equations in order to obtain
in the local limit the frequency () dependent macroscopic dielectric
response of metamaterials made of natural constituents
with any geometrical shape repeated periodically with any structure. We
illustrate the formalism calculating for several
structures. For dielectric rectangular inclusions within a conducting material
we obtained a very anisotropic response which changes along one direction from
conductor-like at low to a resonant dielectric-like at large ,
attaining a very small reflectance at intermediate frequencies unrelated to
surface plasmon excitation and which can be tuned through geometrycal
tayloring. A similar behavior is obtained for other shapes close to the
percolation threshold.Comment: 16 pages 7 figures. Accepted in Phys. Rev. B (2009-06-08
Simulation Studies of Nanomagnet-Based Architecture
We report a simulation study on interacting ensembles of Co nanomagnets that
can perform basic logic operations and propagate logic signals, where the state
variable is the magnetization direction. Dipole field coupling between
individual nanomagnets drives the logic functionality of the ensemble and
coordinated arrangements of the nanomagnets allow for the logic signal to
propagate in a predictable way. Problems with the integrity of the logic signal
arising from instabilities in the constituent magnetizations are solved by
introducing a biaxial anisotropy term to the Gibbs magnetic free energy of each
nanomagnet. The enhanced stability allows for more complex components of a
logic architecture capable of random combinatorial logic, including horizontal
wires, vertical wires, junctions, fanout nodes, and a novel universal logic
gate. Our simulations define the focus of scaling trends in nanomagnet-based
logic and provide estimates of the energy dissipation and time per nanomagnet
reversal
Plant and Sward Characteristics to Achieve High Intake in Ruminants
Key points Intake is affected by complex interactions between signals from the digestive tract, intermediary metabolism and energy supply, and behavioural signals associated with learned behaviours or sensory signals. The ideal sward needs to have characteristics that are similar to total mixed rations to achieve high intake and animal performance. Genetic manipulation of plants may offer an accelerated rate of plant improvement, but benefits need to be demonstrated in a systems context
Relationships Between the Nutrient Content of Irrigated Pasture on Offer and that Selected by Grazing Dairy Cows
An experiment was conducted to determine the dry matter, energy, crude protein and fibre intake by dairy cows in late lactation when grazing perennial pasture offered at allocations of 15, 20, 30 and 40 kg DM/cow.day. The cows consistently selected a diet 10% higher in digestibility than that on offer. In contrast, cows selected diets with crude protein levels increasing from 22% to 40%, above that in pasture on offer, as pasture allocation increased. A similar pattern was observed for the intake of white clover (Trifolium repens L). The increase in the concentration of crude protein in the diet (as allocation increased) was due to the consumption of white clover and a decrease in the consumption of the dead components of the sward. On the other hand, the neutral detergent fibre content of the diet selected was lower than that on offer, and declined as pasture allocation increased
Exact solution for random walks on the triangular lattice with absorbing boundaries
The problem of a random walk on a finite triangular lattice with a single
interior source point and zig-zag absorbing boundaries is solved exactly. This
problem has been previously considered intractable.Comment: 10 pages, Latex, IOP macro
First principles theory of chiral dichroism in electron microscopy applied to 3d ferromagnets
Recently it was demonstrated (Schattschneider et al., Nature 441 (2006),
486), that an analogue of the X-ray magnetic circular dichroism (XMCD)
experiment can be performed with the transmission electron microscope (TEM).
The new phenomenon has been named energy-loss magnetic chiral dichroism (EMCD).
In this work we present a detailed ab initio study of the chiral dichroism in
the Fe, Co and Ni transition elements. We discuss the methods used for the
simulations together with the validity and accuracy of the treatment, which
can, in principle, apply to any given crystalline specimen. The dependence of
the dichroic signal on the sample thickness, accuracy of the detector position
and the size of convergence and collection angles is calculated.Comment: 9 pages, 6 figures, submitted to Physical Review
Ca II H and K Chromospheric Emission Lines in Late K and M Dwarfs
We have measured the profiles of the Ca II H and K chromospheric emission
lines in 147 main sequence stars of spectral type M5-K7 (0.30-0.55 solar
masses) using multiple high resolution spectra obtained during six years with
the HIRES spectrometer on the Keck 1 telescope. Remarkably, the average FWHM,
equivalent widths, and line luminosities of Ca II H and K increase by a factor
of 3 with increasing stellar mass over this small range of stellar masses. We
fit the H and K lines with a double Gaussian model to represent both the
chromospheric emission and the non-LTE central absorption. Most of the sample
stars display a central absorption that is typically redshifted by ~0.1 km/s
relative to the emission, but the nature of this velocity gradient remains
unknown. The FWHM of the H and K lines increase with stellar luminosity,
reminiscent of the Wilson-Bappu effect in FGK-type stars. Both the equivalent
widths and FWHM exhibit modest temporal variability in individual stars. At a
given value of M_v, stars exhibit a spread in both the equivalent width and
FWHM of Ca II H and K, due both to a spread in fundamental stellar parameters
including rotation rate, age, and possibly metallicity, and to the spread in
stellar mass at a given M_v. The K line is consistently wider than the H line,
as expected, and its central absorption is more redshifted, indicating that the
H and K lines form at slightly different heights in the chromosphere where the
velocities are slightly different. The equivalent width of H-alpha correlates
with Ca II H and K only for stars having Ca II equivalent widths above ~2
angstroms, suggesting the existence of a magnetic threshold above which the
lower and upper chromospheres become thermally coupled.Comment: 40 pages including 12 figures and 17 pages of tables, accepted for
publication in PAS
Universal Scaling of Optimal Current Distribution in Transportation Networks
Transportation networks are inevitably selected with reference to their
global cost which depends on the strengths and the distribution of the embedded
currents. We prove that optimal current distributions for a uniformly injected
d-dimensional network exhibit robust scale-invariance properties, independently
of the particular cost function considered, as long as it is convex. We find
that, in the limit of large currents, the distribution decays as a power law
with an exponent equal to (2d-1)/(d-1). The current distribution can be exactly
calculated in d=2 for all values of the current. Numerical simulations further
suggest that the scaling properties remain unchanged for both random injections
and by randomizing the convex cost functions.Comment: 5 pages, 5 figure
Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf
We report the detection of periodic (p = 1.96 hours) bursts of extremely
bright, 100% circularly polarized, coherent radio emission from the M9 dwarf
TVLM 513-46546. Simultaneous photometric monitoring observations have
established this periodicity to be the rotation period of the dwarf. These
bursts, which were not present in previous observations of this target, confirm
that ultracool dwarfs can generate persistent levels of broadband, coherent
radio emission, associated with the presence of kG magnetic fields in a
large-scale, stable configuration. Compact sources located at the magnetic
polar regions produce highly beamed emission generated by the electron
cyclotron maser instability, the same mechanism known to generate planetary
coherent radio emission in our solar system. The narrow beams of radiation pass
our line of sight as the dwarf rotates, producing the associated periodic
bursts. The resulting radio light curves are analogous to the periodic light
curves associated with pulsar radio emission highlighting TVLM 513-46546 as the
prototype of a new class of transient radio source.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter
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