bluuebunny/crossref_metadata_2025 huggingface dataset:
Nondissipative kinetic simulation and analytical solution of three-mode equations of the ion temperature gradient instability
Authors: Watanabe T.-H. , Sugama H. , Sato T.
Published: July 2002

A nondissipative drift kinetic simulation scheme, which rigorously satisfies the time-reversibility, is applied to the three-mode coupling problem of the ion temperature gradient (ITG) instability. It is found from the simulation that the three-mode ITG system repeats growth and decay with a period which shows a logarithmic divergence for infinitesimal initial perturbations. Accordingly, time average of the mode amplitude vanishes, as the initial amplitude approaches zero. An exact solution is analytically given for a class of initial conditions. An excellent agreement is confirmed between the analytical solution and numerical results. The results obtained here provide a useful reference for basic benchmarking of theories and simulations of the ITG modes.
Shear-flow induced stabilization of kinklike modes
Authors: Volponi Francesco , Yoshida Zensho , Tatsuno Tomoya
Published: July 2002

The stabilizing effect of a shear flow on a current-driven instability has been studied by means of Kelvin’s representation of spatially-inhomogeneous Galilean transform. Even though conspicuous transient growth may occur, the mixing effect of the shear flow overcomes the instability and damps kinklike modes.
Near-infrared sensitive electrophotographic photoconductors using chloroindium chlorophthalocyanine
Authors: Kato M. , Nishioka Y. , Kaifu K. , Kawamura K. , Ohno S.
Published: July 2002

Layered electrophotographic photoconductors using chloroindium chlorophthalocyanine have been investigated. An evaporated film of chloroindium chlorophthalocyanine has a broad and strong light absorption band in the visible to near-infrared region and its crystal structure is very stable. The photoconductor using chloroindium chlorophthalocyanine as a carrier generation layer and polyvinylcarbazole as a carrier transport layer is highly sensitive in a wide spectral region (500–900 nm) without any special treatment during its preparation. The typical value of sensitivity is 2.5 cm2/μJ at 800–900 nm, so that this photoconductor is most suitable for a laser printer equipped with a AlGaAs laser diode.
Temperature independent Faraday rotation near the band gap in Cd1−xMnxTe
Authors: Kullendorff Nils , Hök Bertil
Published: July 2002

Faraday rotation spectra have been measured for Cd1−xMnxTe, 0.10≤x≤0.20, for temperatures from 213 to 373 K. At wavelengths well above the fundamental absorption edge the rotation decreases with increasing temperature. The opposite temperature dependence is found for shorter wavelengths since the rotation increases strongly when the wavelength approaches the edge and the edge moves towards longer wavelengths with increasing temperature. For each x a wavelength with temperature independent rotation can be found due to a cancellation of these effects. The large and temperature independent rotation is of great interest for optical devices such as magnetic field sensors, isolators, and modulators. By choosing 0.10≤x≤0.25 these devices would be compatible with GaAlAs emitting diodes and lasers. Optical transmission was also measured and was found to be fairly high at wavelengths of interest. A sensor design is presented, allowing dc response and very small physical dimensions.
Strain-dependent defect formation kinetics and a correlation between flatband voltage and nitrogen distribution in thermally nitrided SiOxNy/Si structures
Authors: Vasquez R. P. , Madhukar A.
Published: July 2002

A correlation between the nitridation condition dependent nitrogen distribution and the magnitude of the flatband voltage shift in SiOxNy/Si structures is identified and an explanation proposed in terms of a strain-dependent kinetics of formation of defects near the interface. The proposed explanation derives from, and is consistent with, the recent demonstration of the influence of the interfacial strain on the nitridation kinetics of thermally nitrided SiOxNy/Si films.
Spatially resolved luminescence near dislocations in In-alloyed Czochralski-grown GaAs
Authors: Hunter A. T.
Published: July 2002

Spatially resolved maps of the near-band-edge band-to-acceptor photoluminescence intensity near isolated dislocations in semi-insulating Czochralski-grown In-alloyed GaAs are presented and discussed. Bright rings ∼150 μm in radius surround 100 μm dark spots centered on dislocations. Far from dislocations the luminescence intensity is two to three orders of magnitude lower than in the rings surrounding dislocations. Spectra from bright and dark regions do not indicate a major difference in In content. The magnitude of the luminescence contrast suggests that it is the lifetime of free carriers rather than the concentration of acceptors involved in the luminescence that directly determines the amount of contrast. The lifetime of electrons may be controlled in turn by the concentration of the positive charge state of the defect causing EL2.
Silicon diffusion at polycrystalline-Si/GaAs interfaces
Authors: Kavanagh K. L. , Mayer J. W. , Magee C. W. , Sheets J. , Tong J. , Woodall J. M.
Published: July 2002

Polycrystalline-Si/GaAs interfaces have been prepared by depositing hydrogenated amorphous Si (a-Si:H) onto GaAs in a silane plasma at 450 °C and annealing at temperatures between 600 and 1020 °C. Rutherford backscattering, secondary ion mass spectroscopy, and transmission electron microscopy show that the resulting polycrystalline-Si/GaAs interface is metallurgically stable when the Si is undoped while significant interdiffusion occurs when 12 at. % As is added to the Si. The data are consistent with the Greiner/Gibbons theory [Appl. Phys. Lett. 44, 750 (1984)] that high concentrations of Si diffuse in GaAs in the form of Si-Si substitutional pairs via Ga and As vacancies.
Observations of argon emission lines used for fluorine atom actinometry in low power rf discharges
Authors: Savas S. E.
Published: July 2002

Anomalous power and frequency dependences of some argon emission lines are observed in a low power rf discharge in a 2% admixture of argon in CF4. Frequency ranges from 0.12 to 13.56 MHz for pressures of 25 and 50 mTorr and 20 W power to the plasma. Power and frequency dependences of the 7504 Å line differ from those for the 6416 Å, 6871 Å, and 7126 Å lines. These differences and argon excitation cross sections are used to examine the validity of actinometry assumptions. The 6871 Å or 6416 Å lines work better under these conditions as actinometer references, while the 7504 Å seems preferable for etch plasmas.
High resolution electron microscopic and spectroscopic characterization of semi-insulating polycrystalline silicon and its interface with single-crystal silicon
Authors: Wong J. , Jefferson D. A. , Sparrow T. G. , Thomas J. M. , Milne R. H. , Howie A. , Koch E. F.
Published: July 2002

A combination of high resolution electron microscopy (HREM), electron energy loss spectroscopy (EELS), energy dispersive x-ray analysis (EDXA), and scanning transmission electron microscopy (STEM) techniques has been employed to characterize the structure and composition of vapor-deposited thin films of semi-insulating polycrystalline silicon (SIPOS) and their interfaces with Si. SIPOS layers containing ∼30 at. % of oxygen are amorphous when deposited at Si substrate temperature of 625 °C. Upon annealing at 900 °C in N2 for 30 min, Si crystallites up to ∼100 Å in size appear throughout the SIPOS layer, the microstructure of which appears to be a dispersion of Si crystallites in an amorphous matrix. The plasmon-loss spectra show a characteristic energy for each material: 16.7 eV for Si, 17.6 eV for SIPOS, and 23.2 eV for SiO2, which correlate empirically with increasing oxygen content in these materials. A combination of HREM and STEM revealed the existence of a 25–30-Å native oxide layer between the SIPOS and Si substrate. This native oxide yields a plasmon loss at 22 eV using a 10-Å probe in the STEM.
Observation of double light pulses in thin-film ZnS:Mn electroluminescent devices
Authors: Bringuier E. , Geoffroy A.
Published: July 2002

Thin-film ZnS:Mn electroluminescent devices can exhibit, under pulsed electrical excitation, a double light pulse, as has been previously observed in other phosphors. We specify the conditions under which this phenomenon appears and relate it to the basic mechanism of carrier emission in the active layer.
Scanning electron beam method for examining surfaces in a short focal length magnetic lens
Authors: Broers A. N. , Coane P. A.
Published: July 2002

A new scanning electron microscope method for examining the surface of bulk samples is described that uses a short focal length (1 mm), low aberration, magnetic electron lens. The minimum beam size produced by the lens is 2–3 times smaller than the minimum beam size in a conventional secondary electron scanning electron microscope. The new method, in which backscattered electrons are detected with silicon diodes located inside the pole-piece gap of the lens, allows large area samples such as silicon integrated circuit wafers to be examined, despite the constricted sample region of the lens. The new method also makes it possible to use this type of short focal length lens for electron beam lithography, because it allows alignment marks on the wafers to be detected while the wafers are immersed in the lens. A 1 mm focal length lens produces a beam current density that is more than 20 times higher than that produced by the longer focal length lenses used in conventional electron beam lithography systems.
Electron states in narrow gate-induced channels in Si
Authors: Laux Steven E. , Stern Frank
Published: July 2002

Self-consistent numerical solutions of the Poisson and Schrödinger equations have been obtained for the states of electrons under a narrow gate or under a narrow slit in a metal-oxide-silicon structure with an inner and an outer gate. For the cases considered (30-nm-wide gate and 80-nm-wide slit), the states of motion parallel to the Si-SiO2 interface are more closely spaced than the states for motion perpendicular to the interface. It should be possible, for the structures considered, to achieve a dynamically one-dimensional system with an electron density ≲106 cm−1.
Low-loss electron images of uncoated photoresist in the scanning electron microscope
Authors: Wells Oliver C.
Published: July 2002

Low voltage scanning electron microscopy is an important part of microelectronic inspection technique. This makes it possible to examine devices without changing the electrical properties, and to examine nonconducting samples such as photoresist without the use of a surface metal layer. The secondary electron imaging method suffers, however, from the difficulty that the image can be spoiled by slight charging of the specimen by the incident electron beam. This problem can be solved by the use of the low-loss electron image.
Free-carrier effects on luminescence linewidths in quantum wells
Authors: Skolnick M. S. , Nash K. J. , Saker M. K. , Bass S. J. , Claxton P. A. , Roberts J. S.
Published: July 2002

The effects of free-carrier broadening on luminescence linewidths in InGaAs-InP quantum wells (QW’s) are demonstrated using Schottky barrier depletion and magnetic field quantization of the conduction-band-energy levels. After removal of free-carrier broadening, linewidths of 5 meV for 100-Å, and 3.5 meV for 150-Å QW’s grown by metalorganic chemical vapor deposition are obtained. Widths of 3.4 meV for a 110-Å QW grown by molecular beam epitaxy (MBE) on an n+ substrate, and only 2.0 meV at 6.2 T for a similar MBE sample grown on a semi-insulating substrate are also reported.
Continuous wave operation of a surface-emitting AlGaAs/GaAs multiquantum well distributed Bragg reflector laser
Authors: Kojima Keisuke , Noda Susumu , Mitsunaga Kazumasa , Kyuma Kazuo , Hamanaka Koichi
Published: July 2002

The first cw operation of an AlGaAs/GaAs distributed Bragg reflector laser was achieved at room temperature with a threshold current as low as 38 mA. Surface emission exceeding 4 mW was obtained with an external differential quantum efficiency of 9% and a beam divergence of 0.17°×10°. A two-dimensional laser array with 3×4 gratings was also fabricated and an output power exceeding 500 mW was obtained under pulsed condition.
Optical frequency-selective amplification in a distributed feedback type semiconductor laser amplifier
Authors: Kawaguchi Hitoshi , Magari Katsuaki , Oe Kunishige , Noguchi Yoshio , Nakano Yoshinori , Motosugi George
Published: July 2002

Optical frequency-selective amplification of an injected optical signal in a distributed feedback type semiconductor laser amplifier is studied. Based on this mechanism an optical demultiplexer with optical gain for optical frequency-division multiplexing is developed. A 9 GHz spectrum selectivity and a 11 dB extinction ratio are observed.
Profile control of SiH radicals by cross magnetic field in plasma processing
Authors: Fujiyama H. , Yamashita T. , Takahashi T. , Matsuo H.
Published: July 2002

The distributions of optical emission intensity along the discharge axis in multiple-parallel-plate ac glow discharges were measured in the presence of a magnetic field perpendicular to the discharge electric field. In a SiH4(10%)/Ar discharge, time-averaged spectroscopic measurements showed that profile control of not only plasmas but also SiH* was possible by varying the field strength. The emission intensity of SiH* was increased with the field strength. These results suggest that the present new configuration with cross magnetic field can make thin films on a large-area substrate outside the multielectrodes uniform and can enhance the deposition rate.
Carbon monoxide laser excited by radio-frequency discharge
Authors: Pearson G. N. , Hall D. R.
Published: July 2002

We report the operation of a carbon monoxide waveguide laser excited by a transverse radio-frequency discharge at 145 MHz. Single mode output powers up to 10 W with efficiencies up to 6.5% have been obtained from a sealed off device having a square section alumina (Al2O3) waveguide and an electrode temperature ≥−23 °C.
Tunable electroabsorption in gallium arsenide doping superlattices
Authors: Chang-Hasnain C. J. , Hasnain G. , Johnson N. M. , Dohler G. H. , Miller J. N. , Whinnery J. R. , Dienes A.
Published: July 2002

Tuning of the optical absorption coefficient by electrical bias in doping superlattices (n-i-p-i crystals) is demonstrated for the first time by both photoconductivity and transmission measurements at room temperature. The absorption coefficient at 1.04 μm obtained from the photoconductivity measurement varies by a factor of 240 when the p-n junction bias is changed from 0.4 to −1.5 V. A change of transmission up to 22% at 0.89 μm wavelength through a 2.1-μm-thick n-i-p-i crystal is achieved by varying bias merely between 0.6 and −2.0 V. Moreover, the change of transmission is nearly linear with p-n junction bias.
Photoluminescence characterization of single heterojunction quantum well structures
Authors: Aina O. , Mattingly M. , Juan F. Y. , Bhattacharya P. K.
Published: July 2002

A photoluminescence emission band at 830 nm has been detected in single heterojunction quantum well structures (modulation-doped structures) in the range of 250–400 K. This emission band is observed neither in heterojunction structures without a two-dimensional electron gas (2DEG), nor in n+ AlGaAs and GaAs. The intensity of the emission band increases as the mobility of the samples with 2DEG and shows excitonic behavior in its variation with incident laser excitation intensity. This photoluminescence emission was observed in samples grown by both molecular beam epitaxy and by organometallic vapor phase epitaxy. This effect may be useful as a rough identification of high quality, modulation-doped heterostructures.
Electric field distribution of electron emitter surfaces
Authors: Tagawa M. , Takenobu S. , Ohmae N. , Umeno M.
Published: July 2002

The electric field distribution of a tungsten field emitter surface and a LaB6 thermionic emitter surface has been studied. The computer simulation of electric field distribution on the emitter surface was carried out with a charge simulation method. The electric field distribution of the LaB6 thermionic emitter was experimentally evaluated by the Schottky plot. Two independent equations are necessary for obtaining local electric field and work function; the Fowler–Nordheim equation and the equation of total energy distribution of emitted electron being used to evaluate the electric field distribution of the tungsten field emitter. The experimental results agreed with the computer simulation.
Phase-locked operation of a grating-surface-emitting diode laser array
Authors: Carlson N. W. , Evans G. A. , Hammer J. M. , Lurie M. , Palfrey S. L. , Dholakia A.
Published: July 2002

A linear array of surface-emitting distributed Bragg reflector AlGaAs diode lasers has been demonstrated. The intensity pattern for five emitting elements had structure with an angular divergence as low as 0.05°, indicating phase locking between the elements of the array.
Abnormal solid solution and activation behavior in Ga-implanted Si(100)
Authors: Matsuo Jiro , Kato Ichiro , Horie Hiroshi , Nakayama Noriaki , Ishikawa Hajime
Published: July 2002

Rapid thermal annealing (RTA) and furnace annealing of Ga-implanted Si were studied. Ga atoms implanted into Si are located on substitutional lattice sites in concentration above the solid solubility limit after short-time and low-temperature annealing. Low-resistivity shallow p+ junctions can be fabricated using this metastable layer. However, precipitation and redistribution of the Ga atoms were observed after high-temperature or longer time annealing. Shallow p+ junctions fabricated by Ga implantation and RTA are suitable for submicron complementary-metal-oxide-semiconductor devices.
Longitudinal electric field effects on GaAs-AlAs type-II superlattices
Authors: Danan G. , Ladan F. R. , Mollot F. , Planel R.
Published: July 2002

We have studied photoluminescence of direct-gap and ‘‘indirect-gap’’ GaAs-AlAs superlattices under electric field perpendicular to the layers. In the former case, the quantum confined Stark effect is observed as expected. In the latter case, reverse effects are found, providing evidence that X-like electron states are confined in the AlAs slabs.
Thermal gas effusion from hydrogenated amorphous carbon films
Authors: Wild Ch. , Koidl P.
Published: July 2002

Hydrogenated amorphous carbon (a-C:H) films, deposited onto negatively biased substrates in a 13.56-MHz hydrocarbon glow discharge system, have been investigated by mass spectroscopic thermal effusion measurements. Depending on the bias voltage UB as the most important deposition parameter, hydrocarbons and/or H2 molecules are desorbed at threshold temperatures between 300 and 600 °C. The threshold temperature increases with increasing bias voltage while the mass of the desorbed molecules decreases. a-C:H films deposited at low bias release H2, CH4, and higher hydrocarbons whereas from hard a-C:H films deposited at UB≳500 V, only H2 is released. Using double-layer a-C:H/a-C:D films, it is shown that H2 and CH4 molecules are formed in the volume of the film followed by molecular diffusion through the a-C:H network. For high-bias voltages (UB≳500 V), the reduced pore size of the strongly crosslinked a-C:H network is shown to prevent diffusion of hydrocarbon molecules, while the films are still permeable for hydrogen.
Fast optical switching and optical bistability in a PbSnSe étalon
Authors: Reid J. J. E. , Kar A. K. , MacKenzie H. A. , Chua P. L. , Grisar R. , Preier H. M.
Published: July 2002

We report the observation of optical bistability with nanosecond optical switching in the lead chalcogenide semiconductor Pb1−xSnxSe. The observed refractive optical nonlinearities were generated by the absorption of radiation from a pulsed CO2 laser at incident intensities of 80 kW cm−2.
Wavelength selective interlayer directionally grating-coupled InP/InGaAsP waveguide photodetection
Authors: Koch T. L. , Corvini P. J. , Tsang W. T. , Koren U. , Miller B. I.
Published: July 2002

We present the first demonstration of interlayer, or vertical, directional grating coupling using a permanently incorporated corrugated layer grating. Based on an InP/InGaAsP antiresonant reflecting optical waveguide (ARROW) geometry grown by chemical beam epitaxy and metalorganic chemical vapor deposition, we have achieved a large mode contradirectional grating-coupled waveguide photodetector with a spectral response peak 9 Å wide. The ‘‘optical bus’’ capabilities of the vertically coupled ARROW structure make this device suitable for optical integration.
Interface states in Bi/Bi1−xSbx heterojunctions
Authors: Agassi D. , Chu T. K.
Published: July 2002

A novel, band-inverted semiconductor junction is proposed. The heterojunction consists of two column V semimetals that have undergone a semimetal-semiconductor transition, i.e., a thin Bi film of thickness ∼100 Å and a Bi1−xSbx alloy with 0.06<x<0.3. Such a junction will support the recently predicted interfacial states. Results of calculations on the dispersion relation and optical transitions pertaining to the interfacial states are presented. It is shown that the optical transition rate between an interfacial state and the conduction state is comparable to that of a conduction-valence band transition. It is also shown that the optical transitions have very interesting polarization dependences.
Masked, anisotropic thermal etching and regrowth for i
n s
i
t
u patterning of compound semiconductors
Authors: Warren A. C. , Woodall J. M. , Fossum E. R. , Pettit G. D. , Kirchner P. D. , McInturff D. T.
Published: July 2002

To produce well-defined device structures having ultrasmall areas in compound semiconductors, it is necessary that lateral variations in doping and composition be realizable having interfacial quality comparable to that presently achieved vertically with techniques such as molecular beam epitaxy (MBE). This letter describes a novel scheme involving the controlled, patterned sublimation of GaAs, termed submicron lithography by molecular evaporation (SUBLIME), which is performed in situ within a MBE growth chamber. The strong orientation dependence of the removal rate from (100) substrates is discussed and final morphology is shown to be critically dependent on both substrate temperature and incident As flux. Several materials have been used successfully as patterned masks, and Al0.3Ga0.7As has been demonstrated to be a versatile SUBLIME ‘‘etch stop.’’ High-resolution micrographs of the resulting submicron structures are shown and initial results from studies of dopant redistribution and in situ regrowth after patterning are presented. Finally, some implications for future device fabrication are discussed.
Room-temperature exciton transitions in partially intermixed GaAs/AlGaAs superlattices
Authors: Ralston J. D. , O’Brien S. , Wicks G. W. , Eastman L. F.
Published: July 2002

Substantial increases are observed in the energies of room-temperature exciton transitions in GaAs/AlGaAs superlattices which have been partially intermixed via the impurity-free vacancy diffusion process. Localized intermixing of the layered structure was accomplished by selective deposition of a SiO2 capping layer followed by rapid thermal annealing at temperatures between 850 and 950 °C for 15 s. In the samples studied, the above process allows continuously variable energy shifts of at least 61 meV while still maintaining clearly resolved excitonic behavior. Shifting and broadening of the exciton transitions are studied using room-temperature photoluminescence and photocurrent spectroscopies. A transmission resonance calculation is used to determine the interdiffusion coefficient as a function of temperature from the measured energy shifts.
Extended infrared response of InAsSb strained-layer superlattices
Authors: Kurtz S. R. , Osbourn G. C. , Biefeld R. M. , Dawson L. R. , Stein H. J.
Published: July 2002

Strained-layer superlattices of InAsSb were grown with low densities of dislocations and microcracks for optical characterization to determine the suitability of these structures for infrared photodetectors. Infrared transmission measurements revealed absorption throughout the 8–12 μm region and extended to longer wavelengths than predicted from consideration of the tensile strain-induced band-gap shift in a type-I superlattice. We conclude that a type-II superlattice occurs in the InAsSb system for alloy compositions >60% Sb.
Investigations of the quantum photovoltaic effect in InAs-GaSb semiconductor superlattices
Authors: Bleuse J. , Voisin P. , Voos M. , Munekata H. , Chang L. L. , Esaki L.
Published: July 2002

We report the observation in InAs-GaSb semiconductor superlattices of a photovoltaic effect that has a quantum origin, as it arises from the spatial separation of the electron and hole ground-state wave functions in these type II superlattices. The results are consistent with the simplest theoretical predictions, except for the voltage sign, which evidences a depopulation of the surface layer.
Structure and superconducting properties of sputtered Gd-Ba-Cu-O thin films
Authors: Kamada Takeshi , Setsune Kentaro , Hirao Takashi , Wasa Kiyotaka
Published: July 2002

Superconducting Gd-Ba-Cu-O thin films were deposited on MgO substrates by rf magnetron sputtering. Preparation temperature dependence on the crystallization behavior and on the superconductive properties of this film have been investigated in correlation with the target-substrate spacing. A sharp superconducting transition with an onset temperature at 94 K and zero resistance (zero ρ) at 86 K was obtained at a substrate temperature of 600 °C with a target-substrate spacing of 25 mm.
Dose effects during solid phase epitaxial regrowth of boron-implanted, germanium-amorphized silicon induced by rapid thermal annealing
Authors: Adekoya W. O. , Hage-Ali M. , Muller J. C. , Siffert P.
Published: July 2002

The effect of implanted boron concentration on the solid phase epitaxial regrowth (SPER) kinetics during rapid thermal annealing (RTA) of 73Ge+-amorphized 〈100〉 silicon in the temperature range 500–575 °C has been studied using Rutherford backscattering and channeling measurements. Relatively flat 11B+ profiles at 5×1019, 1020, and 3×1020 cm−3 were implanted up to a depth ≂0.18 μm into different 73Ge+-preamorphized 75-cm-diam wafers. Following a RTA anneal at 800 °C/10 s (in order to recrystallize the amorphized layer and activate the implanted boron), the wafers were reamorphized with 73Ge+ ions (constant concentration ≂3×1020 cm−3 up to a depth ≂0.2 μm) and the SPER kinetics studied as indicated above. This procedure ensured that the defect densities in the wafers were the same in spite of their different boron doses. Our results clearly show an activation energy reduction from 2.86 eV at 5×1019 atoms/cm3 to 2.6 eV at 3×1020 cm−3 (in spite of the constant defect concentration in the wafers), with a linear relation between the growth rate and the implanted dose throughout the temperature range of study.
Tunneling measurement of the density of states of a superlattice
Authors: England P. , Hayes J. R. , Harbison J. P. , Hwang D. M. , Florez L. T.
Published: July 2002

We have studied the current-voltage characteristics of GaAs/AlGaAs samples consisting of an electron injector, a wide AlGaAs tunnel barrier, and a strongly coupled superlattice. The bias determines the energy of electrons injected into the superlattice, and the resulting tunnel current shows pronounced structure associated with minibands and band gaps. The position of the bands is in quantitative agreement with theory, and we resolve minibands with energies both below and above the AlGaAs conduction-band offset.
InGaAsP/InP buried-heterostructure lasers with concurrent fabrication of the stripes and mirrors
Authors: Yap D. , Walpole J. N. , Liau Z. L.
Published: July 2002

Concurrent fabrication of InGaAsP/InP buried-heterostructure laser stripes and mirrors has been demonstrated by the use of dry etching and mass transport. In comparison to previous processes in which the laser stripes and mirrors are fabricated separately, this process offers the advantages of simplicity, built-in alignment, and improved control of device dimensions. Lasers with a cylindrical and a flat end mirror have threshold currents as low as 22 mA and differential quantum efficiencies as high as 24% per facet. Beam divergences as narrow as 13° are obtained for the cylindrical mirrors.
Structure of neurolysin reveals a deep channel that limits substrate access
Authors: Brown C. Kent , Madauss Kevin , Lian Wei , Beck Moriah R. , Tolbert W. David , Rodgers David W.
Published: July 2002

The zinc metallopeptidase neurolysin is shown by x-ray crystallography to have large structural elements erected over the active site region that allow substrate access only through a deep narrow channel. This architecture accounts for specialization of this neuropeptidase to small bioactive peptide substrates without bulky secondary and tertiary structures. In addition, modeling studies indicate that the length of a substrate N-terminal to the site of hydrolysis is restricted to approximately 10 residues by the limited size of the active site cavity. Some structural elements of neurolysin, including a five-stranded β-sheet and the two active site helices, are conserved with other metallopeptidases. The connecting loop regions of these elements, however, are much extended in neurolysin, and they, together with other open coil elements, line the active site cavity. These potentially flexible elements may account for the ability of the enzyme to cleave a variety of sequences.
Peptide interfacial adsorption is kinetically limited by the thermodynamic stability of self association
Authors: Middelberg Anton P. J. , Radke Clay J. , Blanch Harvey W.
Published: July 2002

We present a study of the adsorption of two peptides at the octane–water interface. The first peptide, Lac21, exists in mixed monomer–tetramer equilibrium in bulk solution with an appreciable monomer concentration. The second peptide, Lac28, exists as a tetramer in solution, with minimal exposed hydrophobic surface. A kinetic limitation to interfacial adsorption exists for Lac28 at moderate to high surface coverage that is not observed for Lac21. We estimate the potential energy barrier for Lac28 adsorption to be 42 kJ/mol and show that this is comparable to the expected free energy barrier for tetramer dissociation. This finding suggests that, at moderate to high surface coverage, adsorption is kinetically limited by the availability of interfacially active monomeric “domains” in the subinterfacial region. We also show how the commonly used empirical equation for protein adsorption dynamics can be used to estimate the potential energy barrier for adsorption. Such an approach is shown to be consistent with a formal description of diffusion–adsorption, provided a large potential energy barrier exists. This work demonstrates that the dynamics of interfacial adsorption depend on protein thermodynamic stability, and hence structure, in a quantifiable way.
Inhibition of chymotrypsin through surface binding using nanoparticle-based receptors
Authors: Fischer Nicholas O. , McIntosh Catherine M. , Simard Joseph M. , Rotello Vincent M.
Published: July 2002

Efficient binding of biomacromolecular surfaces by synthetic systems requires the effective presentation of complementary elements over large surface areas. We demonstrate here the use of mixed monolayer protected gold clusters (MMPCs) as scaffolds for the binding and inhibition of chymotrypsin. In these studies anionically functionalized amphiphilic MMPCs were shown to inhibit chymotrypsin through a two-stage mechanism featuring fast reversible inhibition followed by a slower irreversible process. This interaction is very efficient, with aK\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{i}^{app}}}\end{equation*}\end{document}= 10.4 ± 1.3 nM. The MMPC–protein complex was characterized by CD, demonstrating an almost complete denaturation of the enzyme over time. Dynamic light scattering studies confirm that inhibition proceeds without substantial MMPC aggregation. The electrostatic nature of the engineered interactions provides a level of selectivity: little or no inhibition of function was observed with elastase, β-galactosidase, or cellular retinoic acid binding protein.
CPP1, a DNA-binding protein involved in the expression of a soybean
leghemoglobin c3
gene
Authors: Cvitanich Cristina , Pallisgaard Niels , Nielsen Kirsten A. , Hansen Anette Chemnitz , Larsen Knud , Pihakaski-Maunsbach Kaarina , Marcker Kjeld A. , Jensen Erik Østergaard
Published: July 2002

Nodulin genes are specifically expressed in the nitrogen-fixing root nodules. We have identified a novel type of DNA-binding protein (CPP1) interacting with the promoter of the soybean leghemoglobin gene
Gmlbc3
. The DNA-binding domain of CPP1 contains two similar Cys-rich domains with 9 and 10 Cys, respectively. Genes encoding similar domains have been identified in
Arabidopsis thaliana, Caenorhabditis elegans,
the mouse, and human. The domains also have some homology to a Cys-rich region present in some polycomb proteins. The
cpp1
gene is induced late in nodule development and the expression is confined to the distal part of the central infected tissue of the nodule. A constitutively expressed
cpp1
gene reduces the expression of a
Gmlbc3
promoter–
gusA
reporter construct in
Vicia hirsuta
roots. These data therefore suggest that CPP1 might be involved in the regulation of the
leghemoglobin
genes in the symbiotic root nodule.
Solution structures of the cytoplasmic tail complex from platelet integrin αIIb- and β3-subunits
Authors: Weljie Aalim M. , Hwang Peter M. , Vogel Hans J.
Published: July 2002

Integrin adhesion receptors constitute a cell-signaling system whereby interactions in the small cytoplasmic domains of the heterodimeric α- and β-subunits provoke major functional alterations in the large extracellular domains. With two-dimensional NMR spectroscopy, we examined two synthetic peptides [αIIb(
987
MWKVGFFKRNR) and β3(
716
KLLITIHDRKEFAKFEEERARAKWD)] encompassing the membrane-proximal regions of the cytoplasmic domain motifs from the platelet integrin complex αΙΙbβ3. These membrane-proximal regions contain two conserved motifs, represented by
989
KVGFFKR in the αIIb-subunit, and
716
KLLITIHDR in the β3-subunit. The dimer interaction consists of two adjacent helices with residues V990 and F993 of the αΙΙb-subunit heavily implicated in the dimer interfacial region, as is I719 of β3. These residues are situated within the conserved motifs of their respective proteins. Further structural analysis of this unique peptide heterodimer suggests that two distinct conformers are present. The major structural difference between the two conformers is a bend in the β3-peptide between D723 and A728, whereas the helical character in the other regions remains intact. Earlier mutational analysis has shown that a salt bridge between the side chains of αΙΙb(R955) and β3(D723) is formed. When this ion pair was modeled into both conformers, increased nuclear Overhauser effect violations suggested that the more bent structure was less able to accommodate this interaction. These results provide a molecular level rationalization for previously reported biochemical studies, as well as a basis for an atomic level understanding of the intermolecular interactions that regulate integrin activity.
Diffractive optics-based heterodyne-detected four-wave mixing signals of protein motion: From “protein quakes” to ligand escape for myoglobin
Authors: Dadusc Gami , Ogilvie Jennifer P. , Schulenberg Peter , Marvet Una , Miller R. J. Dwayne
Published: July 2002

Ligand transport through myoglobin (Mb) has been observed by using
optically heterodyne-detected transient grating spectroscopy.
Experimental implementation using diffractive optics has provided
unprecedented sensitivity for the study of protein motions by enabling
the passive phase locking of the four beams that constitute the
experiment, and an unambiguous separation of the Real and Imaginary
parts of the signal. Ligand photodissociation of carboxymyoglobin
(MbCO) induces a sequence of events involving the relaxation of the
protein structure to accommodate ligand escape. These motions show up
in the Real part of the signal. The ligand (CO) transport process
involves an initial, small amplitude, change in volume, reflecting the
transit time of the ligand through the protein, followed by a
significantly larger volume change with ligand escape to the
surrounding water. The latter process is well described by a single
exponential process of 725 ± 15 ns at room temperature. The
overall dynamics provide a distinctive signature that can be understood
in the context of segmental protein fluctuations that aid ligand escape
via a few specific cavities, and they suggest the existence of discrete
escape pathways.
WWOX
: A candidate tumor suppressor gene involved in multiple tumor types
Authors: Paige Adam J. W. , Taylor Karen J. , Taylor Claire , Hillier Stephen G. , Farrington Susan , Scott Diane , Porteous David J. , Smyth John F. , Gabra Hani , Watson J. E. Vivienne
Published: July 2002

We previously reported the construction of a P1-derived artificial
chromosome (PAC) contig encompassing a set of homozygous deletions of
chromosome 16q23–24.1 found in primary ovarian tumor material and
several tumor cell lines. Using these PAC clones in a cDNA selection
experiment, we have isolated a
Sau
3A fragment homologous
to the
WWOX
transcript (GenBank accession no.
AF211943
)
from normal human ovarian surface epithelial (HOSE) cells. We
demonstrate the homozygous deletion of
WWOX
exons from
ovarian cancer cells and three different tumor cell lines. We also
identify an internally deleted
WWOX
transcript from a
further primary ovarian tumor. In three of these samples the deletions
result in frameshifts, and in each case the resulting
WWOX
transcripts lack part, or all, of the short chain
dehydrogenase domain and the putative mitochondrial localization
signal. Sequencing revealed several missense polymorphisms in tumor
cell lines and identified a high level of single nucleotide
polymorphism (SNP) within the
WWOX
gene. This evidence
strengthens the case for
WWOX
as a tumor suppressor gene
in ovarian cancer and other tumor types.
Structure of rat BCKD kinase: Nucleotide-induced domain communication in a mitochondrial protein kinase
Authors: Machius Mischa , Chuang Jacinta L. , Wynn R. Max , Tomchick Diana R. , Chuang David T.
Published: July 2002

Mitochondrial protein kinases (mPKs) are molecular switches that
down-regulate the oxidation of branched-chain α-ketoacids and
pyruvate. Elevated levels of these metabolites are implicated in
disease states such as insulin-resistant Type II diabetes,
branched-chain ketoaciduria, and primary lactic acidosis. We report a
three-dimensional structure of a member of the mPK family, rat
branched-chain α-ketoacid dehydrogenase kinase (BCK). BCK features a
characteristic nucleotide-binding domain and a four-helix bundle
domain. These two domains are reminiscent of modules found in protein
histidine kinases (PHKs), which are involved in two-component signal
transduction systems. Unlike PHKs, BCK dimerizes through direct
interaction of two opposing nucleotide-binding domains. Nucleotide
binding to BCK is uniquely mediated by both potassium and magnesium.
Binding of ATP induces disorder–order transitions in a loop region
at the nucleotide-binding site. These structural changes lead to the
formation of a quadruple aromatic stack in the interface between the
nucleotide-binding domain and the four-helix bundle domain, where they
induce a movement of the top portion of two helices. Phosphotransfer
induces further ordering of the loop region, effectively trapping the
reaction product ADP, which explains product inhibition in mPKs. The
BCK structure is a prototype for all mPKs and will provide a framework
for structure-assisted inhibitor design for this family of kinases.
Poly(A)-independent regulation of maternal
hunchback
translation in the
Drosophila
embryo
Authors: Chagnovich Daniel , Lehmann Ruth
Published: July 2002

Development of the
Drosophila
abdomen requires
repression of maternal
hunchback
(
hb
)
mRNA translation in the posterior of the embryo. This regulation
involves at least four components:
nanos
response
elements within the
hb
3′ untranslated region and the
activities of Pumilio (PUM), Nanos (NOS), and Brain tumor. To study
this regulation, we have developed an RNA injection assay that
faithfully recapitulates the regulation of the endogenous
hb
message. Previous studies have suggested that NOS and
PUM can regulate translation by directing poly(A) removal. We have
found that RNAs that lack a poly(A) tail and cannot be polyadenylated
and RNAs that contain translational activating sequences in place of
the poly(A) tail are still repressed in the posterior. These data
demonstrate that the poly(A) tail is not required for regulation and
suggest that NOS and PUM can regulate
hb
translation by
two mechanisms: removal of the poly(A) tail and a poly(A)-independent
pathway that directly affects translation.
A chemical inhibitor of N-WASP reveals a new mechanism for targeting protein interactions
Authors: Peterson Jeffrey R. , Lokey R. Scott , Mitchison Timothy J. , Kirschner Marc W.
Published: July 2002

Cell morphology and motility are governed largely by complex signaling networks that ultimately engage the actin cytoskeleton. Understanding how individual circuits contribute to the process of forming cellular structures would be aided greatly by the availability of specific chemical inhibitors. We have used a novel chemical screen in
Xenopus
cell-free extracts to identify compounds that inhibit signaling pathways regulating actin polymerization. Here we report the results of a high-throughput screen for compounds that inhibit phosphatidylinositol 4,5-bisphosphate (PIP
2
)-induced actin assembly and the identification of the first compound, a cyclic peptide, known to block actin assembly by inhibiting an upstream signaling component. We identify the target of this compound as N-WASP, a protein that has been investigated for its role as a node interconnecting various actin signaling networks. We show that this compound prevents activation of the Arp2/3 complex by N-WASP by allosterically stabilizing the autoinhibited conformation of N-WASP.
Temporal distribution of the ganglion cell volleys in the normal rat optic nerve
Authors: Galambos Robert , Szabó-Salfay Orsolya , Barabás Péter , Pálhalmi János , Szilágyi Nóra , Juhász Gábor
Published: July 2002

We describe experiments on behaving rats with electrodes implanted
on the cornea, in the optic chiasm, and on the visual cortex; in
addition, two red light-emitting diodes (LED) are permanently attached
to the skull over the left eye. Recordings timelocked to the LED
flashes reveal both the local events at each electrode site and the
orderly transfer of visual information from retina to cortex. The major
finding is that every stimulus, regardless of its luminance, duration,
or the state of retinal light adaptation, elicits an optic nerve volley
with a latency of about 10 ms and a duration of about 300 ms. This
phenomenon has not been reported previously, so far as we are aware. We
conclude that the retina, which originates from the forebrain of the
developing embryo, behaves like a typical brain structure: it
translates, within a few hundred milliseconds, the chemical information
in each pattern of bleached photoreceptors into a corresponding pattern
of ganglion cell neuronal information that leaves via the optic nerve.
The attributes of each rat ganglion cell appear to include whether the
retinal neuropile calls on it to leave after a stimulus and, if so
when, within a 300-ms poststimulus epoch. The resulting retinal
analysis of the scene, on arrival at the cortical level, is presumed to
participate importantly in the creation of visual perceptual
experiences.
Expression of a single-chain HLA class I molecule in a human cell line: presentation of exogenous peptide and processed antigen to cytotoxic T lymphocytes.
Authors: Toshitani K , Braud V , Browning M J , Murray N , McMichael A J , Bodmer W F
Published: July 2002

We have synthesized a recombinant gene encoding a single-chain HLA-A2/beta 2-microglobulin (beta 2m) molecule by linking beta 2m through its carboxyl terminus via a short peptide spacer to HLA-A2 (A*0201). This gene has been expressed in the beta 2m-deficient colorectal tumor cell line DLD-1. Transfection of this cell with the single-chain construct was associated with conformationally correct cell surface expression of a class I molecule of appropriate molecular mass. The single-chain HLA class I molecule presented either exogenously added peptide or (after interferon-gamma treatment) endogenously processed antigen to an influenza A matrix-specific, HLA-A2-restricted cytotoxic T-lymphocyte line. The need for interferon gamma for the processing and presentation of endogenous antigen suggests that DLD-1 has an antigen-processing defect that can be up-regulated, a feature that may be found in other carcinomas. Our data indicate that single-chain HLA class I constructs can form functional class I molecules capable of presenting endogenously processed antigens. Such molecules should be of use for functional studies, as well as providing potential anticancer immunotherapeutic agents or vaccines.
Predominant role of alpha 4-integrins for distinct steps of lymphoma metastasis.
Authors: Gosslar U , Jonas P , Luz A , Lifka A , Naor D , Hamann A , Holzmann B
Published: July 2002

To analyze the role of alpha4-integrins in lymphoma metastasis, sublines of the T-cell lymphoma LB were generated by retrovirus-mediated gene transfer that differ exclusively in the expression of alpha4-integrins. Using LB-alpha4 and control LB-NTK cells, we demonstrate that expression of alpha4-integrins strongly suppresses metastasis formation of LB lymphoma cells in secondary lymphoid organs such as spleen, mesenteric and peripheral lymph nodes, or Peyer's patches after i.v. injection into syngeneic BALB/c mice. Moreover, alpha4-integrin expression inhibited development of metastatic tumors in liver, lung, and kidney. Expansion of LB lymphoma cells in bone marrow was not affected by alpha4-integrin expression. In vivo migration assays using 51Cr-labeled lymphoma cells demonstrated that low-metastatic LB-alpha4 cells accumulated with the same efficiency as high-metastatic LB-NTK cells in all target organs examined and were even enriched in mucosal lymphoid organs. Collectively, these results indicate that alpha4-integrins inhibit metastasis formation of lymphoma cells at a stage subsequent to the invasion of target organs.
Mutations in the rotated abdomen locus affect muscle development and reveal an intrinsic asymmetry in Drosophila.
Authors: Martín-Blanco E , García-Bellido A
Published: July 2002

In bilateral animals, the left and right sides of the body usually present asymmetric structures, the genetic bases of whose generation are still largely unknown [CIBA Foundation (1991) Biological Asymmetry and Handedness, CIBA Foundation Symposium 162 (Wiley, New York), pp. 1-327]. In Drosophila melanogaster, mutations in the rotated abdomen (rt) locus cause a clockwise helical rotation of the body. Even null alleles are viable but exhibit defects in embryonic muscle development, rotation of the whole larval body, and helical staggering of cuticular patterns in abdominal segments of the adult. rotated abdomen is expressed in the embryonic mesoderm and midgut but not in the ectoderm; it encodes a putative integral membrane glycoprotein (homologous to key yeast mannosyltransferases). Mesodermal cells defective in O-glycosylation lead to an impaired larval muscular system. We propose that the staggering of the adult abdominal segments would be a consequence of the relaxation of intrinsic rotational torque of muscle architecture, preventing the colateral alignment of the segmental histoblast cells during their proliferation at metamorphosis.