11,165 research outputs found
Development and performance of IR detectors in the 1.5 to 2.4 micrometer region that operate at 240 K
High performance 1.5 to 2.4 micrometers (Hg,Cd)Te photodetectors for operating at 240 K or above are discussed. The detailed characterization of the detector with respect to detector temperature and background flux led to a development of an empirical model for minority carrier trapping. The concept of detective time constant is presented and successfully demonstrated by the four detectors delivered on this contract. An alternative approach is presented with the use of photovoltaic (Hg,Cd)Te detectors
Phase-sensitive quantum effects in Andreev conductance of the SNS system of metals with macroscopic phase breaking length
The dissipative component of electron transport through the doubly connected
SNS Andreev interferometer indium (S)-aluminium (N)-indium (S) has been
studied. Within helium temperature range, the conductance of the individual
sections of the interferometer exhibits phase-sensitive oscillations of
quantum-interference nature. In the non-domain (normal) state of indium
narrowing adjacent to NS interface, the nonresonance oscillations have been
observed, with the period inversely proportional to the area of the
interferometer orifice. In the domain intermediate state of the narrowing, the
magneto-temperature resistive oscillations appeared, with the period determined
by the coherence length in the magnetic field equal to the critical one. The
oscillating component of resonance form has been observed in the conductance of
the macroscopic N-aluminium part of the system. The phase of the oscillations
appears to be shifted by compared to that of nonresonance oscillations.
We offer an explanation in terms of the contribution into Josephson current
from the coherent quasiparticles with energies of order of the Thouless energy.
The behavior of dissipative transport with temperature has been studied in a
clean normal metal in the vicinity of a single point NS contact.Comment: 9 pages, 7 figures, to be published in Low Temp. Phys., v. 29, No.
12, 200
An ultrafast 1 x M all-optical WDM packet-switched router based on the PPM header address
This paper presents an all-optical 1 x M WDM router architecture for packet routing at multiple wavelengths simultaneously, with no wavelength conversion modules. The packet header address adopted is based on the pulse position modulation (PPM) format, thus enabling the use of only a singlebitwise optical AND gate for fast header address correlation. It offers multicast as well as broadcast capabilities. It is shown that a high speed packet routing at 160 Gb/s can be achieved with a low channel crosstalk (CXT) of ~ -27 dB at a channel spacing of greater than 0.4 THz and a demultiplexer bandwidth of 500 GHz
ISO LWS Spectra of T Tauri and Herbig AeBe stars
We present an analysis of ISO-LWS spectra of eight T Tauri and Herbig AeBe young stellar objects.
Some of the objects are in the embedded phase of star-formation, whereas others have cleared their environs
but are still surrounded by a circumstellar disk. Fine-structure lines of [OI] and [CII] are most likely excited by
far-ultraviolet photons in the circumstellar environment rather than high-velocity outflows, based on comparisons
of observed line strengths with predictions of photon-dominated and shock chemistry models. A subset of our
stars and their ISO spectra are adequately explained by models constructed by Chiang & Goldreich (1997) and
Chiang et al. (2001) of isolated, passively heated, flared circumstellar disks. For these sources, the bulk of the
LWS flux at wavelengths longward of 55 µm arises from the disk interior which is heated diffusively by reprocessed
radiation from the disk surface. At 45 µm, water ice emission bands appear in spectra of two of the coolest stars,
and are thought to arise from icy grains irradiated by central starlight in optically thin disk surface layers
Spectral Energy Distributions of Gamma Ray Bursts Energized by External Shocks
Sari, Piran, and Narayan have derived analytic formulas to model the spectra
from gamma-ray burst blast waves that are energized by sweeping up material
from the surrounding medium. We extend these expressions to apply to general
radiative regimes and to include the effects of synchrotron self-absorption.
Electron energy losses due to the synchrotron self-Compton process are also
treated in a very approximate way. The calculated spectra are compared with
detailed numerical simulation results. We find that the spectral and temporal
breaks from the detailed numerical simulation are much smoother than the
analytic formulas imply, and that the discrepancies between the analytic and
numerical results are greatest near the breaks and endpoints of the synchrotron
spectra. The expressions are most accurate (within a factor of ~ 3) in the
optical/X-ray regime during the afterglow phase, and are more accurate when
epsilon_e, the fraction of swept-up particle energy that is transferred to the
electrons, is <~ 0.1. The analytic results provide at best order-of-magnitude
accuracy in the self-absorbed radio/infrared regime, and give poor fits to the
self-Compton spectra due to complications from Klein-Nishina effects and
photon-photon opacity.Comment: 16 pages, 7 figures, ApJ, in press, 537, July 1, 2000. Minor changes
in response to referee report, corrected figure
Enhanced Trabeculectomy: The Moorfields Safer Surgery System
Trabeculectomy with antifibrotic treatment is still the most popular incisional procedure for glaucoma filtration surgery (GFS) worldwide. The advent of antifibrotic agents reduced failure due to scarring but resulted in increased complications. Advances in trabeculectomy surgery have been driven by the need to minimise the risk of: (1) complications and (2) surgical failure. This chapter covers preoperative, intraoperative, and postoperative strategies, which improve the outcome of GFS. Strategies to reduce the risk of complications centre on the prevention of postoperative hypotony by minimising the risk of overdrainage, postoperative wound leaks, and poor bleb morphology. Surgical techniques to reduce the risk of postoperative fibrosis by the use of antifibrotic agents (including mitomycin C) are discussed in detail. These techniques are based on a combination of considerable clinical experience, observation, and laboratory research. The need to address pre-, intra-, and postoperative issues in each individual patient is emphasised. These changes are embodied in the system we call the "Moorfields Safer Surgery System." The use of these strategies has considerably reduced the incidence of major complications, including hypotony, cystic blebs, and endophthalmitis in practices around the world. Most of these techniques are simple, require minimal equipment, and can be easily mastered. They are associated with an improvement in overall outcome and it is hoped that this chapter will help the reader benefit from these advances
Classical double-layer atoms: artificial molecules
The groundstate configuration and the eigenmodes of two parallel
two-dimensional classical atoms are obtained as function of the inter-atomic
distance (d). The classical particles are confined by identical harmonic wells
and repel each other through a Coulomb potential. As function of d we find
several structural transitions which are of first or second order. For first
(second) order transitions the first (second) derivative of the energy with
respect to d is discontinuous, the radial position of the particles changes
discontinuously (continuously) and the frequency of the eigenmodes exhibit a
jump (one mode becomes soft, i.e. its frequency becomes zero).Comment: 4 pages, RevTex, 5 ps figures, to appear in Phys.Rev.Let
On the Antenna Beam Shape Reconstruction Using Planet Transit
The calibration of the in-flight antenna beam shape and possible
beamdegradation is one of the most crucial tasks for the upcoming Planck
mission. We examine several effects which could significantly influence the
in-flight main beam calibration using planet transit: the problems of the
variability of the Jupiter's flux, the antenna temperature and passing of the
planets through the main beam. We estimate these effects on the antenna beam
shape calibration and calculate the limits on the main beam and far sidelobe
measurements, using observations of Jupiter and Saturn. We also discuss
possible effects of degradation of the mirror surfaces and specify
corresponding parameters which can help us to determine these effects.Comment: 10 pages, 8 figure
Hitting Time of Quantum Walks with Perturbation
The hitting time is the required minimum time for a Markov chain-based walk
(classical or quantum) to reach a target state in the state space. We
investigate the effect of the perturbation on the hitting time of a quantum
walk. We obtain an upper bound for the perturbed quantum walk hitting time by
applying Szegedy's work and the perturbation bounds with Weyl's perturbation
theorem on classical matrix. Based on the definition of quantum hitting time
given in MNRS algorithm, we further compute the delayed perturbed hitting time
(DPHT) and delayed perturbed quantum hitting time (DPQHT). We show that the
upper bound for DPQHT is actually greater than the difference between the
square root of the upper bound for a perturbed random walk and the square root
of the lower bound for a random walk.Comment: 9 page
PAN AIR: A computer program for predicting subsonic or supersonic linear potential flows about arbitrary configurations using a higher order panel method. Volume 4: Maintenance document (version 1.1)
The Maintenance Document is a guide to the PAN AIR software system, a system which computes the subsonic or supersonic linear potential flow about a body of nearly arbitrary shape, using a higher order panel method. The document describes the over-all system and each program module of the system. Sufficient detail is given for program maintenance, updating and modification. It is assumed that the reader is familiar with programming and CDC (Control Data Corporation) computer systems. The PAN AIR system was written in FORTRAN 4 language except for a few COMPASS language subroutines which exist in the PAN AIR library. Structured programming techniques were used to provide code documentation and maintainability. The operating systems accommodated are NOS 1.2, NOS/BE and SCOPE 2.1.3 on the CDC 6600, 7600 and Cyber 175 computing systems. The system is comprised of a data management system, a program library, an execution control module and nine separate FORTRAN technical modules. Each module calculates part of the posed PAN AIR problem. The data base manager is used to communicate between modules and within modules. The technical modules must be run in a prescribed fashion for each PAN AIR problem. In order to ease the problem of supplying the many JCL cards required to execute the modules, a separate module called MEC (Module Execution Control) was created to automatically supply most of the JCL cards. In addition to the MEC generated JCL, there is an additional set of user supplied JCL cards to initiate the JCL sequence stored on the system
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