2,804 research outputs found
Tunable two-dimensional plasmon resonances in an InGaAs/InP high electron mobility transistor
Voltage-tunable plasmon resonances in the two-dimensional electron gas (2DEG) of a high electron mobility transistor (HEMT) fabricated from the InGaAs/InP materials system are reported. The device was fabricated from a commercial HEMT wafer by depositing source and drain contacts using standard photolithography and a semitransparent gate contact that consisted of a 0.5 mu m period transmission grating formed by electron-beam lithography. Narrow-band resonant absorption of terahertz radiation was observed in transmission in the frequency range of 10-50 cm(-1). The resonance frequency depends on the gate-tuned sheet charge density of the 2DEG. The observed separation of resonance fundamental from its harmonics and their shift with gate bias are compared with theory
Jets of nuclear matter from high energy heavy ion collisions
The fluid dynamical model is used to study the reactions 20Ne+238U and 40Ar+40Ca at Elab=390 MeV/nucleon. The calculated double differential cross sections dÂČĂ°â/dΩdE exhibit sidewards maxima in agreement with recent experimental data. The azimuthal dependence of the triple differential distributions, to be obtained from an event-by-event analysis of 4Ï; exclusive experiments, can yield deeper insight into the collision process: Jets of nuclear matter are predicted with a strongly impact-parameter-dependent thrust angle Ξâjet(b). NUCLEAR REACTIONS Ar+Ca, Ne+U, Elab=393 MeV/nucleon, fluid dynamics with thermal breakup, double differential cross sections, azimuthal dependence of triple differential cross sections, event-by-event thrust analysis of 4π exclusive experiments
Fragment emission in high-energy heavy-ion reactions
We present a theoretical description of nuclear collisions which consists of a three-dimensional fluid-dynamical model, a chemical equilibrium breakup calculation for local light fragment (i.e., p, n, d, t, 3He, and 4He) production, and a final thermal evaporation of these particles. The light fragment cross sections and some properties of the heavy target residues are calculated for the asymmetric system Ne+U at 400 MeV/N. The results of the model calculations are compared with recent experimental data. Several observable signatures of the collective hydrodynamical processes are consistent with the present data. An event-by-event analysis of the flow patterns of the various clusters is proposed which can yield deeper insight into the collision dynamics
Surface depletion mediated control of inter-sub-band absorption in GaAs/AlAs semiconductor quantum well systems
The modification of quantum well inter-sub-band absorption properties due to surface depletion induced band bending is reported. Fourier transform infrared spectroscopy measurements of a GaAs/AlAs multiple quantum well system reveal a reduction in the characteristic absorption resonance in correlation with wet chemical etching. High resolution transmission electron microscopy confirms the presence of the quantum wells after etching, suggesting the quantum wells are positioned within the surface depletion region of the structure. This method of inter-sub-band absorption modification could be used for the formation of quantum dots from a quantum well system with the precise, deterministic control of their location
Analysis, Test and Simulation of Landing System Touchdown Dynamics
Future exploration missions pose demanding requirements towards the access by vehicles to scientifically interesting sites on planetary surfaces. These stem particularly from the need of more flexibility in site selection, improved payload to vehicle mass ratios and higher mission success probabilities.
The Landing Technology group of the DLR Institute of Space Systems is focusing on the development and verification of experimental and analytical methods for the investigation of the touchdown dynamics of landing system, its capabilities the embedding into the landing site assessment.
Core element for the experimental investigation is the Landing & Mobility Test Facility (LAMA), which allows touchdown testing under Earth gravity and under a reduced gravitational environment using an active off-loading device. The test article for investigation of legged landing systems is a modular Lander Engineering Model (LEM) designed by the Astrium ST (Bremen), representing today's European mission scenarios to the Moon and Mars such as the ESA Lunar Lander or the ESA Mars Precision Lander. Another test object recently under retesting is the Rosetta lander Philae representing a touch down system concept developed for small body landings.
Usually not all relevant environmental properties of the target landing site can be provided in one single and complete test, any verification approach has to be supported by adequate numerical analyses. Thus, another key topic for the verification of the touchdown performance of a landing system is the accurate analytical and numerical representation of the flight system, its touchdown conditions and the landing site. In this area the research focuses on the development of high fidelity engineering simulations of the vehicle-to-terrain/soil interaction.
The landing site characterization and assessment focuses on the development of landing site assessment methods and tools and to provide terrain models for engineering simulations (both touchdown dynamics and/or hazard detection& avoidance simulations). In return landing system performance limits are mapped onto cartographic landing site representations to support the landing safety assessment.
This poster outlines the test facility, simulation and analysis tools developed by the working group and used in recent landing missions
Simple, efficient protocols for the Pd-catalyzed cross-coupling reaction of aryl chlorides and dimethylamine
Simple and efficient procedures for the Pd-catalyzed cross-coupling reaction of aryl chlorides and dimethylamine are described. At room temperature with a strong base, t-BuXPhos is employed as the supporting ligand; at 110 °C with a weak base, XPhos is employed as the supporting ligand. In each of these cases, commercially available solutions constitute the source of the dimethylamine, and recently disclosed precatalysts constitute the source of the ligand and Pd. This work further expands the utility of these precatalysts in reactions that benefit from an easily activated source of L[subscript 1]Pd(0).National Institutes of Health (U.S.) (GM-058160)National Institutes of Health (U.S.) (Postdoctoral Fellowship GM-F32-75685)Amgen Inc.Merck & Co., Inc.Boehringer Ingelheim PharmaceuticalsMassachusetts Institute of Technology. Undergraduate Research Opportunities Program (Summer Fellowship
Insomnia Symptoms and Cardiovascular Disease among Older American Indians: The Native Elder Care Study
Background. Cardiovascular disease (CVD) is the leading cause of death among American Indians. It is not known if symptoms of insomnia are associated with CVD in this population.
Methods. We examined 449 American Indians aged â„55 years from the Native Elder Care Study. The main outcome-of-interest was self-reported CVD. Results. Short sleep duration, daytime sleepiness, and difficulty falling asleep were positively associated with CVD after adjusting for demographic, lifestyle, and clinical risk factors. Compared with a sleep duration of 7âh, the multivariable odds ratio (OR) (95% confidence interval [CI]) of CVD among those with sleep duration â€5âh was 2.89 (1.17â7.16). Similarly, the multivariable OR (95% CI) of CVD was 4.45 (1.85â10.72) and 2.60 (1.25â5.42) for daytime sleepiness >2âh and difficulty falling asleep often/always.
Conclusion. Symptoms of insomnia including short sleep duration, daytime sleepiness, and difficulty falling asleep are independently associated with CVD in American Indians aged â„55 years
Development of a Marslander with crushable shock absorber by virtual and experimental testing
Since the beginning of space exploration, probes have been sent to other planets or moons with the associated
challenge of landing on these bodies. For a soft landing several damping methods like landing legs or airbags have
been used. A new and potentially less complex and lighter way to reduce the shock loads at touchdown is the use of
a crushable shield underneath the lander platform. This crushable shield could be made for example out of an energy
absorbing materials like an aluminum honeycomb core with a High Performance Polyethylene cover sheet. The
design is particularly advantageous since no moving parts nor other mechanisms are required, thus making the shield
very robust and fail safe. The only mission that is currently planned to use this technique is the ESA-mission
âExoMarsâ which is planned to start in 2016.
The development of such a crushable shock absorber implies and requires assessment of materials,
manufacturing processes, the setup of a numerical simulation and the experimental validation in a test lab. In an
independent research project (Marslander1) a representative engineering mockup of the landing platform has been
build and tested at the Landing & Mobility Test Facility (LAMA) to support the numerical simulation model with
experimental data. The simulations are based on the explicit Finite Element Method, which discretizes the structure
into a defined number of elements, such that each element is assigned a set of equations describing the material
properties and applied loads. The goal is to generate a simplified but still accurate model to predict landing scenarios
by running Monte Carlo simulations.
Results of the above stated development and testing processes will be presented and discussed in this paper
Adjustable microchip ring trap for cold atoms and molecules
We describe the design and function of a circular magnetic waveguide produced
from wires on a microchip for atom interferometry using deBroglie waves. The
guide is a two-dimensional magnetic minimum for trapping weak-field seeking
states of atoms or molecules with a magnetic dipole moment. The design consists
of seven circular wires sharing a common radius. We describe the design, the
time-dependent currents of the wires and show that it is possible to form a
circular waveguide with adjustable height and gradient while minimizing
perturbation resulting from leads or wire crossings. This maximal area geometry
is suited for rotation sensing with atom interferometry via the Sagnac effect
using either cold atoms, molecules and Bose-condensed systems
Atom chips on direct bonded copper substrates
We present the use of direct bonded copper (DBC) for the straightforward
fabrication of high power atom chips. Atom chips using DBC have several
benefits: excellent copper/substrate adhesion, high purity, thick (> 100
microns) copper layers, high substrate thermal conductivity, high aspect ratio
wires, the potential for rapid (< 8 hr) fabrication, and three dimensional atom
chip structures. Two mask options for DBC atom chip fabrication are presented,
as well as two methods for etching wire patterns into the copper layer. The
wire aspect ratio that optimizes the magnetic field gradient as a function of
power dissipation is determined to be 0.84:1 (height:width). The optimal wire
thickness as a function of magnetic trapping height is also determined. A test
chip, able to support 100 A of current for 2 s without failing, is used to
determine the thermal impedance of the DBC. An assembly using two DBC atom
chips to provide magnetic confinement is also shown.Comment: 8 pages, 5 figure
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