154,337 research outputs found
Adaptive and Supertwisting Adaptive Spacecraft Orbit Control Around Asteroids
This paper addresses the development of control systems for the orbit control of spacecraft around irregularly shaped rotating asteroids with uncertain parameters. The objective is to steer the spacecraft along prescribed orbits. First, a nonlinear adaptive law for orbit control was designed. This was followed by the design of a supertwisting adaptive (STWA) control system. In the closed-loop system, which includes the adaptive law or the STWA law, all the signals remain bounded, and the trajectory tracking error asymptotically converges to zero for any initial condition. Finally, under the assumption of boundedness of the derivative of the uncertain functions of the model in a region of the state space, a supertwisting control (STW) law for finite-time convergence of the trajectory was obtained. Based on the Lyapunov theory, stability properties of the closed-loop systems were analyzed. Simulation results for 433 Eros and Ida asteroids were presented for illustration. The results showed that control of spacecraft along closed orbits or to a fixed point is accomplished using each of these controllers, despite uncertainties in the parameters of the asteroid models
Polarization Structures in the Thomson-Scattered Emission Lines in Active Galactic Nuclei
A line photon incident in an electron-scattering medium is transferred in a
diffusive way both in real space and in frequency space, and the mean number of
scatterings changes as the wavelength shifts from the line center. This leads
to the profile broadening and polarization dependence on the wavelength shift
as a function of the Thomson optical depth . We find that the
polarization of the Thomson-scattered emission lines has a dip around the line
center when does not exceed a few. Various structures such as the
polarization flip are also seen. An application to an ionized halo component
surrounding the broad emission line region in active galactic nuclei is
considered and it is found that the polarization structures may still persist.
Brief discussions on observational implications are given.Comment: 14 pages, 3 figures, accepted for publication in ApJ Letter
Computer programs for prediction of structural vibrations due to fluctuating pressure environments. Volume 1 - Theoretical analyses Final report
Theoretical analyses for computer program to calculate random vibrations of reinforced rectangular cylindrical panels in fluctuating pressure environmen
Conquest of the ghost pyramid of the superstring
We give a new Becchi-Rouet-Stora-Tyutin operator for the superstring. It
implies a quadratic gauge-fixed action, and a new gauge-invariant action with
first-class constraints. The infinite pyramid of spinor ghosts appears in a
simple way through ghost gamma matrices.Comment: 30 pages, 1 figure, Late
State space collapse and diffusion approximation for a network operating under a fair bandwidth sharing policy
We consider a connection-level model of Internet congestion control,
introduced by Massouli\'{e} and Roberts [Telecommunication Systems 15 (2000)
185--201], that represents the randomly varying number of flows present in a
network. Here, bandwidth is shared fairly among elastic document transfers
according to a weighted -fair bandwidth sharing policy introduced by Mo
and Walrand [IEEE/ACM Transactions on Networking 8 (2000) 556--567] []. Assuming Poisson arrivals and exponentially distributed document
sizes, we focus on the heavy traffic regime in which the average load placed on
each resource is approximately equal to its capacity. A fluid model (or
functional law of large numbers approximation) for this stochastic model was
derived and analyzed in a prior work [Ann. Appl. Probab. 14 (2004) 1055--1083]
by two of the authors. Here, we use the long-time behavior of the solutions of
the fluid model established in that paper to derive a property called
multiplicative state space collapse, which, loosely speaking, shows that in
diffusion scale, the flow count process for the stochastic model can be
approximately recovered as a continuous lifting of the workload process.Comment: Published in at http://dx.doi.org/10.1214/08-AAP591 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Ferromagnetism below 10 K in Mn doped BiTe
Ferromagnetism is observed below 10 K in [Bi0.75Te0.125Mn0.125]Te. This
material has the BiTe structure, which is made from the stacking of two
Te-Bi-Te-Bi-Te blocks and one Bi-Bi block per unit cell. Crystal structure
analysis shows that Mn is localized in the Bi2 blocks, and is accompanied by an
equal amount of TeBi anti-site occupancy in the Bi2Te3 blocks. These TeBi
anti-site defects greatly enhance the Mn solubility. This is demonstrated by
comparison of the [Bi1-xMnx]Te and [Bi1-2xTexMnx]Te series; in the former, the
solubility is limited to x = 0.067, while the latter has xmax = 0.125. The
magnetism in [Bi1-xMnx]Te changes little with x, while that for
[Bi1-2xTexMnx]Te shows a clear variation, leading to ferromagnetism for x >
0.067. Magnetic hysteresis and the anomalous Hall Effect are observed for the
ferromagnetic samples.Comment: Accepted for publication in Phys. Rev.
Modulation of the Curie Temperature in Ferromagnetic/Ferroelectric Hybrid Double Quantum Wells
We propose a ferromagnetic/ferroelectric hybrid double quantum well
structure, and present an investigation of the Curie temperature (Tc)
modulation in this quantum structure. The combined effects of applied electric
fields and spontaneous electric polarization are considered for a system that
consists of a Mn \delta-doped well, a barrier, and a p-type ferroelectric well.
We calculate the change in the envelope functions of carriers at the lowest
energy sub-band, resulting from applied electric fields and switching the
dipole polarization. By reversing the depolarizing field, we can achieve two
different ferromagnetic transition temperatures of the ferromagnetic quantum
well in a fixed applied electric field. The Curie temperature strongly depends
on the position of the Mn \delta-doped layer and the polarization strength of
the ferroelectric well.Comment: 9 pages, 5 figures, to be published in Phys. Rev. B (2006) minor
revision: One of the line types is changed in Fig.
The 1D interacting Bose gas in a hard wall box
We consider the integrable one-dimensional delta-function interacting Bose
gas in a hard wall box which is exactly solved via the coordinate Bethe Ansatz.
The ground state energy, including the surface energy, is derived from the
Lieb-Liniger type integral equations. The leading and correction terms are
obtained in the weak coupling and strong coupling regimes from both the
discrete Bethe equations and the integral equations. This allows the
investigation of both finite-size and boundary effects in the integrable model.
We also study the Luttinger liquid behaviour by calculating Luttinger
parameters and correlations. The hard wall boundary conditions are seen to have
a strong effect on the ground state energy and phase correlations in the weak
coupling regime. Enhancement of the local two-body correlations is shown by
application of the Hellmann-Feynman theorem.Comment: 23 pages, 7 figures. Improved version. Extra figure added for the
weak coupling regime. New expression for the interaction-dependent cloud size
and additional reference
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