bluuebunny/crossref_metadata_2025 huggingface dataset:
Space charge effects in projection charged particle lithography systems
Authors: Harriott L. R. , Berger S. D. , Liddle J. A. , Watson G. P. , Mkrtchyan M. M.
Published: July 2002

Charged particle image projection lithography systems have been proposed and are currently under development for design rules of 0.18 μm and below. Although charged particle projection lithography systems do not suffer from diffraction as a limit to spatial resolution as in photolithography, image degradation due to the effects of mutual repulsion of particles in the beam, space charge, will ultimately limit the performance of these systems. Space charge effects increase with increasing beam current. The uncorrectable image blur caused by space charge effectively reduces the dose latitude in projection charged particle lithography and therefore limits the ultimate throughput of such systems. We will describe the effects of space charge in charged particle projection lithography systems using a model we have previously developed. We will compare the predictions of the model with experimental data for an ion projection system and predict the performance of electron and ion beam systems under development. The predictions for image blur caused by space charge are used to calculate the dose latitude as a function of beam current and thus throughput for the SCALPEL(R) electron beam lithography system.
High resolution x-ray diffraction studies of AlGaP grown by gas-source molecular-beam epitaxy
Authors: Bi W. G. , Deng F. , Lau S. S. , Tu C. W.
Published: July 2002

High-resolution x-ray measurements have been performed to study the lattice deformation and the Al composition determination of AlGaP grown on (100) GaP by gas-source molecular-beam epitaxy. By using {511} asymmetric reflections, lattice constants in both the growth direction a⊥ and the growth plane direction a∥ were determined. The results show that the AlGaP unit cell deforms from cubic to tetragonal with a⊥ being larger than a∥, and that correlations are needed when using x-ray (400) reflection to determine the Al composition. By using the elastic theory and Rutherford backscattering spectrometry technique, the Al mole concentration and its relationship with the perpendicular lattice mismatch measured from x-ray (400) reflections have been determined.
Structure imaging by atomic force microscopy and transmission electron microscopy of different light emitting species of porous silicon
Authors: Sassaki R. Massami , Douglas R. A. , Kleinke M. U. , Teschke O.
Published: July 2002

The complex pattern of the nanowire skeletons of different light emitting porous silicon structures is investigated by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Diffraction lines and dark field images are used to identify and determine the crystallite specimen long range order. TEM images give the size and particle orientation, and AFM images show a three-dimensional pattern formed by an interconnecting skeleton of particles. Near infrared photoluminescent porous silicon (0.006 Ω cm) structures show a skeleton of nanosized silicon aggregates which form domains of spatially oriented crystallites. For red photoluminescent samples (4.9 Ω cm) the electron diffraction spots are discontinuously split into tiny intensity maxima. The diameter of the wire structure forming porous silicon as measured by TEM allows us to estimate the distortion of the AFM images due to the finite size of the tip radius. A critical angle α0=2 arctan[K/(1−K)]1/2, where K is the ratio of the height of the structure to the tip diameter was defined and it was shown that for structure walls steeper than α0 the distortion may be substantial.
Interface states of ZnSe/GaAs interface
Authors: Yang Z. , Sou I. K. , Yeung Y. H. , Wong G. K. L. , Wang Jie , Jin Cai-xia , Hou Xiao-yuan
Published: July 2002

The in-plane anisotropy (εx≠εy) and the off-plane anisotropy (εx≠εz) of ZnSe/GaAs interfaces formed under conditions that promote the formation of either Zn–As or Ga–Se bonds are studied by reflectance difference spectroscopy. Two resonance lines, one at 2.70 eV and the other around 3.0 eV, have been observed. The in-plane anisotropy is along the [110] and the [11_0] principal axes. The dependence of the resonances on interface formation conditions, the results of the photoreflectance spectroscopy, and the annealing experiments all suggest that the anisotropy is not due to the electro-optic effect resulting from an interface electric field. The reflectance difference spectroscopy results are consistent with the assumption that the anisotropy is the intrinsic properties of ordered ZnSe/GaAs heterovalent interface, that the resonance at 2.70 eV is associated with the interface state of Zn–As bonds and the resonance near 3.0 eV is associated with the interface state of Ga–Se bonds. The presence of a thin layer of S at the ZnSe/GaAs interfaces results in a third resonance at 3.2 eV, probably due to the Ga–S bonds. The resonance line shape changes as the top layer thickness varies. Such change can be well explained by a three-layer system with the conventional Fresnel optics and is understood as due to the ZnSe top layer. It is our hope that our results will stimulate more interest in the theoretical study of this interface.
Intrinsic defects at TiO2(110) surfaces studied with scanning tunneling microscopy
Authors: Fischer Stefan , Munz Andreas W. , Schierbaum Klaus-Dieter , Göpel Wolfgang
Published: July 2002

Intrinsic defects of the nearly stoichiometric (110) surface of TiO2 have been studied by means of scanning tunneling microscopy. Temperature treatment at 870≤T≤1070 K in ultrahigh vacuum induces changes of the orientation of single steps. Line defects along [001] and oxygen vacancies are formed simultaneously as predominant surface defects under these conditions. Atomically resolved scanning tunneling microscopy images make it possible to determine their geometric structures. Simple structural models are derived to explain the high stability of electrostatically neutral [001] single steps and [001] line defects at TiO2(110).
Electron cyclotron resonance etching of III–V nitrides in IBr/Ar plasmas
Authors: Vartuli C. B. , Pearton S. J. , Lee J. W. , MacKenzie J. D. , Abernathy C. R. , Shul R. J.
Published: July 2002

The etch characteristics of GaN, InN, InAlN, AlN, and InGaN were measured for a new plasma chemistry, IBr/Ar, in an electron cyclotron resonance plasma discharge. The effects of plasma composition (12.5%–100% IBr), microwave (400–1000 W) and rf power (50–250 W) on the etch rates for these materials were examined. The etch rates for GaN depended strongly on plasma composition, while the etch rates were only weakly dependent on microwave power in the range 400–800 W for all materials. The etch rates for all materials generally increased with increasing rf power, indicating that higher ion energies are much more efficient in enhancing sputter desorption of the etch products. While the etch rates were slower than with either ICl/Ar or Cl2/Ar, the etched surface of GaN was found to be extremely smooth, with little loss of N from the surface at low rf powers, and no significant residue on the surface.
Development of a metal patterned cantilever for scanning capacitance microscopy and its application to the observation of semiconductor devices
Authors: Yamamoto Takuma , Suzuki Yoshihiko , Miyashita Masayuki , Sugimura Hiroyuki , Nakagiri Nobuyuki
Published: July 2002

The performance of scanning capacitance microscopy (SCM) strongly depends on the probe used. We have developed an original probe suitable for SCM (SCM probe), which uses a pyramid-shaped metal tip and metal lead line patterned on a silicon nitride cantilever. We installed the SCM probe on a SCM based on a commercial atomic force microscope (AFM). Differences in silicon oxide thickness and differences of dopant types and densities in a silicon substrate with a thermal oxide layer were successfully imaged by SCM simultaneously with AFM using the SCM probe. Boundaries between different dopant types and densities, which were not recognizable by AFM, were clearly observed by SCM. Signal-to-noise ratio and reproducibility were improved in the SCM images obtained with the SCM probe when compared with images obtained with a metal-coated silicon nitride cantilever.
Explanation of the linear correlation between barrier heights and ideality factors of real metal-semiconductor contacts by laterally nonuniform Schottky barriers
Authors: Schmitsdorf R. F. , Kampen T. U. , Mönch W.
Published: July 2002

A new and simple-to-use method to obtain homogeneous Schottky barrier heights from effective barrier heights and ideality factors that are determined from current-voltage (I-V) characteristics of metal-semiconductor contacts is presented. This approach is justified by a theory of metal-semiconductor interfaces with laterally inhomogeneous distributions of barrier heights. Effective barrier heights and ideality factors were determined from I-V characteristics of Si and GaN Schottky contacts and a linear reduction of the effective barrier heights with increasing ideality factors was always observed. These findings are explained by numerical simulations of inhomogeneous Schottky contacts which are based on theoretical results by Tung [Phys. Rev. B 45, 13509 (1992)]. The homogeneous barrier heights of metal-semiconductor contacts are obtained by a linear extrapolation of the effective barrier heights to nif ≅ 1.01, the value of the ideality factor characteristic for image-force lowering of Schottky barriers only.
Role of Al content on surface structure evolution of low temperature AlxGa1−xAs and its effect on critical thickness
Authors: Carlin J. A. , Ringel S. A. , Sacks R. N. , Yap K. S.
Published: July 2002

Reflection high energy electron diffraction (RHEED) has been used to track the evolution of both low temperature (LT) GaAs and LT-AlxGa1−xAs growth surfaces. LT-AlxGa1−xAs surfaces have been found to roughen by a pyramidal defect formation similar to that reported for LT-GaAs over the entire range of aluminum compositions. Correlations between RHEED and transmission electron microscopy (TEM) results indicate that the critical thickness found using the formation of extra fourth order diffraction spots as a marker correlate well to the nucleation of pyramidal defects. Additionally, results for the critical thickness of layers grown over the range of aluminum compositions indicate an exponential decrease in the critical thickness with increasing aluminum composition under similar growth conditions. While the mechanism for this dependence is still unclear, it does not appear to be related to an increase in the excess arsenic incorporation with increasing Al composition.
Reduced 980 nm laser facet absorption by band gap shifted extended cavities
Authors: Piva P. G. , Goldberg R. D. , Mitchell I. V. , Fafard S. , Dion M. , Buchanan M. , Charbonneau S. , Hillier G. , Miner C.
Published: July 2002

Reflectance modulation thermography has been used to determine facet temperatures of InGaAs/GaAs double quantum well (QW) GRINSCH ridge-waveguide lasers possessing band gap shifted extended cavities (BSECs). The incorporation of BSECs produced by mega-electron-volt ion-implantation enhanced QW intermixing, significantly decreased the laser facet temperatures and should result in increased device longevity prior to the onset of catastrophic mirror failure. Low energy implants in Al-free InGaAs/InGaAsP/InGaP laser structures exhibited large effective diffusivities of intermixing enhancing defects from the implant damage regions. This latter material system is particularly well suited for the implementation of BSECs as end of range damage from the implant can be kept spatially isolated from the optical mode regions.
Self-heating effects in a InP/CdS/LaS cold cathode
Authors: Malhotra Aashish , Modukuru Yamini , Cahay Marc
Published: July 2002

We analyze the importance of self-heating effects in a new cold cathode emitter which consists of a thin region of cadmium sulfide (CdS) sandwiched between a heavily doped indium phosphide (InP) substrate and a low work function lanthanum sulfide (LaS) semimetallic thin film. We identify the mechanisms leading to power dissipation in a cathode with an emission window of rectangular geometry, including the effects of inelastic scattering in the CdS and LaS layers. The latter are modeled using a temperature dependent mean-free path approach. The analysis includes the effects of current crowding in the emission window due to the finite resistivity of the LaS layer. We determine the cathode parameters which minimize current crowding and self-heating effects in the cathode. We also calculate the relative contribution of the ballistic and inelastic portions of the electron current emitted into vacuum.
Deuterium sintering of silicon-on-insulator structures: D diffusion and replacement reactions at the SiO2/Si interface
Authors: Wallace R. M. , Chen P. J. , Archer L. B. , Anthony J. M.
Published: July 2002

We use dynamic secondary ion mass spectrometry (SIMS) to examine the mechanism of H (D) incorporation into and retention within a buried SiO2 film at 625 °C. We find that diffusion of H2 (D2) through the Si/SiO2/Si structure at this temperature is facile and that isotopic exchange occurs at the interfaces upon subsequent forming gas anneals at 625 °C. A detailed examination of the isotopic exchange process indicates that the interfaces do not exhibit equivalent behavior. We also describe the artifacts observed in the SIMS profiles by comparing positive and negative secondary ion profiles.
Fabrication of three-dimensional microstructures by high resolution x-ray lithography
Authors: Cuisin C. , Chen Y. , Decanini D. , Chelnokov A. , Carcenac F. , Madouri A. , Lourtioz J. M. , Launois H.
Published: July 2002

We describe the fabrication of three-dimensional (3D) microstructures by high-resolution x-ray lithography (XRL) as the first step in the realization of photonic band gap (PBG) materials for infrared wavelengths. In the soft x-ray range (λ∼1 nm), conventional XRL mask technology can be adopted which allowed us to pattern 3D features with relatively thick resist layers. We have checked the possibility of making diamond-like PBG microstructures by three consecutive exposures under tilt angle of 35.26° with respect to the incoming beam and a rotation angle of 120° between each exposure. Exposures were done in a vacuum chamber with a broadband synchrotron radiation (SuperACO) and three types of experimental configurations. As results, we obtain diamond-like structures showing several (111) lattice periods in polymethyl methacrylate resist.
p-type doping of (Zn,Mg,Cd)Se alloys using a radio frequency discharge nitrogen plasma source
Authors: Lin W. , Guo S. P. , Tamargo M. C.
Published: July 2002

(Zn,Mg,Cd)Se quaternary alloys lattice matched to InP, a novel material for the fabrication of light emitting devices with light emission spanning the visible range, are difficult to dope p-type while their n-type doping is relatively easy. To address this issue, the p-type doping of the Zn1−xMgxSe and Zn1−yCdySe ternaries was investigated as a function of composition using a radio frequency discharge nitrogen plasma source. Besides conventional (uniform) doping, a type of δ doping that involves codoping with N and Te [(N+Te)δ doping] was performed. The p-type doping level was increased by the (N+Te)δ doping, however, the net acceptor concentration (NA–ND) of both ternaries decreased with the addition of either Mg or Cd, indicating that both Mg and Cd atoms hinder the p-type doping of the alloys. To overcome these doping difficulties, we propose the growth of a Zn1−x′−y′Mgx′Cdy′Se/ZnSe:N superlattice structure, in which the p-type doping is selectively performed only in the ZnSe region, as a substitute for the quaternary alloy.
Organic Carbon Composition of Marine Sediments: Effect of Oxygen Exposure on Oil Generation Potential
Authors: Gélinas Yves , Baldock Jeffrey A. , Hedges John I.
Published: July 2002

Anaerobic sedimentary conditions have traditionally been linked to the generation of the source rocks for petroleum formation. However, the influence of sedimentary redox conditions on the composition of freshly deposited organic matter (OM) is not clear. We assessed the effect of in situ exposure time to oxic conditions on the composition of OM accumulating in different coastal and deep-sea sediments using solid-state
13
C nuclear magnetic resonance (NMR).
13
C NMR spectra were resolved into mixtures of model components to distinguish between alkyl carbon present in protein and nonprotein structures. There is an inverse relation between the length of exposure to oxic conditions and the relative abundance of nonprotein alkyl (alkyl
NP
) carbon, whose concentration is two orders of magnitude higher in coastal sediments with short exposure times than in deep-sea sediments with long exposure times. All alkyl
NP
-rich samples contain a physically separate polymethylene component similar in composition to algaenans and kerogens in type I oil shales. The duration of exposure to oxic conditions appears to directly influence the quality and oil generation potential of OM in marine shales.
Cold Dark Matter
Authors: Dodelson Scott , Gates Evalyn I. , Turner Michael S.
Published: July 2002

Motivated by inflation, the theory of big-bang nucleosynthesis, and the quest for a deeper understanding of fundamental forces and particles, a paradigm for the development of structure in the universe has evolved. It holds that most of the matter exists in the form of slowly moving elementary particles left over from the earliest moments—cold dark matter—and that the small density inhomogeneities that seed structure formation arose from quantum fluctuations around 10
−34
seconds after the big bang. A flood of observations, from determinations of the Hubble constant to measurements of the anisotropy of cosmic background radiation, are now testing the cold dark matter paradigm.
Signaling Across Membranes—A One and a Two and a …
Authors: Stock Jeff
Published: July 2002

The usually accepted mechanisms for transduction of a signal across the membrane require that the receptor exist as a dimer. New results in this issue by Tatsuno
et al
. (
p. 423
) and Gardina
et al
. (
p. 425
) show that the bacterial chemotaxis receptor Tar can actually function quite well if its intracellular domain is monomeric. In his Perspective, Stock describes how current assumptions about the role of dimers in receptor signal transduction may be too simple, and that at least sometimes, monomeric forms of the receptor may be the functionally relevant molecule.
Time-Resolved Observations of Jupiter's Far-Ultraviolet Aurora
Authors: Ballester Gilda E. , Clarke John T. , Trauger John T. , Harris Walter M. , Stapelfeldt Karl R. , Crisp David , Evans Robin W. , Burgh Eric B. , Burrows Christopher J. , Casertano Stefano , Gallagher John S. , Griffiths Richard E. , Hester J. Jeff , Hoessel John G. , Holtzman Jon A. , Krist John E. , Meadows Vikki , Mould Jeremy R. , Sahai Raghvendra , Scowen Paul A. , Watson Alan M. , Westphal James A.
Published: July 2002

Simultaneous imaging and spectroscopic observations of Jupiter's far-ultraviolet aurora covering half a jovian rotation were made on 31 May 1994. The Hubble Space Telescope Wide Field Planetary Camera 2 images revealed dramatic and rapidly changing auroral features, including discrete longitudinal structures along the auroral ovals, with variable contrast; a poleward offset in a north oval sector, showing equatorward motion near dusk; emissions polewards of the ovals, apparently co-rotating; and a bright event developing near the dawn limb. Viewing geometry effects explain the rotational intensity modulation observed by the International Ultraviolet Explorer, without intrinsic longitudinal asymmetries.
The Genome Program's Conscience
Authors: Marshall Eliot
Published: July 2002

A research program on the ethical, legal, and social implications of genome studies, launched as an “afterthought” to the Human Genome Project, is now the world's biggest bioethics program. Experts are divided, however, on how effective the program has been in helping develop policies to guide the genetics revolution.
Coming to Grips With Genes and Risk
Authors: Kahn Patricia
Published: July 2002

The identification of two genes,
BRCA1
and
BRCA2
, linked to hereditary breast and ovarian cancers raised hopes for improved prevention and treatment. But now that tests for mutations in these genes are coming on the market, reality is setting in. Too little is known about the mutations to give women carrying them a precise assessment of what they mean: Are these women almost certain to get cancer, or are the odds less grim? Clear answers will require huge studies on large numbers of women and their families, which are just beginning.
Duality in Perspective
Authors: Girvin S. M.
Published: July 2002

Electric and magnetic fields are related by a property known as duality symmetry. Likewise, in superconductors, the electron pairs that give rise to electric current without resistance are related to objects known as vortices by a dual symmetry. In his Perspective, Girvin discusses results reported by Shahar
et al
. (
p. 589
) that show evidence for this dual symmetry in superconducting films. The experimental demonstration of this symmetry provides fundamental insights at the critical point where an insulator switches to a superconductor.
Far-Infrared Emission Spectra of Selected Gas-Phase PAHs: Spectroscopic Fingerprints
Authors: Zhang K. , Guo B. , Colarusso P. , Bernath P. F.
Published: July 2002

The emission spectra of the gaseous polycyclic aromatic hydrocarbons (PAHs) naphthalene, chrysene, and pyrene were recorded in the far-infrared (far-IR) region. The vibrational bands that lie in the far IR are unique for each PAH molecule and allow discrimination among the three PAH molecules. The far-IR PAH spectra, therefore, may prove useful in the assignment of unidentified spectral features from astronomical objects.
Seismic Evidence for a Low-Velocity Zone in the Upper Crust Beneath Mount Vesuvius
Authors: Zollo A. , Gasparini P. , Virieux J. , le Meur H. , de Natale G. , Biella G. , Boschi E. , Capuano P. , de Franco R. , dell'Aversana P. , de Matteis R. , Guerra I. , Iannaccone G. , Mirabile L. , Vilardo G.
Published: July 2002

A two-dimensional active seismic experiment was performed on Mount Vesuvius: Explosive charges were set off at three sites, and the seismic signal along a dense line of 82 seismometers was recorded. A high-velocity basement, formed by Mesozoic carbonates, was identified 2 to 3 kilometers beneath the volcano. A slower (
P
-wave velocity

V
P

≃ 3.4 to 3.8 kilometers per second) and shallower high-velocity zone underlies the central part of the volcano. Large-amplitude late arrivals with a dominant horizontal wave motion and low-frequency content were identified as a
P
to
S
phase converted at a depth of about 10 kilometers at the top of a low-velocity zone (
V

P

< 3 kilometers per second), which might represent a melting zone.
An Asymmetric Model for the Nucleosome: A Binding Site for Linker Histones Inside the DNA Gyres
Authors: Pruss Dmitry , Bartholomew Blaine , Persinger Jim , Hayes Jeffrey , Arents Gina , Moudrianakis Evangelos N. , Wolffe Alan P.
Published: July 2002

Histone-DNA contacts within a nucleosome influence the function of trans-acting factors and the molecular machines required to activate the transcription process. The internal architecture of a positioned nucleosome has now been probed with the use of photoactivatable cross-linking reagents to determine the placement of histones along the DNA molecule. A model for the nucleosome is proposed in which the winged-helix domain of the linker histone is asymmetrically located inside the gyres of DNA that also wrap around the core histones. This domain extends the path of the protein superhelix to one side of the core particle.
The Spell of Sonoluminescence
Authors: Glanz James
Published: July 2002

Physicists have known for decades that ultrasound beamed into a flask of water can spark brief bursts of visible and ultraviolet light from a bubble, but the phenomenon is still a mystery. In a flurry of theorizing, physicists are trying to explain it by invoking everything from miniature implosions to tiny lightning storms to energy stolen from empty space.
Protein Test Favors BSE-CJD Link
Authors: O'Brien Claire
Published: July 2002

London—British scientists have published the first experimental evidence of biochemical similarities between recent anomalous cases of Creutzfeldt-Jakob disease (CJD) in humans and bovine spongiform encephalopathy (BSE) in cows, mice, and other animals. The new work strengthens the case that beef infected with BSE, or “mad cow disease,” is the cause of this new variant of CJD, but scientists do not think the case has been proved yet. Medicine
Building a Bridge Between The Big Bang and Biology
Authors: Lawler Andrew
Published: July 2002

Space scientists from a host of disciplines met last week in Washington to build a case for protecting and expanding work into the origins of the universe, planetary systems, and life itself. Their goal: to convince the Clinton Administration that further cuts to NASA's science budget will endanger efforts to understand how life emerged.
A Shocking View of the Permo-Triassic
Authors: Kerr Richard A.
Published: July 2002

Denver—The biggest extinction of all time was 250 million years ago at the end of the Permian period, and has been considered a long, drawn-out “event.” Now grains of quartz from this time may show the telltale shock fractures that indicate an asteroid impact—and an almost instantaneous extinction.
The Supple Synapse: An Affair That Remembers
Authors: Roush Wade
Published: July 2002

In a recent flurry of work, researchers have found that some of the same genes and proteins involved in the development of the nervous system also participate in the remodeling of the synapses between nerve and muscle cells. The work may shed light on such fundamental issues in neurobiology as how synapses are strengthened during learning.
Early Adaptive Radiation of Birds: Evidence from Fossils from Northeastern China
Authors: Hou Lianhai , Martin Larry D. , Zhou Zhonghe , Feduccia Alan
Published: July 2002

Late Jurassic and Early Cretaceous birds from northeastern China, including many complete skeletons of
Confuciusornis
, provide evidence for a fundamental dichotomy in the class Aves that may antedate the temporal occurrence of the Late Jurassic
Archaeopteryx
. The abundance of
Confuciusornis
may provide evidence of avian social behavior. Jurassic skeletal remains of an ornithurine bird lend further support to the idea of an early separation of the line that gave rise to modern birds.
Chaoyangia
, an ornithurine bird from the Early Cretaceous of China, has premaxillary teeth.
Quasar Pairs: A Redshift Puzzle?
Authors: Schilling Govert
Published: July 2002

A pair of German astronomers has proposed that quasars are not, as commonly thought, extremely distant galactic nuclei but are in fact ejected from relatively nearby active galaxies. Their assertion challenges the conventional understanding of redshift, the cosmological ruler which is astronomers' number one tool.
PER Protein in Silkmoths Marches to Different Drummer
Authors: Barinaga Marcia
Published: July 2002

After working out the mechanism that drives the circadian clock of the fruit fly, researchers expected that what they learned would be applicable to other species. But new work on the silkmoth shows that there's more than one way to drive a clock—even when the same key proteins form the mechanism
Solar Wind Magnetic Field Bending of Jovian Dust Trajectories
Authors: Zook H. A. , Grün E. , Baguhl M. , Hamilton D. P. , Linkert G. , Liou J.-C. , Forsyth R. , Phillips J. L.
Published: July 2002

From September 1991 to October 1992, the cosmic dust detector on the Ulysses spacecraft recorded 11 short bursts, or streams, of dust. These dust grains emanated from the jovian system, and their trajectories were strongly affected by solar wind magnetic field forces. Analyses of the on-board measurements of these fields, and of stream approach directions, show that stream-associated dust grain masses are of the order of 10
−18
gram and dust grain velocities exceed 200 kilometers per second. These masses and velocities are, respectively, about 10
3
times less massive and 5 to 10 times faster than earlier reported.
Myc and Max Homologs in
Drosophila
Authors: Gallant Peter , Shiio Yuzuru , Cheng Pei Feng , Parkhurst Susan M. , Eisenman Robert N.
Published: July 2002

The proteins encoded by the
myc
proto-oncogene family are involved in cell proliferation, apoptosis, differentiation, and neoplasia. Myc acts through dimerization with Max to bind DNA and activate transcription. Homologs of the
myc
and
max
genes were cloned from the fruit fly
Drosophila melanogaster
and their protein products (dMyc and dMax) were shown to heterodimerize, recognize the same DNA sequence as their vertebrate homologs, and activate transcription. The dMyc protein is likely encoded by the
Drosophila
gene
diminutive
(
dm
), a mutation in which results in small body size and female sterility caused by degeneration of the ovaries. These findings indicate a potential role for Myc in germ cell development and set the stage for genetic analysis of Myc and Max.
Turbulence May Sink Titanic Reactor
Authors: Glanz James
Published: July 2002

Denver—The $10 billion International Thermonuclear Experimental Reactor is meant to demonstrate that fusion is a practical energy source. If all goes as planned, the reactor will cage million-degree ions for long enough to fuse and generate copious amounts of power—enough to ignite a self-sustaining fusion burn. But a physics-based theory says that turbulence could turn that flame into a fizzle, cooling the plasma and drastically reducing its power output.
Did a Plate Tectonic Surge Flood Earth?
Authors: Kerr Richard A.
Published: July 2002

About 100 million years ago, the seas ruled the Earth, rising perhaps 200 meters higher than today. One theory has suggested that a rising plume of hot material from deep within the Earth triggered these surging seas, as well as other unexplained Cretaceous events, such as a warm climate and bursts of volcanism. But some geophysicists now challenge that idea, saying that the drowned continents of the Cretaceous were due to ordinary plate motions rather than deep convulsions of the Earth.
A Cyclophilin Function in Hsp90-Dependent Signal Transduction
Authors: Duina Andrea A. , Chang Hui-Chen Jane , Marsh James A. , Lindquist Susan , Gaber Richard F.
Published: July 2002

Cpr6 and Cpr7, the
Saccharomyces cerevisiae
homologs of cyclophilin-40 (CyP-40), were shown to form complexes with Hsp90, a protein chaperone that functions in several signal transduction pathways. Deletion of
CPR7
caused severe growth defects when combined with mutations that decrease the amount of Hsp90 or Sti1, another component of the Hsp90 chaperone machinery. The activities of two heterologous Hsp90-dependent signal transducers expressed in yeast, glucocorticoid receptor and pp60
v−

src

kinase, were adversely affected by
cpr7
null mutations. These results suggest that CyP-40 cyclophilins play a general role in Hsp90-dependent signal transduction pathways under normal growth conditions.
Paleontology and Chronology of Two Evolutionary Transitions by Hybridization in the Bahamian Land Snail
Cerion
Authors: Goodfriend Glenn A. , Gould Stephen Jay
Published: July 2002

The late Quaternary fossil record of the Bahamian land snail
Cerion
on Great Inagua documents two transitions apparently resulting from hybridization. In the first, a localized modern population represents the hybrid descendants of a 13,000-year-old fossil form from the same area, introgressed with the modern form now characteristic of the adjacent regions. In the second case, a chronocline spanning 15,000 to 20,000 years and expressing the transition of an extinct fossil form to the modern form found on the south coast was documented by morphometry of fossils dated by amino acid racemization and radiocarbon. Hybrid intermediates persisted for many thousands of years.
CD5-Mediated Negative Regulation of Antigen Receptor-Induced Growth Signals in B-1 B Cells
Authors: Bikah Gabriel , Carey Jacqueline , Ciallella John R. , Tarakhovsky Alexander , Bondada Subbarao
Published: July 2002

A subset of B lymphocytes present primarily in the peritoneal and pleural cavities is defined by the expression of CD5 and is elevated in autoimmune diseases. Upon signaling through membrane immunoglobulin M (mIgM), splenic B lymphocytes (B-2) proliferate, whereas peritoneal B cells (B-1) undergo apoptosis. However, in CD5-deficient mice, B-1 cells responded to mIgM crosslinking by developing a resistance to apoptosis and entering the cell cycle. In wild-type B-1 cells, prevention of association between CD5 and mIgM rescued their growth response to mIgM crosslinking. Thus the B cell receptor-mediated signaling is negatively regulated by CD5 in normal B-1 cells.
Supercritical Solvent Comes Into Its Own
Authors: Kaiser Jocelyn
Published: July 2002

Chemists have long thought that liquefied, or ”supercritical,“ carbon dioxide could be the perfect, environmentally friendly solvent: It's essentially harmless to living things—and cheap. But it's also so inert that few substances will dissolve in it. Now, however, as reported in this issue on
page 2049
, researchers have devised a surfactant—a sort of soap—that should help carbon dioxide dissolve a host of greasy compounds that are important in industry.
Prime Formula Weds Number Theory and Quantum Physics
Authors: Cipra Barry
Published: July 2002

A problem in number theory and a problem in physics may turn out to be two sides of the same numerical coin. Quantum physicists think a mathematical beast known as the Riemann zeta function, which encodes information about prime numbers, could provide a key to understanding the behavior of complex atomic systems. At the same time, the connection with physics could break a longtime logjam in pure mathematics by leading to a proof of a century-old problem.
A Ligand-Receptor Mechanism in Plant-Pathogen Recognition
Authors: Lamb Chris
Published: July 2002

Two reports in this issue (
p. 2060
and
p. 2063
) illustrate the molecular mechanism underlying the specificity of plant pathogens for certain plant species. In his Perspective, Lamb explains what these results teach us about signal transduction pathways in plants and how they provide a new tool for engineering crop plants.
A Mechanism of Drug Action Revealed by Structural Studies of Enoyl Reductase
Authors: Baldock Clair , Rafferty John B. , Sedelnikova Svetlana E. , Baker Patrick J. , Stuitje Antoine R. , Slabas Antoni R. , Hawkes Timothy R. , Rice David W.
Published: July 2002

Enoyl reductase (ENR), an enzyme involved in fatty acid biosynthesis, is the target for antibacterial diazaborines and the front-line antituberculosis drug isoniazid. Analysis of the structures of complexes of
Escherichia coli
ENR with nicotinamide adenine dinucleotide and either thienodiazaborine or benzodiazaborine revealed the formation of a covalent bond between the 2′ hydroxyl of the nicotinamide ribose and a boron atom in the drugs to generate a tight, noncovalently bound bisubstrate analog. This analysis has implications for the structure-based design of inhibitors of ENR, and similarities to other oxidoreductases suggest that mimicking this molecular linkage may have generic applications in other areas of medicinal chemistry.
Biogeochemical Evidence for Dinoflagellate Ancestors in the Early Cambrian
Authors: Moldowan J. Michael , Talyzina Nina M.
Published: July 2002

Dinoflagellates are single-celled organisms that reflect the ecological conditions in modern oceans and lakes. Ultrastructural data and molecular phylogeny suggest that they originated in the Neoproterozoic, yet dinoflagellate ancestors are classified only to the Middle Triassic (∼240 million years ago). Examination of dinoflagellate-specific biological markers (dinosteranes and 4α-methyl-24-ethylcholestane) in concentrated microfossils with known morphology identified ancient dinoflagellate ancestors from the Early Cambrian (∼520 million years ago).
Electronic Structure of Mott Insulators Studied by Inelastic X-ray Scattering
Authors: Hasan M. Z. , Isaacs E. D. , Shen Z.-X. , Miller L. L. , Tsutsui K. , Tohyama T. , Maekawa S.
Published: July 2002

The electronic structure of Mott insulators continues to be a major unsolved problem in physics despite more than 50 years of research. Well-developed momentum-resolved spectroscopies such as photoemission or neutron scattering cannot probe the full Mott gap. High-resolution resonant inelastic x-ray scattering revealed dispersive charge excitations across the Mott gap in a high–critical temperature parent cuprate (Ca
2
CuO
2
Cl
2
), shedding light on the anisotropy of the Mott gap. These charge excitations across the Mott gap can be described within the framework of the Hubbard model.
In Vitro Pharmacodynamic Characteristics of Nystatin Including Time-Kill and Postantifungal Effect
Authors: Gunderson Shana M. , Hoffman Holly , Ernst Erika J. , Pfaller Michael A. , Klepser Michael E.
Published: July 2002

Four
Candida albicans
isolates and six non-
albicans Candida
isolates were evaluated by time-kill methods to characterize the relationship between nystatin concentrations, the rate and extent of fungicidal activity, and the postantifungal effect (PAFE). Against
Candida
species, nystatin exhibits concentration-dependent fungicidal activity and a pronounced PAFE.
Characterization of 4-Hydroxyphenylacetate 3-Hydroxylase (HpaB) of
Escherichia coli
as a Reduced Flavin Adenine Dinucleotide-Utilizing Monooxygenase
Authors: Xun Luying , Sandvik Erik R.
Published: July 2002

4-Hydroxyphenylacetate 3-hydroxylase (HpaB and HpaC) of
Escherichia coli
W has been reported as a two-component flavin adenine dinucleotide (FAD)-dependent monooxygenase that attacks a broad spectrum of phenolic compounds. However, the function of each component in catalysis is unclear. The large component (HpaB) was demonstrated here to be a reduced FAD (FADH
2
)-utilizing monooxygenase. When an
E. coli
flavin reductase (Fre) having no apparent homology with HpaC was used to generate FADH
2
in vitro, HpaB was able to use FADH
2
and O
2
for the oxidation of 4-hydroxyphenylacetate. HpaB also used chemically produced FADH
2
for 4-hydroxyphenylacetate oxidation, further demonstrating that HpaB is an FADH
2
-utilizing monooxygenase. FADH
2
generated by Fre was rapidly oxidized by O
2
to form H
2
O
2
in the absence of HpaB. When HpaB was included in the reaction mixture without 4-hydroxyphenylacetate, HpaB bound FADH
2
and transitorily protected it from rapid autoxidation by O
2
. When 4-hydroxyphenylacetate was also present, HpaB effectively competed with O
2
for FADH
2
utilization, leading to 4-hydroxyphenylacetate oxidation. With sufficient amounts of HpaB in the reaction mixture, FADH
2
produced by Fre was mainly used by HpaB for the oxidation of 4-hydroxyphenylacetate. At low HpaB concentrations, most FADH
2
was autoxidized by O
2
, causing uncoupling. However, the coupling of the two enzymes' activities was increased by lowering FAD concentrations in the reaction mixture. A database search revealed that HpaB had sequence similarities to several proteins and gene products involved in biosynthesis and biodegradation in both bacteria and archaea. This is the first report of an FADH
2
-utilizing monooxygenase that uses FADH
2
as a substrate rather than as a cofactor.
Expression in
Escherichia coli
of Native and Chimeric Phenolic Acid Decarboxylases with Modified Enzymatic Activities and Method for Screening Recombinant
E. coli
Strains Expressing These Enzymes
Authors: Barthelmebs Lise , Diviès Charles , Cavin Jean-François
Published: July 2002

Four bacterial phenolic acid decarboxylases (PAD) from
Lactobacillus plantarum
,
Pediococcus pentosaceus
,
Bacillus subtilis
, and
Bacillus pumilus
were expressed in
Escherichia coli
, and their activities on
p
-coumaric, ferulic, and caffeic acids were compared. Although these four enzymes displayed 61% amino acid sequence identity, they exhibit different activities for ferulic and caffeic acid metabolism. To elucidate the domain(s) that determines these differences, chimeric PAD proteins were constructed and expressed in
E. coli
by exchanging their individual carboxy-terminal portions. Analysis of the chimeric enzyme activities suggests that the C-terminal region may be involved in determining PAD substrate specificity and catalytic capacity. In order to test phenolic acid toxicity, the levels of growth of recombinant
E. coli
displaying and not displaying PAD activity were compared on medium supplemented with different concentrations of phenolic acids and with differing pHs. Though these acids already have a slight inhibitory effect on
E. coli
, vinyl phenol derivatives, created during decarboxylation of phenolic acids, were much more inhibitory to the
E. coli
control strain. To take advantage of this property, a solid medium with the appropriate pH and phenolic acid concentration was developed; in this medium the recombinant
E. coli
strains expressing PAD activity form colonies approximately five times smaller than those formed by strains devoid of PAD activity.
Multiphasic Kinetics of Transformation of 1,2,4-Trichlorobenzene at Nano- and Micromolar Concentrations by
Burkholderia
sp. Strain PS14
Authors: Rapp Peter
Published: July 2002

The transformation of 1,2,4-trichlorobenzene (1,2,4-TCB) at initial concentrations in nano- and micromolar ranges was studied in batch experiments with
Burkholderia
sp. strain PS14. 1,2,4-TCB was metabolized from nano- and micromolar concentrations to below its detection limit of 0.5 nM. At low initial 1,2,4-TCB concentrations, a first-order relationship between specific transformation rate and substrate concentration was observed with a specific affinity (
a
0
A
) of 0.32 liter · mg (dry weight)
−1
· h
−1
followed by a second one at higher concentrations with an
a
o
A
of 0.77 liter · mg (dry weight)
−1
· h
−1
. This transition from the first-order kinetics at low initial 1,2,4-TCB concentrations to the second first-order kinetics at higher 1,2,4-TCB concentrations was shifted towards higher initial 1,2,4-TCB concentrations with increasing cell mass. At high initial concentrations of 1,2,4-TCB, a maximal transformation rate of approximately 37 nmol · min
−1
· mg (dry weight)
−1
was measured, irrespective of the cell concentration.
T-Cell Receptor Vβ Repertoire CDR3 Length Diversity Differs within CD45RA and CD45RO T-Cell Subsets in Healthy and Human Immunodeficiency Virus-Infected Children
Authors: Kou Zhong Chen , Puhr Joshua S. , Rojas Mabel , McCormack Wayne T. , Goodenow Maureen M. , Sleasman John W.
Published: July 2002

The T-cell receptor (TCR) CDR3 length heterogeneity is formed during recombination of individual Vβ gene families. We hypothesized that CDR3 length diversity could be used to assess the fundamental differences within the TCR repertoire of CD45RA and CD45RO T-cell subpopulations. By using PCR-based spectratyping, nested primers for all 24 human Vβ families were developed to amplify CDR3 lengths in immunomagnetically selected CD45RA and CD45RO subsets within both CD4+and CD8+T-cell populations. Umbilical cord blood mononuclear cells or peripheral blood mononuclear cells obtained from healthy newborns, infants, and children, as well as human immunodeficiency virus (HIV)-infected children, were analyzed. All T-cell subsets from newborn and healthy children demonstrated a Gaussian distribution of CDR3 lengths in separated T-cell subsets. In contrast, HIV-infected children had a high proportion of predominant CDR3 lengths within both CD45RA and CD45RO T-cell subpopulations, most commonly in CD8+CD45RO T cells. Sharp differences in clonal dominance and size distributions were observed when cells were separated into CD45RA or CD45RO subpopulations. These differences were not apparent in unfractionated CD4+or CD8+T cells from HIV-infected subjects. Sequence analysis of predominant CDR3 lengths revealed oligoclonal expansion within individual Vβ families. Analysis of the CDR3 length diversity within CD45RA and CD45RO T cells provides a more accurate measure of disturbances in the TCR repertoire than analysis of unfractionated CD4 and CD8 T cells.
Systemic Neutralization of Interleukin-8 Markedly Reduces Neutrophilic Pleocytosis during Experimental Lipopolysaccharide-Induced Meningitis in Rabbits
Authors: Dumont Rachel A. , Car Bruce D. , Voitenok Nikolai N. , Junker Ursula , Moser Bernhard , Zak Oto , O'Reilly Terence
Published: July 2002

Interleukin-8 (IL-8) is elevated in the cerebrospinal fluid (CSF) of patients with meningitis and is proposed to participate in subarachnoid-space pleocytosis. However, intracisternal injection of IL-8 into rabbits failed to induce indices typical of meningitis (leukocyte, tumor necrosis factor, or protein accumulation in the CSF or histopathological changes), indicating that merely increasing the CSF level of this chemokine is insufficient to induce inflammation in this anatomical site. IL-8 treatment did not affect inflammatory responses to subsequently intracisternally administered lipopolysaccharide (LPS). IL-8 was chemotactic for rabbit neutrophils in vitro, and subcutaneous injection of IL-8 (diluted in buffer or CSF) proved the in vivo activity of this peptide and suggested the absence of an IL-8 inhibitor in normal rabbit CSF. LPS-dependent pleocytosis was only slightly diminished by intracisternally administered murine anti-rabbit IL-8 monoclonal antibody (MAb) WS-4 but was dramatically reduced by intravenously administered MAb. Therefore, elevated CSF IL-8 levels may contribute to, but cannot solely account for, neutrophil influx into the subarachnoid space during meningitis. However, inhibition of IL-8 activity of the bloodstream side of the blood-brain barrier effectively reduces pleocytosis, indicating a central role of IL-8 in neutrophil influx into CSF during bacterial meningitis. Thus, inhibition of IL-8 is a possible therapeutic target for adjunct treatment of meningitis.
A Shift from Oral to Blood pH Is a Stimulus for Adaptive Gene Expression of
Streptococcus gordonii
CH1 and Induces Protection against Oxidative Stress and Enhanced Bacterial Growth by Expression of
msrA
Authors: Vriesema Aldwin J. M. , Dankert Jacob , Zaat Sebastian A. J.
Published: July 2002

Viridans group streptococci (VS) from the oral cavity entering the bloodstream may initiate infective endocarditis (IE). We aimed to identify genes expressed in response to a pH increase from slightly acidic (pH 6.2) to neutral (pH 7.3) as encountered by VS entering the bloodstream from the oral cavity. Using a recently developed promoter-screening vector, we isolated five promoter fragments from the genomic DNA of
Streptococcus gordonii
CH1 responding to this stimulus. No common regulatory sequences were identified in these promoter fragments that could account for the coordinate expression of the corresponding genes. One of the isolated fragments contained the promoter region and 5′ end of a gene highly homologous to the methionine sulfoxide reductase gene (
msrA
) of various bacterial and eukaryotic species. This gene has been found to be activated in
S. gordonii
strain V288 in a rabbit model of IE (A. O. Kiliç, M. C. Herzberg, M. W. Meyer, X. Zhao, and L. Tao, Plasmid 42:67–72, 1999). We isolated and characterized the
msrA
gene of
S. gordonii
CH1 and constructed a chromosomal insertion mutant. This mutant was more sensitive to hydrogen peroxide, suggesting a role for the streptococcal MsrA in protecting against oxidative stress. Moreover, MsrA appeared to be important for the growth of
S. gordonii
CH1 under aerobic and anaerobic conditions. Both these properties of MsrA may contribute to the ability of
S. gordonii
to cause IE.