16,792 research outputs found
Reduction of the radar cross section of arbitrarily shaped cavity structures
The problem of the reduction of the radar cross section (RCS) of open-ended cavities was studied. The issues investigated were reduction through lossy coating materials on the inner cavity wall and reduction through shaping of the cavity. A method was presented to calculate the RCS of any arbitrarily shaped structure in order to study the shaping problem. The limitations of this method were also addressed. The modal attenuation was studied in a multilayered coated waveguide. It was shown that by employing two layers of coating, it was possible to achieve an increase in both the magnitude of attenuation and the frequency band of effectiveness. The numerical method used in finding the roots of the characteristic equation breaks down when the coating thickness is very lossy and large in terms of wavelength. A new method of computing the RCS of an arbitrary cavity was applied to study the effects of longitudinal bending on RCS reduction. The ray and modal descriptions for the fields in a parallel plate waveguide were compared. To extend the range of validity of the Shooting and Bouncing Ray (SBR) method, the simple ray picture must be modified to account for the beam blurring
Effects of edge disorder in nano-scale antiferromagnetic clusters
We study the distribution of local magnetic susceptibilities in the
two-dimensional antiferromagnetic S=1/2 Heisenberg model on various random
clusters, in order to determine whether effects of edge disorder could be
detected in NMR experiments (through the line shape, as given by the
distribution of local Knight shifts). Although the effects depend strongly on
the nature of the edge and the cluster size, our results indicate that line
widths broader than the average shift should be expected even in clusters as
large as lattice spacing in diameter. Experimental
investigations of the NMR line width should give insights into the magnetic
structure of the edges.Comment: 4 pages, 4 figure
Analysis and control of bifurcation and chaos in averaged queue length in TCP/RED model
This paper studies the bifurcation and chaos phenomena in averaged queue length in a
developed Transmission Control Protocol (TCP) model with Random Early Detection
(RED) mechanism. Bifurcation and chaos phenomena are nonlinear behaviour in network
systems that lead to degradation of the network performance. The TCP/RED model used
is a model validated previously. In our study, only the average queue size k q
−
is
considered, and the results are based on analytical model rather than actual measurements.
The instabilities in the model are studied numerically using the conventional nonlinear
bifurcation analysis. Extending from this bifurcation analysis, a modified RED algorithm
is derived to prevent the observed bifurcation and chaos regardless of the selected
parameters. Our modification is for the simple scenario of a single RED router carrying
only TCP traffic. The algorithm neither compromises the throughput nor the average
queuing delay of the system
Measurement back-action on the quantum spin-mixing dynamics of a spin-1 Bose-Einstein condensate
We consider a small F=1 spinor condensate inside an optical cavity driven by
an optical probe field, and subject the output of the probe to a homodyne
detection, with the goal of investigating the effect of measurement back-action
on the spin dynamics of the condensate. Using the stochastic master equation
approach, we show that the effect of back-action is sensitive to not only the
measurement strength but also the quantum fluctuation of the spinor condensate.
The same method is also used to estimate the atom numbers below which the
effect of back-action becomes so prominent that extracting spin dynamics from
this cavity-based detection scheme is no longer practical
Salient Object Detection via Structured Matrix Decomposition
Low-rank recovery models have shown potential for salient object detection, where a matrix is decomposed into a low-rank
matrix representing image background and a sparse matrix identifying salient objects. Two deficiencies, however, still exist. First,
previous work typically assumes the elements in the sparse matrix are mutually independent, ignoring the spatial and pattern relations
of image regions. Second, when the low-rank and sparse matrices are relatively coherent, e.g., when there are similarities between the
salient objects and background or when the background is complicated, it is difficult for previous models to disentangle them. To
address these problems, we propose a novel structured matrix decomposition model with two structural regularizations: (1) a
tree-structured sparsity-inducing regularization that captures the image structure and enforces patches from the same object to have
similar saliency values, and (2) a Laplacian regularization that enlarges the gaps between salient objects and the background in feature
space. Furthermore, high-level priors are integrated to guide the matrix decomposition and boost the detection. We evaluate our model
for salient object detection on five challenging datasets including single object, multiple objects and complex scene images, and show
competitive results as compared with 24 state-of-the-art methods in terms of seven performance metrics
Orbital symmetry fingerprints for magnetic adatoms in graphene
In this paper, we describe the formation of local resonances in graphene in
the presence of magnetic adatoms containing localized orbitals of arbitrary
symmetry, corresponding to any given angular momentum state. We show that
quantum interference effects which are naturally inbuilt in the honeycomb
lattice in combination with the specific orbital symmetry of the localized
state lead to the formation of fingerprints in differential conductance curves.
In the presence of Jahn-Teller distortion effects, which lift the orbital
degeneracy of the adatoms, the orbital symmetries can lead to distinctive
signatures in the local density of states. We show that those effects allow
scanning tunneling probes to characterize adatoms and defects in graphene.Comment: 15 pages, 11 figures. Added discussion about the multi-orbital case
and the validity of the single orbital picture. Published versio
Robust pinning of magnetic moments in pyrochlore iridates
Pyrochlore iridates A2Ir2O7 (A = rare earth elements, Y or Bi) hold great
promise for realizing novel electronic and magnetic states owing to the
interplay of spin-orbit coupling, electron correlation and geometrical
frustration. A prominent example is the formation of all-in/all-out
(AIAO)antiferromagnetic order in the Ir4+ sublattice that comprises of
corner-sharing tetrahedra. Here we report on an unusual magnetic phenomenon,
namely a cooling-field induced shift of magnetic hysteresis loop along
magnetization axis, and its possible origin in pyrochlore iridates with
non-magnetic Ir defects (e.g. Ir3+). In a simple model, we attribute the
magnetic hysteresis loop to the formation of ferromagnetic droplets in the AIAO
antiferromagnetic background. The weak ferromagnetism originates from canted
antiferromagnetic order of the Ir4+ moments surrounding each non-magnetic Ir
defect. The shift of hysteresis loop can be understood quantitatively based on
an exchange-bias like effect in which the moments at the shell of the FM
droplets are pinned by the AIAO AFM background via mainly the Heisenberg (J)
and Dzyaloshinsky-Moriya (D) interactions. The magnetic pinning is stable and
robust against the sweeping cycle and sweeping field up to 35 T, which is
possibly related to the magnetic octupolar nature of the AIAO order.Comment: 16 pages, 4 figure
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