90,391 research outputs found
Pattern Synthesis of Dual-band Shared Aperture Interleaved Linear Antenna Arrays
This paper presents an approach to improve the efficiency of an array aperture by interleaving two different arrays in the same aperture area. Two sub-arrays working at different frequencies are interleaved in the same linear aperture area. The available aperture area is efficiently used. The element positions of antenna array are optimized by using Invasive Weed Optimization (IWO) to reduce the peak side lobe level (PSLL) of the radiation pattern. To overcome the shortness of traditional methods which can only fulfill the design of shared aperture antenna array working at the same frequency, this method can achieve the design of dual-band antenna array with wide working frequency range. Simulation results show that the proposed method is feasible and efficient in the synthesis of dual-band shared aperture antenna array
An {\it ab initio} study of the magnetic and electronic properties of Fe, Co, and Ni nanowires on Cu(001) surface
Magnetism at the nanoscale has been a very active research area in the past
decades, because of its novel fundamental physics and exciting potential
applications. We have recently performed an {\it ab intio} study of the
structural, electronic and magnetic properties of all 3 transition metal
(TM) freestanding atomic chains and found that Fe and Ni nanowires have a giant
magnetic anisotropy energy (MAE), indicating that these nanowires would have
applications in high density magnetic data storages. In this paper, we perform
density functional calculations for the Fe, Co and Ni linear atomic chains on
Cu(001) surface within the generalized gradient approximation, in order to
investigate how the substrates would affect the magnetic properties of the
nanowires. We find that Fe, Co and Ni linear chains on Cu(001) surface still
have a stable or metastable ferromagnetic state. When spin-orbit coupling (SOC)
is included, the spin magnetic moments remain almost unchanged, due to the
weakness of SOC in 3 TM chains, whilst significant orbital magnetic moments
appear and also are direction-dependent. Finally, we find that the MAE for Fe,
and Co remains large, i.e., being not much affected by the presence of Cu
substrate.Comment: 4 pages, 2 figure
Magnetic moment and magnetic anisotropy of linear and zigzag 4{\it d} and 5{\it d} transition metal nanowires: First-principles calculations
An extensive {\it ab initio} study of the physical properties of both linear
and zigzag atomic chains of all 4 and 5 transition metals (TM) within the
GGA by using the accurate PAW method, has been carried out. All the TM linear
chains are found to be unstable against the corresponding zigzag structures.
All the TM chains, except Nb, Ag and La, have a stable (or metastable) magnetic
state in either the linear or zigzag or both structures. Magnetic states appear
also in the sufficiently stretched Nb and La linear chains and in the largely
compressed Y and La chains. The spin magnetic moments in the Mo, Tc, Ru, Rh, W,
Re chains could be large (1.0 /atom). Structural transformation
from the linear to zigzag chains could suppress the magnetism already in the
linear chain, induce the magnetism in the zigzag structure, and also cause a
change of the magnetic state (ferromagnetic to antiferroamgetic or vice verse).
The calculations including the spin-orbit coupling reveal that the orbital
moments in the Zr, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir and Pt chains could be
rather large (0.1 /atom). Importantly, large magnetic anisotropy
energy (1.0 meV/atom) is found in most of the magnetic TM chains,
suggesting that these nanowires could have fascinating applications in
ultrahigh density magnetic memories and hard disks. In particular, giant
magnetic anisotropy energy (10.0 meV/atom) could appear in the Ru, Re,
Rh, and Ir chains. Furthermore, the magnetic anisotropy energy in several
elongated linear chains could be as large as 40.0 meV/atom. A
spin-reorientation transition occurs in the Ru, Ir, Ta, Zr, La and Zr, Ru, La,
Ta and Ir linear chains when they are elongated. Remarkably, all the 5 as
well as Tc and Pd chains show the colossal magnetic anisotropy (i.e., it is
impossible to rotate magnetization into certain directions). Finally, the
electronic band structure and density of states of the nanowires have also been
calculated in order to understand the electronic origin of the large magnetic
anisotropy and orbital magnetic moment as well as to estimate the conduction
electron spin polarization.Comment: To appear in Phys. Rev.
On the Cauchy problem for the magnetic Zakharov system
In this paper, we study the Cauchy problem of the magnetic type Zakharov
system which describes the pondermotive force and magnetic field generation
effects resulting from the non-linear interaction between plasma-wave and
particles. By using the energy method to derive a priori bounds and an
approximation argument for the construction of solutions, we obtain local
existence and uniqueness results for the magnetic Zakharov system in the case
of
Observation of an in-plane magnetic-field-driven phase transition in a quantum Hall system with SU(4) symmetry
In condensed matter physics, the study of electronic states with SU(N)
symmetry has attracted considerable and growing attention in recent years, as
systems with such a symmetry can often have a spontaneous symmetry-breaking
effect giving rise to a novel ground state. For example, pseudospin quantum
Hall ferromagnet of broken SU(2) symmetry has been realized by bringing two
Landau levels close to degeneracy in a bilayer quantum Hall system. In the past
several years, the exploration of collective states in other multi-component
quantum Hall systems has emerged. Here we show the conventional pseudospin
quantum Hall ferromagnetic states with broken SU(2) symmetry collapsed rapidly
into an unexpected state with broken SU(4) symmetry, by in-plane magnetic field
in a two-subband GaAs/AlGaAs two-dimensional electron system at filling factor
around . Within a narrow tilting range angle of 0.5 degrees, the
activation energy increases as much as 12 K. While the origin of this puzzling
observation remains to be exploited, we discuss the possibility of a
long-sought pairing state of electrons with a four-fold degeneracy.Comment: 13 pages, 4 figure
Cutout reinforcements for shear loaded laminate and sandwich composite panels
This paper presents the numerical and experimental studies of shear loaded
laminated and sandwich carbon/epoxy composite panels with cutouts and
reinforcements aiming at reducing the cutout stress concentration and increasing
the buckling stability of the panels. The effect of different cutout sizes and
the design and materials of cutout reinforcements on the stress and buckling
behaviour of the panels are evaluated. For the sandwich panels with a range of
cutout size and a constant weight, an optimal ratio of the core to the face
thickness has been studied for the maximum buckling stability. The finite
element method and an analytical method are employed to perform parametric
studies. In both constant stress and constant displacement shear loading
conditions, the results are in very good agreement with those obtained from
experiment for selected cutout reinforcement cases. Conclusions are drawn on the
cutout reinforcement design and improvement of stress concentration and buckling
behaviour of shear loaded laminated and sandwich composite panels with cutouts
Bijections behind the Ramanujan Polynomials
The Ramanujan polynomials were introduced by Ramanujan in his study of power
series inversions. In an approach to the Cayley formula on the number of trees,
Shor discovers a refined recurrence relation in terms of the number of improper
edges, without realizing the connection to the Ramanujan polynomials. On the
other hand, Dumont and Ramamonjisoa independently take the grammatical approach
to a sequence associated with the Ramanujan polynomials and have reached the
same conclusion as Shor's. It was a coincidence for Zeng to realize that the
Shor polynomials turn out to be the Ramanujan polynomials through an explicit
substitution of parameters. Shor also discovers a recursion of Ramanujan
polynomials which is equivalent to the Berndt-Evans-Wilson recursion under the
substitution of Zeng, and asks for a combinatorial interpretation. The
objective of this paper is to present a bijection for the Shor recursion, or
and Berndt-Evans-Wilson recursion, answering the question of Shor. Such a
bijection also leads to a combinatorial interpretation of the recurrence
relation originally given by Ramanujan.Comment: 18 pages, 7 figure
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