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981.
F. M. Steranka D. C. Defevere Camras M.D. S. L. Rudaz D. K. Mc Elfresh L. W. Cook W. L. Snyder M. G. Craford 《Journal of Electronic Materials》1995,24(10):1407-1412
Electroluminescent decay and internal quantum efficiency measurements are made as a function of temperature on two double
heterostructure AIGaAs light emitting diodes (LEDs) that emit in the visible (red) portion of the spectrum. The electroluminescent
lifetimes increase by more than a factor of ten and the internal quantum efficiency falls by a factor of three as the temperature
is raised from 90 to 400K. By analyzing the data with a model that accounts for the transfer with increasing temperature of
the minority-carrier electrons from the direct-gap to the indirect-gap minima in the p-type active layer of these near-crossover
LEDs, values for the radiative and nonradiative lifetimes as a function of temperature are obtained. A fit to the radiative-lifetime
data results in an estimate of 1.3 × 10−10 cm3s−1 for the room-temperature radiative recombination coefficient of Al0.39Ga0.61As, which is very similar to values reported for GaAs. The nonradiative lifetimes are found to be nearly independent of temperature
from 220 to 400K and provide upper limits of 940 and 1250 cms−1 for the interface recombination velocities of the two samples. These values are roughly an order of magnitude lower than
any previously reported values for high-Al-content (x > 0.3) AlxGa1−xAs heterostructures. 相似文献
982.
Subramanian S Murugesan R Devasahayam N Cook JA Afeworki M Pohida T Tschudin RG Mitchell JB Krishna MC 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1999,137(2):379-388
Design strategies, system configuration, and operation of a dual-channel data acquisition system for a radiofrequency (RF) time-domain electron paramagnetic resonance (EPR) spectrometer/imager operating at 300 MHz are described. This system wasconfigured to incorporate high-speed analog-to-digital conversion (ADC) and summation capabilities with both internal and external triggering via GPIB interface. The sampling rate of the ADC is programmable up to a maximum of 1 GS/s when operating in a dual-channel mode or 2 GS/s when the EPR data are collected in a single-channel mode. By using high-speed flash ADCs, a pipelined 8-bit adder, and a 24-bit accumulator, a repetition rate of 230 kHz is realized to sum FIDs of 4096 points. The record length is programmable up to a maximum of 8K points and a large number of FIDs (2(24)) can be summed without overflow before the data can be transferred to a host computer via GPIB interface for further processing. The data acquisition system can operate in a two-channel (quadrature) receiver mode for the conventional mixing to baseband. For detection using the single-channel mode, the resonance signals around the center frequency of 300 MHz were mixed with a synchronized local oscillator of appropriate frequency leading to an intermediate frequency (IF) which is sampled at a rate of 2 GS/s. Comparison of quadrature-mode and an IF-mode operation for EPR detection is presented by studying the FID signal intensity across a bandwidth of 10 MHz and as a function of transmit RF power. Imaging of large-sized phantoms accommodated in appropriately sized resonators indicates that IF-mode operation can be used to obtain distortion-free images in resonators of size 50 mm diameter and 50 mm length. 相似文献
983.
On the non-productivity of normality in Moore spaces 总被引:1,自引:0,他引:1
Under Martin's Axiom and the denial of the Continuum Hypothesis, the authors give examples of normal Moore spaces whose squares are not normal.
984.
We consider integer linear programming problems with a fixed coefficient matrix and varying objective function and right-hand-side vector. Among our results, we show that, for any optimal solution to a linear program max{wx: Axb}, the distance to the nearest optimal solution to the corresponding integer program is at most the dimension of the problem multiplied by the largest subdeterminant of the integral matrixA. Using this, we strengthen several integer programming proximity results of Blair and Jeroslow; Graver; and Wolsey. We also show that the Chvátal rank of a polyhedron {x: Axb} can be bounded above by a function of the matrixA, independent of the vectorb, a result which, as Blair observed, is equivalent to Blair and Jeroslow's theorem that each integer programming value function is a Gomory function.Supported by a grant from the Alexander von Humboldt Stiftung.Since September 1985: Department of Operations Research, Upson Hall, Cornell University, Ithaca, NY 14853, USA.Partially supported by the Sonderforschungbereich 21 (DFG), Institut für Ökonometrie und Operations Research of the University of Bonn, FR Germany. 相似文献
985.
This paper describes the ready preparation of 3,6-difluoro-3-deazapurine (4,7-difluoroimidazo[4,5-c]pyridine). This novel base was glycosylated under mild conditions using three different ribose sugar analogs. 3,6-Difluoro-3-deazapurine ribonucleoside analogs underwent direct SNAr amination reactions with liquid ammonia to give 3-fluoro-3-deazaadenosine analogs in excellent yield; in contrast, 6-chloro-3-fluoro-3-deazapurine nucleosides were inert under similar reaction conditions. 相似文献
986.
987.
Baltrusaitis RM Becker JJ Blaylock GT Brown JS Bunnell KO Burnett TH Cassell RE Coffman D Cook V Coward DH Dado S Dorfan DE Dubois GP Duncan AL Einsweiler KF Eisenstein BI Fabrizio R Gladding G Grancagnolo F Hamilton RP Hauser J Heusch CA Hitlin DG Köpke L Lockman WS Mallik U Mockett PM Mozley RF Nappi A Odian A Partridge R Perrier J Plaetzer SA Richman JD Roehrig J Russell JJ Sadrozinski HF Scarlatella M Schalk TL Schindler RH Seiden A Simopoulos C Sleeman JC Spadafora AL Stockdale IE 《Physical review letters》1986,56(20):2136-2139
988.
D. C. Cook 《Hyperfine Interactions》1986,28(1-4):891-894
The study of the corrosion products formed on the surface of Type 316 stainless steel exposed to chlorinated seawater for 18 days shows that ferrihydrite and lepidocrocite are the first oxides to form. The surface also shows the formation of a martensite when it is cleaned by light brushing before exposure.Samples of weathering steel, ASTM A242 Type 1, exposed for 51/2 years to either a marine or inland rural environment have been studied between 100K and 400K using CEMS and XMS. Different amounts of hydroxoxides are present on the surface of each sample. The steel exposed to the inland environment contains more -FeOOH.The author acknowledges the LaQue Center of Corrosion Technology, NC, and the Bethlehem Steel Corporation, PA, for preparing the samples. 相似文献
989.
Jerry D. Cook Norbert Stankiewicz Mark Podany 《Journal of Infrared, Millimeter and Terahertz Waves》1989,10(1):161-177
The machining of slow wave structures for high frequency BWO's is extremely difficult beyond l THz. Recently a microfabrication technique using photolithography and ion-beam assisted etching has been used to construct a prototype BWO operating at 200 to 265 GHz. The output coupler for such tubes remains a problem. Waveguides do not exist or are very lossy at the frequencies of interest (300 to 2000 GHz). This paper discusses several scaled experiments of optical output couplers for submillimeter BWO's. Various designs of planar antennas (Vivaldi horns) and lens-feed systems (hyperhemispherical lens) were constructed and tested between 20 and 100 GHz using a spectrum analyzer. The lens system was also tested at 337 GHz using a CO2 pumped FIR laser. 相似文献
990.