Search Results - (Author, Cooperation:R. Harper)

2013-07-23

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

Computer Simulation/*utilization ; Humans ; Research/*trends ; *Software

2013-05-21

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

Computer Simulation/*utilization ; Humans ; Research/*trends ; *Software

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

By modeling the statistical evolution of an avalanche created by 20 keV protons impacting the input surface of a z-stack microchannel plate (MCP) detector, the mean secondary electron yield γC of avalanche electrons propagating through a MCP channel is measured to equal 1.37 for 760 V per MCP in the z stack. This value agrees with other studies that used MCP gain measurements to infer γC. The technique described here to measure γC is independent of gain saturation effects and simplifying assumptions used in the segmented dynode model, both of which can introduce errors when inferring γC through gain measurements. © 1996 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

We present data demonstrating the influence of an applied electric field E oriented normal to the input surface of a microchannel plate (MCP) detector on the critical operating parameters of the detector, including the quantum detection efficiency, the spatial resolution, and pulse height distribution. The MCP detector response is characterized using 20 keV protons as the primary radiation. An applied electric field E〈−4 V/mm, where a negative value of E corresponds to a nearby object that is biased positive relative to the input surface, results in a high spatial resolution and a quantum detection efficiency that is approximately equal to the open area ratio of the MCP. An electric field −1〈E〈5 V/mm results in low spatial resolution, in which up to 32% of the measured signal appears as a localized noise that extends several millimeters from the point of ion impact, and a maximum quantum detection efficiency of approximately 0.87. Furthermore, a separate peak in the pulse-height distribution arises from ions striking the web of the MCP detector and has a much lower pulse magnitude than that of ions striking channels. For E(approximately-greater-than)5 V/mm, the spatial resolution increases, and the quantum detection efficiency slightly decreases from its maximum value with increasing E. The characteristics of each of these electric field configurations are analyzed in the context of the yield and transport of secondary electrons created at the web of the MCP detector, and the results can be scaled to other ions and energies according to the secondary electron yield of ions striking the web. © 1996 American Institute of Physics.

Electronic Resource

1477-9730

Blackwell Publishing Journal Backfiles 1879-2005

Architecture, Civil Engineering, Surveying

Electronic Resource

1077-3118

AIP Digital Archive

Physics

High-resolution low-energy electron diffraction has been used to study the generation of defects on the surface of commercial Si (111) wafers under high-temperature thermal cycling in ultrahigh vacuum. We observed a gradual increase of single-atomic step density from 0.15 to 0.30% after several thermal annealings at ∼1200 °C. However, a reduction of step density, accompanied by a change of step height from single-atomic to double-atomic step height, was observed after the sample was annealed to near melting temperature (∼1400 °C). At the same time, the surface was broken into micrograins having a small mosaic orientation. Due to the limited instrumental resolution, these low-density step structures and the small-angle mosaic structures cannot be resolved by using the conventional low-energy electron diffraction technique.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the first demonstration of electronic devices, Schottky diodes, and metal-semiconductor field-effect transistors, in a diluted magnetic semiconductor Cd1−xMnxTe. These devices were fabricated using n-type, indium-doped CdMnTe films grown by photoassisted molecular beam epitaxy on (100) CdTe and CdZnTe substrates. Epitaxial layers with carrier concentrations of 1×1017 cm−3 and electron mobilities as large as 720 cm2/V s at 120 K were used. The Schottky diodes have turn-on voltages of 0.8 V, idealities in the range between 1.27 and 1.7, and reverse breakdown voltages from 5.5 to 10.5 V reverse bias. The 100 μm gate length transistors have transconductances of 1 mS/mm.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the successful p-type doping of CdTe films with arsenic using the photoassisted molecular beam epitaxy growth technique. These doped epilayers were grown at substrate temperatures as low as 180 °C. The room-temperature hole concentrations in the CdTe:As layers ranged from 7×1015 to 6.2×1018 cm−3 as determined by van der Pauw–Hall effect measurements. We propose a doping mechanism responsible for the high p-type doping levels observed in the films. The arsenic acceptor ionization energy was found to ∼58–60 meV using low-temperature photoluminescence measurements.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

At North Carolina State University, we have recently employed photoassisted molecular beam epitaxy to successfully prepare p-type modulation-doped HgCdTe. The modulation-doped HgCdTe samples were grown on lattice-matched (100) CdZnTe substrates cut from boules grown at Santa Barbara Research Center. In this letter, we report details of the growth experiments and describe the structural, optical, and electrical properties that this new infrared quantum alloy of HgCdTe possesses.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We observed the growth of epitaxial Al(111) films on Si(111) at room temperature by the partially ionized beam deposition technique. The films were deposited in a conventional vacuum condition without in situ cleaning. The beam contained 0.3% of Al self-ions and a bias potential of 1 kV was applied to the substrate during deposition. X-ray diffraction (pole figure) revealed that one of the two possible twin structures, with the Al〈1¯10〉(parallel)Si〈1¯10〉 orientation, was preferentially grown on the Si substrate.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report the first demonstration of CdTe metal-semiconductor field-effect transistors. These transistors were fabricated using n-type CdTe films grown by photoassisted molecular beam epitaxy. Using this new film deposition technique, it is possible to obtain highly activated n-type or p-type films suitable for device applications. In the present work, transistor structures with 5 or 100 μm gate lengths having channel dopings in the range from 2×1016 to 2×1017 cm−3 were fabricated and tested. The 5 μm gate devices have transconductances as large as 10 mS/mm and pinch-off voltages of 4.0 V.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Electroreflectance measurements are reported for bulk and epitaxial CdTe layers grown by molecular-beam epitaxy (MBE). Spectra are analyzed to determine the E0, E1, and E1+Δ1 transition energies as well as the associated broadening energies. The broadening energies are found to correlate with the structural quality of the epilayers. Additional spectral features near the band edge (1.45–1.51 eV) and in the UV (3.1–4.3 eV) are discussed. The extremely sharp spectra obtained for the MBE-grown epitaxial layers indicate high-quality material.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The piezomodulated reflectivity spectra of CdMnTe-CdTe superlattices grown by photoassisted molecular-beam epitaxy reveal signatures associated with excited confined quantum states. The energies of these signatures are in excellent agreement with the predicted energies for heavy-hole and light-hole states calculated using a Kronig–Penney model in which strain effects are included. For comparison, we present calculations for the cases of a zero valence-band offset and a small valence-band offset which scales with Mn concentration.

Electronic Resource

1749-6632

Blackwell Publishing Journal Backfiles 1879-2005

Natural Sciences in General

Electronic Resource

1365-2427

Blackwell Publishing Journal Backfiles 1879-2005

Biology

SUMMARY. The estimated average number of bacteria in the gut of laboratory-cultured Nais variabilis was 100.4 × 104. This compares with values of 6.7 × 104 and 32.8 × 104, respectively, for N. variabilis and N. barbata obtained from a river.Comparison of the viability of heterotrophic bacteria in the food and gut of N. variabilis indicated a decline in viability of up to 99.6% during the passage of bacteria through the gut. The estimated gut retention time was 39 ± 8 min at 18°C and therefore loss of viability is unlikely to be due to long retention in the gut. Loss of viability is, however, a prerequisite of bacterial digestion by the worm and provides circumstantial evidence that ingested bacteria are used as food.No conclusive evidence was obtained indicating (a) preferential ingestion of certain kinds of bacteria, (b) that the gut environment was selectively hostile to the particular kinds of bacteria studied or (c) that worms had a specialized gut micro-flora.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

In nuclear magnetic resonance (NMR) of I=1/2 nuclei that are scalar coupled to quadrupolar spins, a tensor operator product (TOP) basis set provides a convenient description of the time evolution of the density operator. Expressions for the evolution of equivalent I=1/2 spins, coupled to an arbitrary spin S〉1/2, were obtained by explicit algebraic density operator calculations in Mathematica, and specific examples are given for S=1 and S=3/2. Tensor operators are described by the convenient quantum numbers rank and order and this imparts to the TOP basis features that enable an intuitive understanding of NMR behavior of these spin systems. It is shown that evolution as a result of J coupling alone changes the rank of tensors for the coupling partner, generating higher-rank tensors, which allow efficient excitation of S-spin multiple-quantum coherences. Theoretical predictions obtained using the TOP formalism were confirmed using multiple-quantum filtered heteronuclear spin-echo experiments and were further employed to demonstrate polarization transfer directly to multiple-quantum transitions using the insensitive nucleus enhancement by polarization transfer pulse sequence. This latter experiment is the basis of two-dimensional heteronuclear correlation experiments and direct generation of multiple-quantum S-spin coherences can therefore be exploited to yield greater spectral resolution in such experiments. Simulated spectra and experimental results are presented. © 2001 American Institute of Physics.

Electronic Resource

1365-2621

Blackwell Publishing Journal Backfiles 1879-2005

Process Engineering, Biotechnology, Nutrition Technology

A set of seven soft drinks was profiled using a check list of descriptors and the same set of products was assessed for similarity by a panel of ten assessors. The profile data were used to generate a matrix of interstimulus distances, and this and the dissimilarity matrix obtained directly from the similarity assessment were examined using multidimensional scaling approaches (MDS/INDSCAL). The two methods yielded broadly similar results in terms of the spatial representation of the stimuli. This was further confirmed on analysis of the data using a multidimensional unfolding program which aimed to highlight any differences between the two sets of dissimilarity measures.

Electronic Resource

1365-2621

Blackwell Publishing Journal Backfiles 1879-2005

Process Engineering, Biotechnology, Nutrition Technology

Electronic Resource

1750-3841

Blackwell Publishing Journal Backfiles 1879-2005

Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

Process Engineering, Biotechnology, Nutrition Technology

Electronic Resource

0022-1341

Geography