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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Proton-radiation damage in Gunn oscillators

The irradiation effects of 22 MeV protons on the electrical characteristics of GaAs continuous-wave Gunn oscillators was studied. The radio frequency power output was reduced by 3 decibels at proton fluences in the neighborhood of 1.5 x 10 to the 12th power protons/sq cm. Conductance measurements indicate that the carrier removal rate at high electric fields remained roughly 40 percent less than at low fields. Diode efficiencies of two device groups were found to be monotonically descreasing functions of fluence. Frequency modulation noise was generally unaffected by radiation, but the magnitude of the noise in the noise power spectrum increased significantly. These effects are partially accounted for, in a qualitative fashion, by a model of electron traps having field-dependent net-carrier capture rates and various response times.

Johnson, J. W.↗
NASA NTRS1989

MT's algorithm: A new algorithm to search for the optimum set of modulation indices for simultaneous range, command, and telemetry

MT's algorithm was developed as an aid in the design of space telecommunications systems when utilized with simultaneous range/command/telemetry operations. This algorithm provides selection of modulation indices for: (1) suppression of undesired signals to achieve desired link performance margins and/or to allow for a specified performance degradation in the data channel (command/telemetry) due to the presence of undesired signals (interferers); and (2) optimum power division between the carrier, the range, and the data channel. A software program using this algorithm was developed for use with MathCAD software. This software program, called the MT program, provides the computation of optimum modulation indices for all possible cases that are recommended by the Consultative Committee on Space Data System (CCSDS) (with emphasis on the squarewave, NASA/JPL ranging system).

Nguyen, Tien Manh↗
DOE OSTI2014

Direct Analysis of JV-Curves Applied to an Outdoor-Degrading CdTe Module (Presentation)

We present the application of a phenomenological four parameter equation to fit and analyze regularly measured current density-voltage JV curves of a CdTe module during 2.5 years of outdoor operation. The parameters are physically meaningful, i.e. the short circuit current density Jsc, open circuit voltage Voc and differential resistances Rsc, and Roc. For the chosen module, the fill factor FF degradation overweighs the degradation of Jsc and Voc. Interestingly, with outdoor exposure, not only the conductance at short circuit, Gsc, increases but also the Gsc(Jsc)-dependence. This is well explained with an increase in voltage dependent charge carrier collection in CdTe.

14 SOLAR ENERGY↗
DOE OSTI2020

Strongly Surface State Carrier‐Dependent Spin–Orbit Torque in Magnetic Topological Insulators

Abstract The topological surface states (TSS) in topological insulators (TIs) can exert strong spin–orbit torque (SOT) on adjacent magnetization, offering great potential in implementing energy‐efficient magnetic memory devices. However, there are large discrepancies among the reported spin Hall angle values in TIs, and its temperature dependence still remains elusive. Here, the spin Hall angle in a modulation‐doped Cr‐Bi x Sb 2− x Te 3 (Cr‐BST) film is quantitatively determined via both transport and optic approaches, where consistent results are obtained. A large spin Hall angle of ≈90 in the modulation‐doped Cr‐BST film is demonstrated at 2.5 K, and the spin Hall angle drastically decreases to 0.3–0.5 as the temperature increases. Moreover, by tuning the top TSS carrier concentration, a competition between the top and bottom TSS in contributing to SOT is observed. The above phenomena can account for the large discrepancies among the previously reported spin Hall angle values and reveal the unique role of TSS in generating SOT.

Che, Xiaoyu↗
DOE OSTI2019

Quantum Confined Stark Effect and Temperature Dependencies of Photoluminescence of InAs Quantum Dots Coupled with AlGaAs/GaAs Two Dimensional Electron Gas

In this work, Experimental study of the influence of internal electric field and the temperature on the photoluminescence of InAs self assembled quantum dots inserted in AlGaAs/GaAs modulation doped hetero-structure have been investigated. The internal electric field is controlled by an appropriate design of the hetero-structure. We have observed a red shift of the photoluminescence position peaks result from the quantum confined Stark effect due to the local electric field existing in the structure. Estimation values of the internal electric field have been obtained through carrier’s densities values in interface of AlGaAs/GaAs hetero-junction. An anomalous dependence of the full width at half maximum with temperature has been found, which attributed to the carrier’s dynamics between InAs quantum dots layer and the two-dimensional electron gas.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1993

Performance of the unique-word-reverse-modulation type demodulator for mobile satellite communications

This paper proposes a new type of coherent demodulator, the unique-word (UW)-reverse-modulation type demodulator, for burst signal controlled by voice operated transmitter (VOX) in mobile satellite communication channels. The demodulator has three individual circuits: a pre-detection signal combiner, a pre-detection UW detector, and a UW-reverse-modulation type demodulator. The pre-detection signal combiner combines signal sequences received by two antennas and improves bit energy-to-noise power density ratio (E(sub b)/N(sub 0)) 2.5 dB to yield 10(exp -3) average bit error rate (BER) when carrier power-to-multipath power ratio (CMR) is 15 dB. The pre-detection UW detector improves UW detection probability when the frequency offset is large. The UW-reverse-modulation type demodulator realizes a maximum pull-in frequency of 3.9 kHz, the pull-in time is 2.4 seconds and frequency error is less than 20 Hz. The performances of this demodulator are confirmed through computer simulations and its effect is clarified in real-time experiments at a bit rate of 16.8 kbps using a digital signal processor (DSP).

Dohi, Tomohiro↗
NASA NTRS1987

Charge distribution and response time for a modulation-doped extrinsic infrared detector

The electric charge distribution and response time of a modulation-doped extrinsic infrared detector are determined. First, it is demonstrated theoretically that the photoconductive layer is effectively depleted of ionized majority-impurity charges so that scattering is small and mobility is high for photogenerated carriers. Then, using parameters appropriate to an actual detector, the predicted response time is 10 to the -8th to about 10 to the -9th s, which is much faster than comparable conventional detectors. Thus, the modulation-doped detector design would be valuable for heterodyne applications.

Hadek, Victor↗
DOE OSTI2016

Resistive switching and voltage induced modulation of tunneling magnetoresistance in nanosized perpendicular organic spin valves

Nanoscale multifunctional perpendicular organic spin valves have been fabricated. The devices based on an La{sub 0.7}Sr{sub 0.3}MnO{sub 3}/Alq3/Co trilayer show resistive switching of up to 4-5 orders of magnitude and magnetoresistance as high as -70% the latter even changing sign when voltage pulses are applied. This combination of phenomena is typically observed in multiferroic tunnel junctions where it is attributed to magnetoelectric coupling between a ferromagnet and a ferroelectric material. Modeling indicates that here the switching originates from a modification of the La{sub 0.7}Sr{sub 0.3}MnO{sub 3} surface. This modification influences the tunneling of charge carriers and thus both the electrical resistance and the tunneling magnetoresistance which occurs at pinholes in the organic layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Light-induced Kondo-like exciton-spin interaction in neodymium(II) doped hybrid perovskite

Abstract Tuning the properties of a pair of entangled electron and hole in a light-induced exciton is a fundamentally intriguing inquiry for quantum science. Here, using semiconducting hybrid perovskite as an exploratory platform, we discover that Nd 2+ -doped CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskite exhibits a Kondo-like exciton-spin interaction under cryogenic and photoexcitation conditions. The feedback to such interaction between excitons in perovskite and the localized spins in Nd 2+ is observed as notably prolonged carrier lifetimes measured by time-resolved photoluminescence, ~10 times to that of pristine MAPbI 3 without Nd 2+ dopant. From a mechanistic standpoint, such extended charge separation states are the consequence of the trap state enabled by the antiferromagnetic exchange interaction between the light-induced exciton and the localized 4 f spins of the Nd 2+ in the proximity. Importantly, this Kondo-like exciton-spin interaction can be modulated by either increasing Nd 2+ doping concentration that enhances the coupling between the exciton and Nd 2+ 4 f spins as evidenced by elongated carrier lifetime, or by using an external magnetic field that can nullify the spin-dependent exchange interaction therein due to the unified orientations of Nd 2+ spin angular momentum, thereby leading to exciton recombination at the dynamics comparable to pristine MAPbI 3 .

36 MATERIALS SCIENCE↗
NASA NTRS2001

The Effect of Doppler Frequency Shift, Frequency Offset of the Local Oscillators, and Phase Noise on the Performance of Coherent OFDM Receivers

This paper first shows that the Doppler frequency shift affects the frequencies of the RF carrier, subcarriers, envelope, and symbol timing by the same percentage in an Orthogonal Frequency Division Multiplexing (OFDM) signal or any other modulated signals. Then the SNR degradation of an OFDM system due to Doppler frequency shift, frequency offset of the local oscillators and phase noise is analyzed. Expressions are given and values for 4-, 16-, 64-, and 256-QAM OFDM systems are calculated and plotted. The calculations show that the Doppler shift of the D3 project is about 305 kHz, and the degradation due to it is about 0.01 to 0.04 dB, which is negligible. The degradation due to frequency offset and phase noise of local oscillators will be the main source of degradation. To keep the SNR degradation under 0.1 dB, the relative frequency offset due to local oscillators must be below 0.01 for the 16 QAM-OFDM. This translates to an offset of 1.55 MHz (0.01 x 155 MHz) or a stability of 77.5 ppm (0.01 x 155 MHz/20 GHz) for the DI project. To keep the SNR degradation under 0.1 dB, the relative linewidth (0) due to phase noise of the local oscillators must be below 0.0004 for the 16 QAM-OFDM. This translates to a linewidth of 0.062 MHz (0.0004 x 155 MHz) of the 20 GHz RIF carrier. For a degradation of 1 dB, beta = 0.04, and the linewidth can be relaxed to 6.2 MHz.

Xiong, Fuqin↗
DOE OSTI2015

Quantum effects on compressional Alfven waves in compensated semiconductors

Amplitude modulation of a compressional Alfven wave in compensated electron-hole semiconductor plasmas is considered in the quantum magnetohydrodynamic regime in this paper. The important ingredients of this study are the inclusion of the particle degeneracy pressure, exchange-correlation potential, and the quantum diffraction effects via the Bohm potential in the momentum balance equations of the charge carriers. A modified nonlinear Schrödinger equation is derived for the evolution of the slowly varying amplitude of the compressional Alfven wave by employing the standard reductive perturbation technique. Typical values of the parameters for GaAs, GaSb, and GaN semiconductors are considered in analyzing the linear and nonlinear dispersions of the compressional Alfven wave. Detailed analysis of the modulation instability in the long-wavelength regime is presented. For typical parameter ranges of the semiconductor plasmas and at the long-wavelength regime, it is found that the wave is modulationally unstable above a certain critical wavenumber. Effects of the exchange-correlation potential and the Bohm potential in the wave dynamics are also studied. It is found that the effect of the Bohm potential may be neglected in comparison with the effect of the exchange-correlation potential in the linear and nonlinear dispersions of the compressional Alfven wave.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Voltage Control of a van der Waals Spin-Filter Magnetic Tunnel Junction

Atomically thin chromium triiodide (CrI 3 ) has recently been identified as a layered antiferromagnetic insulator, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled. This unusual magnetic structure naturally comprises a series of antialigned spin filters, which can be utilized to make spin-filter magnetic tunnel junctions with very large tunneling magnetoresistance (TMR). Here we report voltage control of TMR formed by four-layer CrI 3 sandwiched by monolayer graphene contacts in a dual-gated structure. By varying the gate voltages at fixed magnetic field, the device can be switched reversibly between bistable magnetic states with the same net magnetization but drastically different resistance (by a factor of 10 or more). Also, without switching the state, the TMR can be continuously modulated between 17,000% and 57,000%, due to the combination of spin-dependent tunnel barrier with changing carrier distributions in the graphene contacts. Our work shows new kinds of magnetically moderated transistor action and opens up possibilities for voltage-controlled van der Waals spintronic devices.

2D magnets↗
DOE OSTI2016

Development of flexible, scalable, low cost readout for beam tests of the high granularity calorimeter for the CMS endcap

As part of the development of the High Granularity Calorimeter for the CMS Endcap at HL-LHC, The CMS collaboration is conducting a comprehensive series of beam tests. The first beam test, with a single HGC module was carried out in March of 2016 at the Fermilab Test Beam Facility, continuing to a 16 module test in July of 2016. We describe here the development of a low cost readout system that is simple to implement and is able to grow with the system under test. The system is based on the low cost Zynq SoC that allows simple DAQ development in a Linux environment. For this application we used the Digilent ZedBoard, which allows high speed LVDS links and Linux software development on a single commercial board. A small custom FPGA board designed to comply with the VITA 57 Field Programmable Mezzanine Card standard implements the interface to the readout ASIC mounted on the HGC sensor modules and provides the LVDS links to the ZedBoard, either directly over the FMC connector or via a custom carrier card. This architecture provides scalability, ease of development and low cost.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Features of a priori heavy doping of the n-TiNiSn intermetallic semiconductor

The crystal structure, the distribution of electron density, and the energy, kinetic, and magnetic properties of the n-TiNiSn intermetallic semiconductor are investigated. It is shown that a priori doping of n-TiNiSn with donors originates from partial, up to 0.5 at %, redistribution of Ti and Ni atoms in crystallographic sites of Ti atoms. The correlation is established between the donor concentration, amplitude of modulation of the continuous energy bands, and degree of filling of low-scale fluctuation potential wells with charge carriers. The results obtained are discussed within the Shklovskii-Efros model of a heavily doped and compensated semiconductor.

36 MATERIALS SCIENCE↗
NASA NTRS1983

Two-way satellite time transfer using low power CW tones

In the search for an economical means of precise time transfer, the NRC Time Laboratory decided to adapt the techniques used by radio astronomers in an experiment to compare the phases of the local oscillators at widely separated VLBI stations. The objective is to design a system which would use commercial satellites, and which would be of reasonable cost for the ground stations and for operations. Two satellite ground stations were installed at NRC about 100 m from the Time Laboratory. For the preliminary experiment, a channel on the Anik Al 6/4 GHz satellite was made available by TELESAT Canada. Two tones were transmitted + or - MHz from the suppressed carrier. The difference frequency of 32 MHz was recorded using narrow band receivers. A low level 1 MHz phase modulation was added to identify the 32 MHz cycle, giving 1 microsec ambiguity in the time transfer. With less than 1/4 W in each tone, the EIRP is 43 dB below that of a normal TV Earth station, and no frequency dispersion is required. The measurements taken each second for the 32 MHz have an rms scatter of 1 ns.

Costain, C. C.↗
NASA NTRS1988

Doppler and range determination for deep space vehicles using active optical transponders

This paper describes and analyzes two types of laser system employing active transponders that could accurately determine Doppler and range to deep space vehicles from earth-orbiting satellites. The first is a noncoherent optical system in which the Doppler effect on an intensity-modulating subcarrier is measured. The second is a coherent optical system in which the Doppler effect of the optical carrier itself is measured. Doppler and range measurement errors are mathematically modeled and, for three example systems, numerically evaluated.

Kinman, Peter W.↗
NASA NTRS2002

Transmission of RF Signals Over Optical Fiber for Avionics Applications

During flight, aircraft avionics transmit and receive RF signals to/from antennas over coaxial cables. As the density and complexity of onboard avionics increases, the electromagnetic interference (EM) environment degrades proportionately, leading to decreasing signal-to-noise ratios (SNRs) and potential safety concerns. The coaxial cables are inherently lossy, limiting the RF signal bandwidth while adding considerable weight. To overcome these limitations, we have investigated a fiber optic communications link for aircraft that utilizes wavelength division multiplexing (WDM) to support the simultaneous transmission of multiple signals (including RF) over a single optical fiber. Optical fiber has many advantages over coaxial cable, particularly lower loss, greater bandwidth, and immunity to EM. In this paper, we demonstrate that WDM can be successfully used to transmit multiple RF signals over a single optical fiber with no appreciable signal degradation. We investigate the transmission of FM and AM analog modulated signals, as well as FSK digital modulated signals, over a fiber optic link (FOL) employing WDM. We present measurements of power loss, delay, SNR, carrier-to-noise ratio (CNR), total harmonic distortion (THD), and bit error rate (BER). Our experimental results indicate that WDM is a fiber optic technology suitable for avionics applications.

Slaveski, Filip↗
NASA NTRS1979

Bandgap narrowing in moderately to heavily doped silicon

A theoretical model of bandgap narrowing in silicon at high doping levels has been developed. The model takes into account the electrostatic energy of interaction between a minority carrier and the majority carriers surrounding it, which reduces the thermal energy necessary for creation of an electron-hole pair. A pair energy similar to the excitonic binding energy of bound electron-hole pairs in insulators is obtained. Theoretical results are in excellent agreement with experimental results in the doping range from 3 times 10 to the 17th to 1.5 times 10 to the 20th/cu cm at room temperature. These results indicate that at high injection levels such as a transistor biased into the conductivity-modulation regime or a solar cell whose surface is established by ion implantation into an oxide layer, the bandgap narrowing is determined by the injected carrier concentration rather than by the doping level.

Lanyon, H. P. D.↗