4,873 research outputs found
Casimir effect of electromagnetic field in Randall-Sundrum spacetime
We study the finite temperature Casimir effect on a pair of parallel
perfectly conducting plates in Randall-Sundrum model without using scalar field
analogy. Two different ways of interpreting perfectly conducting conditions are
discussed. The conventional way that uses perfectly conducting condition
induced from 5D leads to three discrete mode corrections. This is very
different from the result obtained from imposing 4D perfectly conducting
conditions on the 4D massless and massive vector fields obtained by decomposing
the 5D electromagnetic field. The latter only contains two discrete mode
corrections, but it has a continuum mode correction that depends on the
thicknesses of the plates. It is shown that under both boundary conditions, the
corrections to the Casimir force make the Casimir force more attractive. The
correction under 4D perfectly conducting condition is always smaller than the
correction under the 5D induced perfectly conducting condition. These
statements are true at any temperature.Comment: 20 pages, 4 figure
Origins of ferromagnetism in transition-metal doped Si
We present results of the magnetic, structural and chemical characterizations of Mn<sup>+</sup>-implanted Si displaying <i>n</i>-type semiconducting behavior and ferromagnetic ordering with Curie temperature,T<sub>C</sub> well above room temperature. The temperature-dependent magnetization measured by superconducting quantum device interference (SQUID) from 5 K to 800 K was characterized by three different critical temperatures (T*<sub>C</sub>~45 K, T<sub>C1</sub>~630-650 K and T<sub>C2</sub>~805-825 K). Their origins were investigated using dynamic secondary mass ion spectroscopy (SIMS) and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy (EELS), Z-contrast STEM (scanning TEM) imaging and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn<sup>+</sup> ions, as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed T*<sub>C</sub> is attributed to the Mn<sub>4</sub>Si<sub>7</sub> precipitates identified by electron diffraction. Possible origins of and are also discussed. Our findings raise questions regarding the origin of the high ferromagnetism reported in many material systems without a careful chemical analysis
Structure of Extremely Nanosized and Confined In-O Species in Ordered Porous Materials
Perturbed-angular correlation, x-ray absorption, and small-angle x-ray
scattering spectroscopies were suitably combined to elucidate the local
structure of highly diluted and dispersed InOx species confined in porous of
ZSM5 zeolite. These novel approach allow us to determined the structure of
extremely nanosized In-O species exchanged inside the 10-atom-ring channel of
the zeolite, and to quantify the amount of In2O3 crystallites deposited onto
the external zeolite surface.Comment: 4 pages, 5 postscript figures, REVTEX4, published in Physical Review
Letter
Propagation of Bose-Einstein condensates in a magnetic waveguide
Gaseous Bose-Einstein condensates of 2-3 million atoms were loaded into a
microfabricated magnetic trap using optical tweezers. Subsequently, the
condensates were released into a magnetic waveguide and propagated 12 mm.
Single-mode propagation was observed along homogeneous segments of the
waveguide. Inhomogeneities in the guiding potential arose from geometric
deformations of the microfabricated wires and caused strong transverse
excitations. Such deformations may restrict the waveguide physics that can be
explored with propagating condensates.Comment: 5 pages, 4 figure
Electronic structure studies of Fe- ZnO nanorods by x-ray absorption fine structure
We report the electronic structure studies of well characterized
polycrystalline Zn_{1-x}Fe_xO (x = 0.0, 0.01, 0.03, and 0.05) nanorods
synthesized by a co-precipitation method through x-ray absorption fine
structure (XAFS). X-ray diffraction (XRD) reveals that Fe doped ZnO
crystallizes in a single phase wurtzite structure without any secondary phase.
From the XRD pattern, it is observed that peak positions shift towards lower
2\theta value with Fe doping. The change in the peak positions with increase in
Fe contents clearly indicates that Fe ions are replacing Zn ions in the ZnO
matrix. Linear combination fittings (LCF) at Fe K-edge demonstrate that Fe is
in mixed valent state (Fe3+/Fe2+) with a ratio of ~ 7:3 (Fe3+:Fe2+). XAFS data
is successfully fitted to wurtzite structure using IFEFFIT and Artemis. The
results indicate that Fe substitutes Zn site in the ZnO matrix in tetrahedral
symmetry.Comment: 7 pages, 5 figures, 2 tables, regular articl
Finite temperature Casimir effect for massive scalar field in spacetime with extra dimensions
We compute the finite temperature Casimir energy for massive scalar field
with general curvature coupling subject to Dirichlet or Neumann boundary
conditions on the walls of a closed cylinder with arbitrary cross section,
located in a background spacetime of the form ,
where is the -dimensional Minkowski spacetime and
is an -dimensional internal manifold. The Casimir energy is
regularized using the criteria that it should vanish in the infinite mass
limit. The Casimir force acting on a piston moving freely inside the closed
cylinder is derived and it is shown that it is independent of the
regularization procedure. By letting one of the chambers of the cylinder
divided by the piston to be infinitely long, we obtain the Casimir force acting
on two parallel plates embedded in the cylinder. It is shown that if both the
plates assume Dirichlet or Neumann boundary conditions, the strength of the
Casimir force is reduced by the increase in mass. Under certain conditions, the
passage from massless to massive will change the nature of the force from long
range to short range. Other properties of the Casimir force such as its sign,
its behavior at low and high temperature, and its behavior at small and large
plate separations, are found to be similar to the massless case. Explicit exact
formulas and asymptotic behaviors of the Casimir force at different limits are
derived. The Casimir force when one plate assumes Dirichlet boundary condition
and one plate assumes Neumann boundary condition is also derived and shown to
be repulsive.Comment: 28 pages, 4 figure
A new form of the rotating C-metric
In a previous paper, we showed that the traditional form of the charged
C-metric can be transformed, by a change of coordinates, into one with an
explicitly factorizable structure function. This new form of the C-metric has
the advantage that its properties become much simpler to analyze. In this
paper, we propose an analogous new form for the rotating charged C-metric, with
structure function G(\xi)=(1-\xi^2)(1+r_{+}A\xi)(1+r_{-}A\xi), where r_\pm are
the usual locations of the horizons in the Kerr-Newman black hole. Unlike the
non-rotating case, this new form is not related to the traditional one by a
coordinate transformation. We show that the physical distinction between these
two forms of the rotating C-metric lies in the nature of the conical
singularities causing the black holes to accelerate apart: the new form is free
of torsion singularities and therefore does not contain any closed timelike
curves. We claim that this new form should be considered the natural
generalization of the C-metric with rotation.Comment: 13 pages, LaTe
Local Structure of La1-xSrxCoO3 determined from EXAFS and neutron PDF studies
The combined local structure techniques, extended x-ray absorption fine
structure (EXAFS) and neutron pair distribution function analysis, have been
used for temperatures 4 <= T <= 330 K to rule out a large Jahn-Teller (JT)
distortion of the Co-O bond in La1-xSrxCoO3 for a significant fraction of Co
sites (x <= 0.35), indicating few, if any, JT-active, singly occupied e_g Co
sites exist.Comment: 5 page
Kaluza-Klein Pistons with non-Commutative Extra Dimensions
We calculate the scalar Casimir energy and Casimir force for a
Kaluza-Klein piston setup in which the extra dimensional space contains a
non-commutative 2-sphere, . The cases to be studied are and respectively as extra dimensional spaces, with the
dimensional commutative torus. The validity of the results and the
regularization that the piston setup offers are examined in both cases. Finally
we examine the 1-loop corrected Casimir energy for one piston chamber, due to
the self interacting scalar field in the non-commutative geometry. The
computation is done within some approximations. We compare this case for the
same calculation done in Minkowski spacetime . A discussion on the
stabilization of the extra dimensional space within the piston setup follows at
the end of the article.Comment: 22 page
The control parameterization method for nonlinear optimal control: A survey
The control parameterization method is a popular numerical technique for solving optimal control problems. The main idea of control parameterization is to discretize the control space by approximating the control function by a linear combination of basis functions. Under this approximation scheme, the optimal control problem is reduced to an approximate nonlinear optimization problem with a finite number of decision variables. This approximate problem can then be solved using nonlinear programming techniques. The aim of this paper is to introduce the fundamentals of the control parameterization method and survey its various applications to non-standard optimal control problems. Topics discussed include gradient computation, numerical convergence, variable switching times, and methods for handling state constraints. We conclude the paper with some suggestions for future research
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