369,988 research outputs found
Negative Refraction and Subwavelength Lensing in a Polaritonic Crystal
We show that a two-dimensional polaritonic crystal, made of metallic rods
that support well defined plasmon oscillations, can act in a narrow frequency
range as a medium in which a negative refraction and subwavelength lensing can
occur. We show that surface modes are excited on the surface of the lens, and
that they facilitate restoration of the evanescent waves, which carry the
subwavelength image information. We demonstrate that this can occur in the
visible frequency range, for a wide range of materials, including silver and
aluminum rods, and carbon nanotubes. This flexibility should allow for an
experimental demonstration of this phenomenon in the visible frequency range.Comment: 14 pages; 4 figure
Autonomous Locomotion Mode Transition Simulation of a Track-legged Quadruped Robot Step Negotiation
Multi-modal locomotion (e.g. terrestrial, aerial, and aquatic) is gaining
increasing interest in robotics research as it improves the robots
environmental adaptability, locomotion versatility, and operational
flexibility. Within the terrestrial multiple locomotion robots, the advantage
of hybrid robots stems from their multiple (two or more) locomotion modes,
among which robots can select from depending on the encountering terrain
conditions. However, there are many challenges in improving the autonomy of the
locomotion mode transition between their multiple locomotion modes. This work
proposed a method to realize an autonomous locomotion mode transition of a
track-legged quadruped robot steps negotiation. The autonomy of the
decision-making process was realized by the proposed criterion to comparing
energy performances of the rolling and walking locomotion modes. Two climbing
gaits were proposed to achieve smooth steps negotiation behaviours for energy
evaluation purposes. Simulations showed autonomous locomotion mode transitions
were realized for negotiations of steps with different height. The proposed
method is generic enough to be utilized to other hybrid robots after some
pre-studies of their locomotion energy performances
Figure of Merit for Dark Energy Constraints from Current Observational Data
Choosing the appropriate figure of merit (FoM) for dark energy (DE)
constraints is key in comparing different DE experiments. Here we show that for
a set of DE parameters {f_i}, it is most intuitive to define FoM =
1/\sqrt{Cov(f1,f2,f3,...)}, where Cov(f1,f2,f3,...) is the covariance matrix of
{f_i}. The {f_i} should be minimally correlated. We demonstrate two useful
choices of {f_i} using 182 SNe Ia (compiled by Riess et al. 2007), [R(z_*),
l_a(z_*), \Omega_b h^2] from the five year Wilkinson Microwave Anisotropy Probe
(WMAP) observations, and SDSS measurement of the baryon acoustic oscillation
(BAO) scale, assuming the HST prior of H_0=72+/-8 km/s Mpc^{-1} and without
assuming spatial flatness. We find that the correlation of (w_0,w_{0.5})
[w_0=w_X(z=0), w_{0.5}=w_X(z=0.5), w_X(a) = 3w_{0.5}-2w_0+3(w_0-w_{0.5})a] is
significantly smaller than that of (w_0,w_a) [w_X(a)=w_0+(1-a)w_a]. In order to
obtain model-independent constraints on DE, we parametrize the DE density
function X(z)=\rho_X(z)/\rho_X(0) as a free function with X_{0.5}, X_{1.0}, and
X_{1.5} [values of X(z) at z=0.5, 1.0, and 1.5] as free parameters estimated
from data. If one assumes a linear DE equation of state, current data are
consistent with a cosmological constant at 68% C.L. If one assumes X(z) to be a
free function parametrized by (X_{0.5}, X_{1.0}, X_{1.5}), current data deviate
from a cosmological constant at z=1 at 68% C.L., but are consistent with a
cosmological constant at 95% C.L.. Future DE experiments will allow us to
dramatically increase the FoM of constraints on (w_0,w_{0.5}) and of (X_{0.5},
X_{1.0}, X_{1.5}). This will significantly shrink the DE parameter space to
enable the discovery of DE evolution, or the conclusive evidence for a
cosmological constant.Comment: 7 pages, 3 color figures. Submitte
Localization and band-gap pinning in semiconductor superlattices with layer-thickness fluctuations
We consider (AlAs)_n/(GaAs)_n superlattices with random thickness
fluctuations Delta-n around the nominal period n. Using three-dimensional
pseudopotential plane-wave band theory, we show that (i) any amount Delta-n/n
of thickness fluctuations leads to band-edge wavefunction localization, (ii)
for small Delta-n/n the SL band gap is pinned at the gap level produced by a
single layer with ``wrong'' thickness n + Delta-n, (iii) the bound states due
to monolayer thickness fluctuations lead to significant band-gap reductions,
(iv) AlAs/GaAs SL's with monolayer thickness fluctuations have a direct
band gap, while the ideal SL's are indirect for n<4.Comment: 10 pages, Revtex. 3 figures available at
http://www.cecam.fr/~mader/elstruc.html . Published in Europhys. Lett. 31,
107 (95
Spin and Orbital Splitting in Ferromagnetic Contacted Single Wall Carbon Nanotube Devices
We observed the coulomb blockade phenomena in ferromagnetic contacting single
wall semiconducting carbon nanotube devices. No obvious Coulomb peaks shift was
observed with existing only the Zeeman splitting at 4K. Combining with other
effects, the ferromagnetic leads prevent the orbital spin states splitting with
magnetic field up to 2 Tesla at 4K. With increasing magnetic field further,
both positive or negative coulomb peaks shift slopes are observed associating
with clockwise and anticlockwise orbital state splitting. The strongly
suppressed/enhanced of the conductance has been observed associating with the
magnetic field induced orbital states splitting/converging
Control of Coercivities in (Ga,Mn)As Thin Films by Small Concentrations of MnAs Nanoclusters
We demonstrate that low concentrations of a secondary magnetic phase in
(Ga,Mn)As thin films can enhance the coercivity by factors up to ~100 without
significantly degrading the Curie temperature or saturation magnetisation.
Magnetic measurements indicate that the secondary phase consists of MnAs
nanoclusters, of average size ~7nm. This approach to controlling the coercivity
while maintaining high Curie temperature, may be important for realizing
ferromagnetic semiconductor based devices.Comment: 8 pages,4 figures. accepted for publication in Appl. Phys. Let
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