37,544 research outputs found
Is Gamma-ray Absorption by Induced Electric Fields Important in the Pulsar Magnetospheres?
Although the unified formula for gamma-ray absorption process involving both
the magnetic field and a perpendicular electric field derived by Daugherty &
Lerche (1975) is correct, we argued in this paper that their conclusion that
the induced electric fields are important in the pair formation process in the
pulsar magnetospheres is wrong and misleading. The key point is that usually
the direction of a gamma photon at the emission point observed in the
laboratory frame should be (v/c, 0, [1-(v/c)^2]^{1/2}) rather than (0, 0, 1),
where v is the co-rotating velocity. This emission direction is just the one
which results in zero attenuation coefficient of the gamma photon. Calculation
shows that after the photon has moved a distance, its direction lead to the
result that the induced electric field is also of minor importance. Thus only
gamma-B process is the important mechanism for the pair production in the
pulsar magnetospheres. The implications of the modification by ejecting the
induced electric field are also discussed.Comment: 4 pages, 2 Postscript figures, LaTeX, some miscomments on the
references of Harding et al are modified, Accepted for publication in
Astronomy and Astrophysics Letter
Transition behavior of k-surface from hyperbola to ellipse
The transition behavior of the k-surface of a lossy anisotropic indefinite slab is investigated. It is found that, if the material loss is taken into account, the k-surface does not show a sudden change from hyperbola to the ellipse when one principle element of the permittivity tensor changes from negative to positive. In fact, after introducing a small material loss, the shape of the k-surface can be a combination of a hyperbola and an ellipse, and a selective high directional transmission can be obtained in such a slab
Spin Dynamics in the Second Subband of a Quasi Two Dimensional System Studied in a Single Barrier Heterostructure by Time Resolved Kerr Rotation
By biasing a single barrier heterostructure with a 500nm-thick GaAs layer as
the absorption layer, the spin dynamics for both of the first and second
subband near the AlAs barrier are examined. We find that when simultaneously
scanning the photon energy of both the probe and pump beams, a sign reversal of
the Kerr rotation (KR) takes place as long as the probe photons break away the
first subband and probe the second subband. This novel feature, while stemming
from the exchange interaction, has been used to unambiguously distinguish the
different spin dynamics ( and ) for the first and second
subbands under the different conditions by their KR signs (negative for
and positive for ). In the zero magnetic field, by scanning
the wavelength towards the short wavelength, decreases in accordance
with the D'yakonov-Perel' (DP) spin decoherence mechanism. At 803nm,
(450ps) becomes ten times longer than (50ps). However, the
value of at 803nm is roughly the same as the value of at
815nm. A new feature has been disclosed at the wavelength of 811nm under the
bias of -0.3V (807nm under the bias of -0.6V) that the spin coherence times
( and ) and the effective factors ( and
) all display a sudden change, due to the "resonant" spin exchange
coupling between two spin opposite bands.Comment: 9pages, 3 figure
Consensus of multi-agent systems with faults and mismatches under switched topologies using a delta operator method
© 2018 Elsevier B.V. This paper studies the consensus of multi-agent systems with faults and mismatches under switched topologies using a delta operator method. Since faults and mismatches can result in failure of the consensus even for a fixed topology with a spanning tree, how to reach a consensus is a complicated and challenging problem under such circumstances especially when part topologies have no spanning tree. Although some works studied the influence of faults and mismatches on the consensus, there is little work on reaching a consensus for the multi-agent systems with faults and mismatches. In this paper, we introduce the delta operator to unify the consensus analysis for continuous, discrete, or sampled systems under one framework. We develop the theories on the delta operator systems first and then apply theories of the delta operator systems to the consensus problems. By converting the consensus problems into stability problems, we investigate and prove consensus and the associated conditions for systems 1) without any fault, 2) with a known fault, and 3) with unknown faults, under switching topologies with matching or mismatching coefficients. Numerical examples are provided and validate the effectiveness of the theoretical results
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