bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanometer scale patterning and oxidation of silicon surfaces with an ultrahigh vacuum scanning tunneling microscope
Authors: Lyding J. W. , Abeln G. C. , Shen T.-C. , Wang C. , Tucker J. R.
Published: July 2002

Nanoscale patterning of the Si(100)-2×1 monohydride surface has been achieved by using an ultrahigh vacuum (UHV) scanning tunneling microscope (STM) to selectively desorb the hydrogen passivation. Hydrogen passivation on silicon represents one of the simplest possible resist systems for nanolithography experiments. After preparing high quality H-passivated surfaces in the UHV chamber, patterning is achieved by operating the STM in field emission. The field emitted electrons stimulate the desorption of molecular hydrogen, restoring clean Si(100)-2×1 in the patterned area. This depassivation mechanism seems to be related to the electron kinetic energy for patterning at higher voltages and the electron current for low voltage patterning. The patterned linewidth varies linearly with the applied tip bias achieving a minimum of <10 Å at −4.5 V. The dependence of linewidth on electron dose is also studied. For positive tip biases up to 10 V no patterning occurs. The restoration of clean Si(100)-2×1 is suggestive of selective area chemical modifications. This possibility has been explored by exposing the patterned surface to oxygen and ammonia. For the oxygen case, initial oxidation of the patterned area is observed. Ammonia dosing, on the other hand, repassivates the surface in a manner different from that of atomic hydrogen. In both cases the pattern resolution is retained and the surrounding H-passivated areas remain unaffected by the dosing.
Comparison of etching processes of silicon and germanium in SF6–O2 radio-frequency plasma
Authors: Campo A. , Cardinaud Ch. , Turban G.
Published: July 2002

Reactive ion etching of Si and Ge in SF6–O2 is investigated. Etch rate shows that Si etching is selective with respect to Ge in SF6–O2 (O2<50%); the reverse is observed in SF6–O2 (O2≳50%). In agreement with the compared evolution of the F and O concentration in the plasma and of the etch product formation rate obtained respectively, by optical emission spectroscopy and mass spectrometry, the x-ray photon spectroscopy surface analysis reveals that the growth of a SiOxFy layer quenches the Si etching whereas the formation of GeOxFy does not inhibit Ge etching. Using a simple model, an experimental surface reactivity is defined and expressed in function of the experimental data. Results suggest a different behavior for sulfur and oxygen on both materials.
Analytical and seminumerical models for gated field emitter arrays. I. Theory
Authors: Jensen K. L. , Zaidman E. G. , Kodis M. A. , Goplen B. , Smithe D. N.
Published: July 2002

A simple analytic approach for gated field emission array unit cell modeling, the ‘‘Saturn’’ model, has been developed and serves to show how those parameters which most affect inductive output amplifier performance are dependent on the material and geometry of the unit cell. The Saturn model replaces the field emission tip with a sphere and the gate with a charged ring. A relation then exists between the gate radius, the gate voltage, the sphere radius, the field, and its angular variation on the sphere, from which the total emitted current may be given analytically, and from which the geometric and material dependence of those quantities which most affect amplifier performance (Fowler Nordheim A and B parameters, tip field, capacitance, and field enhancement factor) can be qualitatively determined. The qualitative predictions of this model may be made quantitative (the ‘‘seminumerical model’’) by the inclusion of a subset of parameters normally provided in a full boundary element numerical simulation which accounts for how the field varies along the surface of the emitter. The manner in which a statistical distribution of tip radii affects the least-squares estimate of the A and B parameters is shown.
Quantification of metal trace contaminants on Si wafer surfaces by Laser-SNMS and TOF-SIMS using sputter deposited submonolayer standards
Authors: Schnieders A. , Möllers R. , Terhorst M. , Cramer H.-G. , Niehuis E. , Benninghoven A.
Published: July 2002

We have shown that quantitative determination of metal trace impurities in the range between 109 cm−2 and 1012 cm−2 on top of the native oxide of a Si wafer by laser postionization of sputtered neutrals in combination with time-of-flight mass spectrometry (Laser-SNMS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) is possible. Trace metal standards with concentrations in the range between 109 cm−2 and 1012 cm−2 were used for calibration. These standards were prepared by sputter deposition and were independently controlled by Total Reflection X-Ray Fluorescence Analysis. Relative sensitivity factors for 12 metals on Si for Laser-SNMS were determined. Additionally, we compared postionization with different wavelengths (193 nm and 248 nm). With Laser-SNMS it was then possible to determine the influence of UV/ozone treatment on the measured surface concentration of metal species. The UV/ozone treatment is necessary to achieve high and reproducible useful yields for the metal species in TOF-SIMS. With this knowledge, we were able to determine relative sensitivity factors for the metals on UV/ozone-treated Si wafer surfaces measured by TOF-SIMS. Detection limits down to 108 cm−2 and 1012 cm−2 for sample surface areas of 100 μm in diameter and 1 μm in diameter, respectively, were found for both Laser-SNMS and TOF-SIMS.
Stability of surface atomic structures of ionic crystals studied by atomic force microscopy observation of various faces of CaSO4 crystal in solutions
Authors: Shindo H. , Seo A. , Itasaka M. , Odaki T. , Tanaka K.
Published: July 2002

Dissolution processes on three low index faces of anhydrite (CaSO4) crystal in water were monitored in situ with atomic force microscopy. Well-controlled layer-by-layer dissolution was observed on the most stable (010) surface, where rectangular pits with monolayer depth grew in the directions of the crystal axes until they met each other forming an atom-flat surface. The shape of the pits suggests that the dissolution proceeds row by row, by formation of vacancies at the step edge followed by rapid removal of kink ions. Speeds of the step retreat decreased markedly when sulfate and calcium ions were added to water. The steps got saturated at different concentrations. Stabilities of the steps are discussed in relation to the ionic arrangement. On a highly directional (100) face, marked anisotropy was observed in step stabilities. In addition to the numbers of ionic bonds and charge neutrality as a whole, local polarity greatly affects the stabilities of the step structures. The speeds of the step retreat vary by five orders of magnitude depending on the structures, but they all approach zero toward the bulk saturation concentration of CaSO4 in water.
Analysis of frictional-force image patterns of a graphite surface
Authors: Sasaki Naruo , Tsukada Masaru , Fujisawa Satoru , Sugawara Yasuhiro , Morita Seizo , Kobayashi Katsuyoshi
Published: July 2002

We discuss the mechanism of image patterns of the frictional-force microscopy (FFM) of a graphite surface by using a three-dimensional model comprised of a tip connected to a cantilever and a substrate surface. A simulated FFM image is in good agreement with an experimental one. A stable domain of the tip atom position can be defined in an analytic way. In the frictional-force regime, more than one quasistable tip atom position are mapped into a single cantilever basal position. Part of the boundary of the two-dimensional domain of the cantilever basal position appears as a fringe between the bright and the dark areas along the scan direction of the FFM image. General features of FFM images can be completely understood by this analysis.
Scanning probe nanofabrication of chemically active areas on substrate covered with organosilane monolayers
Authors: Sugimura Hiroyuki , Nakagiri Nobuyuki
Published: July 2002

An organosilane monolayer self-assembled onto Si substrates was patterned using a scanning tunneling microscope or an atomic force microscope with a conductive probe. The tip-scanned area was etched by wet etching, while the unscanned area remained resistant to the etching. Consequently, the tip-scanned patterns were transferred to the substrate Si using the scanned area as an etching window. Moreover, the tip-scanned area preferentially reacted with other organosilane molecules, e.g., 3-aminopropyltrimethoxysilane, to form a monolayer confined to the scanned pattern. Next, onto this amino-terminated monolayer, aldehyde-modified latex nanoparticles were immobilized area selectively through chemical reaction between the amino and aldehyde groups. It was demonstrated that in the areas where the probe tip of the microscopes had been scanned, the surface was terminated with hydroxyl groups and became active to various chemical treatments.
Polymer structure effect on dissolution characteristics and acid diffusion in chemically amplified deep ultraviolet resists
Authors: Itani Toshiro , Yoshino Hiroshi , Hashimoto Shuichi , Yamana Mitsuharu , Samoto Norihiko , Kasama Kunihiko
Published: July 2002

The role of molecular weight dispersion (Mw/Mn) of base phenolic resin was investigated in chemically amplified positive KrF resists. The resists consisted of tert-butoxycarbonyl (t-BOC) blocked polystyrene as base resin and 2,4-dimethylbenzenesulfonic acid derivative as photoacid generator, and the Mw/Mn value was changed as 1.2, 4.0, and 9.0. Not only dissolution rate contrast, but also the slope m of log(dissolution rate)-log(exposure dose) plots increased with decreasing Mw/Mn. In the case of low Mw/Mn, the molecular structure becomes more homogeneous, so resist film could be dissolved uniformly via constant developer penetration into film. This uniform dissolution characteristics may be attributable to the origin of dispersion effects mentioned above. Furthermore, higher acid diffusion property and smooth pattern side wall were obtained without side wall roughness in lower Mw/Mn. This fact indicates that acid diffusion length is also uniform within resist film in the case of low Mw/Mn. Based on the experimental analysis, the clear relationship among Mw/Mn, dissolution characteristics and acid diffusion behavior in resist film was obtained. Moreover, it was found that lower Mw/Mn has advantage for improving inherent resist performance.
Single phase ZnSnAs2 grown by molecular beam epitaxy
Authors: Seryogin G. A. , Nikishin S. A. , Temkin H. , Schlaf R. , Sharp L. I. , Wen Y. C. , Parkinson B. , Elyukhin V. A. , Kudriavtsev Yu. A. , Mintairov A. M. , Faleev N. N. , Baidakova M. V.
Published: July 2002

Epitaxial layers of ZnSnAs2 were grown by molecular beam epitaxy on Si and InP substrates. Growth conditions were investigated by varying the substrate temperature and the Sn, As, and Zn fluxes. The best morphology and stoichiometry was obtained at Ts=300–320 °C and the flux ratio of PAs4/PZn∼1.5–4. The samples were evaluated by secondary neutral mass spectroscopy, high resolution x-ray diffraction, Raman spectroscopy, and atomic force microscopy. Single phase layers of ZnSnAs2 grown on InP(001) substrates show lattice mismatch of −3.4×10−4.
193 nm single layer resist strategies, concepts, and recent results
Authors: Nalamasu O. , Houlihan F. M. , Cirelli R. A. , Timko A. G. , Watson G. P. , Hutton R. S. , Kometani J. M. , Reichmanis E. , Gabor A. , Medina A. , Slater S.
Published: July 2002

Matrix resins used in conventional resists are not suitable for use at 193 nm due to their opacity. Hence new materials that are functionally similar to but structurally different from novolac and poly(hydroxy styrene) are required for 193 nm lithography. We report on the use of alternating copolymers of cycloolefins with maleic anhydride as effective 193 nm matrix resins, with or without dissolution inhibitors based on polyfunctional cholates, for 193 nm lithography. Due to their structural diversity, the required high transparency and etch stability, compatibility with industry standard 0.262 N tetramethyl ammonium hydroxide (TMAH) can be built into the polymer by conventional free redical polymerization techniques. A correlation between the molecular properties of the resist components (matrix resin, dissolution inhibitor, photoacid generator, and base additive) and resist lithographic performance parameters is illustrated. The formulations containing dissolution inhibitors currently show 0.13 μm line/space (L/S) pair and 0.11 μm isolated line resolution at 20–30 mJ/cm2 photospeeds with good process latitude, etch resistance, and environmental stability. These materials are completely compatible with 0.262 N TMAH development, show poly and oxide etch rates comparable to that of deep-ultraviolet resists used in manufacturing today, and exhibit postexposure delay stabilities of greater than 2 h for 0.16 μm features at 13 ppb concentrations of ammonia or N-methyl pyrrolidone. The importance of reflectivity control at the 193 nm imaging wavelength and how it can be handled by a multilayer dielectric antireflective coating (ARC) has been elucidated. An optimized resist and ARC system shows 0.14 μm l/s pair resolution over 100 nm poly topography and exhibits a resolution of 60 nm isolated lines and 80 nm with >0.8 μm depth of focus using a Levenson phase shift mask. The design and utility of photoacid generators that serve both the photoacid generation and amine additive functions in low activation energy resist systems and the base additive function in high activation energy system are also described. The use of these photodecomposable bases (PDBs) has been shown to dramatically alleviate the outgassing during exposure in low activation energy chemically amplified (CA) resists. In high activation energy CA resists, use of a PDB that is basic but generates an acid upon exposure to light or radiation provides some relief in designing 193 nm resists that are fast, yet stable, to environmental and substrate contamination effects.
InGaAsP photonic band gap crystal membrane microresonators
Authors: Scherer A. , Painter O. , D’Urso B. , Lee R. , Yariv A.
Published: July 2002

We have microfabricated two-dimensional (2D) photonic band gap structures in a thin slab of dielectric material to define reflectors and high-Q microresonators. By selectively omitting holes from the 2D photonic crystal, optical microcavities, and in-plane microresonator switches can be defined. We have designed this structure with a finite difference time domain approach, and demonstrate the effect of lithographic 2D band gap tuning on the emission spectra of InGaAs/InGaAsP multiple quantum well material emission wavelength of 1.55 μm.
Formation of low resistance Pt ohmic contacts to p-type GaN using two-step surface treatment
Authors: Jang Ja-Soon , Park Seong-Ju , Seong Tae-Yeon
Published: July 2002

Two-step surface treatment is introduced to obtain low resistance Pt contacts to p-type GaN. The first step is performed after the mesa etching process using buffered oxide etch (BOE) and ammonium sulfide [(NH4)2Sx]. This is followed by the second step using BOE. The Pt contact, that was simply BOE treated, yields 2.1(±0.9)×10−2 Ω cm2. However, the contact which was treated sequentially using ultrasonically boiled BOE (10 min) and boiled (NH4)2Sx (10 min), produces a specific contact resistance of 2.0(±3.5)×10−5 Ω cm2. To the best of our knowledge, this is the lowest contact resistance reported hitherto for the contacts on p-GaN. The effective Schottky barrier heights (SBHs) of the differently surface-treated contacts were determined using the Norde and current–voltage methods. It is shown that the SBHs are dependent upon the surface treatment conditions.
Atomic force microscopy and transmission electron microscopy observations of KOH-etched GaN surfaces
Authors: Shiojima Kenji
Published: July 2002

Clear results on the origin of etch pits are reported. The surfaces of high-quality GaN epitaxial layers were etched with molten KOH and observed by atomic force microscopy (AFM) and plan-view and cross-sectional transmission electron microscopy (TEM). AFM images for as-grown samples show dark spots indicating mixed dislocations. For etched samples, hexagonal-base etch pits were clearly observed, and the dark spots disappeared. TEM observations show that a mixed dislocation terminates at the bottom of each etch pit. The densities of etch pits and mixed dislocations are almost the same, i.e., around 3×108 cm−2. The origin of etch pits is the mixed dislocation, and the combination of KOH etching and AFM is found to be a better approach for two-dimensional evaluation of mixed dislocations.
Study of β-Al2TiO5 Thin Films by XPS
Authors: Armelao Lidia , Martucci Alessandro , Innocenzi Plinio
Published: July 2002

β-Al2TiO5 thin films were prepared by dip-coating on silicon substrates via sol-gel processing. Acid catalyzed sols of aluminum and titanium alkoxides were used as precursors, described in work published previously. The composition of the films and their behavior toward crystallization were studied by x-ray photoelectron spectroscopy and x-ray diffraction as a function of the firing time and temperature. The films were amorphous up to 600 °C and crystalline β-Al2TiO5 was formed at 700 °C. From 800 up to 1200 °C, titania (anatase and rutile) phases were also detected, without decomposition of the metastable β-Al2TiO5 phase. The formation of β-Al2TiO5 at low temperatures is attributed to a diffusion-limited crystallization process, which takes place in the conditions of high homogeneity at molecular level reached in the sol-gel synthesis.
G Protein βγ Subunit-Mediated Presynaptic Inhibition: Regulation of Exocytotic Fusion Downstream of Ca
2+
Entry
Authors: Blackmer Trillium , Larsen Eric C. , Takahashi Michiko , Martin Thomas F. J. , Alford Simon , Hamm Heidi E.
Published: July 2002

The nervous system can modulate neurotransmitter release by neurotransmitter activation of heterotrimeric GTP–binding protein (G protein)–coupled receptors. We found that microinjection of G protein βγ subunits (Gβγ) mimics serotonin's inhibitory effect on neurotransmission. Release of free Gβγ was critical for this effect because a Gβγ scavenger blocked serotonin's effect. Gβγ had no effect on fast, action potential–evoked intracellular Ca
2+
release that triggered neurotransmission. Inhibition of neurotransmitter release by serotonin was still seen after blockade of all classical Gβγ effector pathways. Thus, Gβγ blocked neurotransmitter release downstream of Ca
2+
entry and may directly target the exocytotic fusion machinery at the presynaptic terminal.
Glia-Synapse Interaction Through Ca
2+
-Permeable AMPA Receptors in Bergmann Glia
Authors: Iino Masae , Goto Kaori , Kakegawa Wataru , Okado Haruo , Sudo Makoto , Ishiuchi Shogo , Miwa Akiko , Takayasu Yukihiro , Saito Izumu , Tsuzuki Keisuke , Ozawa Seiji
Published: July 2002

Glial cells express a variety of neurotransmitter receptors. Notably, Bergmann glial cells in the cerebellum have Ca
2+
-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)–type glutamate receptors (AMPARs) assembled without the GluR2 subunit. To elucidate the role of these Ca
2+
-permeable AMPARs, we converted them into Ca
2+
-impermeable receptors by adenoviral-mediated delivery of the GluR2 gene. This conversion retracted the glial processes ensheathing synapses on Purkinje cell dendritic spines and retarded the removal of synaptically released glutamate. Furthermore, it caused multiple innervation of Purkinje cells by the climbing fibers. Thus, the glial Ca
2+
-permeable AMPARs are indispensable for proper structural and functional relations between Bergmann glia and glutamatergic synapses.
Water at Hydrophobic Surfaces: Weak Hydrogen Bonding and Strong Orientation Effects
Authors: Scatena L. F. , Brown M. G. , Richmond G. L.
Published: July 2002

Vibrational studies that selectively probe molecular structure at CCl
4
/H
2
O and hydrocarbon/H
2
O interfaces show that the hydrogen bonding between adjacent water molecules at these interfaces is weak, in contrast to generally accepted models of water next to fluid hydrophobic surfaces that suggest strong hydrogen bonding. However, interactions between these water molecules and the organic phase result in substantial orientation of these weakly hydrogen-bonded water molecules in the interfacial region. The results have important implications for understanding water adjacent to hydrophobic surfaces and the penetration of water into hydrophobic phases.
Success and Virulence in
Toxoplasma
as the Result of Sexual Recombination Between Two Distinct Ancestries
Authors: Grigg Michael E. , Bonnefoy Serge , Hehl Adrian B. , Suzuki Yasuhiro , Boothroyd John C.
Published: July 2002

Toxoplasma gondii
is a common human pathogen causing serious, even fatal, disease in the developing fetus and in immunocompromised patients. Despite its ability to reproduce sexually and its broad geographic and host range,
Toxoplasma
has a clonal population structure comprised principally of three lines. We have analyzed 15 polymorphic loci in the archetypal type I, II, and III strains and found that polymorphism was limited to, at most, two rather than three allelic classes and no polymorphism was detected between alleles in strains of a given type. Multilocus analysis of 10 nonarchetypal isolates likewise clustered the vast majority of alleles into the same two distinct ancestries. These data strongly suggest that the currently predominant genotypes exist as a pandemic outbreak from a genetic mixing of two discrete ancestral lines. To determine if such mixing could lead to the extreme virulence observed for some strains, we examined the F
1
progeny of a cross between a type II and III strain, both of which are relatively avirulent in mice. Among the progeny were recombinants that were at least 3 logs more virulent than either parent. Thus, sexual recombination, by combining polymorphisms in two distinct and competing clonal lines, can be a powerful force driving the natural evolution of virulence in this highly successful pathogen.
Otolith δ18O Record of Mid-Holocene Sea Surface Temperatures in Peru
Authors: Andrus C. Fred T. , Crowe Douglas E. , Sandweiss Daniel H. , Reitz Elizabeth J. , Romanek Christopher S.
Published: July 2002

Peruvian sea catfish (Galeichthys peruvianus) sagittal otoliths preserve a record of modern and mid-Holocene sea surface temperatures (SSTs). Oxygen isotope profiles in otoliths excavated from Ostra [6010 ± 90 years before the present (yr B.P.); 8°55′S] indicate that summer SSTs were ∼3°C warmer than those of the present. Siches otoliths (6450 ± 110 yr B.P.; 4°40′S) recorded mean annual temperatures ∼3° to 4°C warmer than were measured under modern conditions. Trophic level and population diversity and equitability data from these faunal assemblages and other Peruvian archaeological sites support the isotope interpretations and suggest that upwelling of the Peru-Chile current intensified after ∼5000 yr B.P.
Hydrogen Sensors and Switches from Electrodeposited Palladium Mesowire Arrays
Authors: Favier Frédéric , Walter Erich C. , Zach Michael P. , Benter Thorsten , Penner Reginald M.
Published: July 2002

Hydrogen sensors and hydrogen-activated switches were fabricated from arrays of mesoscopic palladium wires. These palladium “mesowire” arrays were prepared by electrodeposition onto graphite surfaces and were transferred onto a cyanoacrylate film. Exposure to hydrogen gas caused a rapid (less than 75 milliseconds) reversible decrease in the resistance of the array that correlated with the hydrogen concentration over a range from 2 to 10%. The sensor response appears to involve the closing of nanoscopic gaps or “break junctions” in wires caused by the dilation of palladium grains undergoing hydrogen absorption. Wire arrays in which all wires possessed nanoscopic gaps reverted to open circuits in the absence of hydrogen gas.
Quantifying Uncertainties in Climate System Properties with the Use of Recent Climate Observations
Authors: Forest Chris E. , Stone Peter H. , Sokolov Andrei P. , Allen Myles R. , Webster Mort D.
Published: July 2002

We derive joint probability density distributions for three key uncertain properties of the climate system, using an optimal fingerprinting approach to compare simulations of an intermediate complexity climate model with three distinct diagnostics of recent climate observations. On the basis of the marginal probability distributions, the 5 to 95% confidence intervals are 1.4 to 7.7 kelvin for climate sensitivity and −0.30 to −0.95 watt per square meter for the net aerosol forcing. The oceanic heat uptake is not well constrained, but ocean temperature observations do help to constrain climate sensitivity. The uncertainty in the net aerosol forcing is much smaller than the uncertainty range for the indirect aerosol forcing alone given in the Intergovernmental Panel on Climate Change Third Assessment Report.
Retrograde Support of Neuronal Survival Without Retrograde Transport of Nerve Growth Factor
Authors: MacInnis Bronwyn L. , Campenot Robert B.
Published: July 2002

Application of nerve growth factor (NGF) covalently cross-linked to beads increased the phosphorylation of TrkA and Akt, but not of mitogen-activated protein kinase, in cultured rat sympathetic neurons. NGF beads or iodine-125–labeled NGF beads supplied to distal axons resulted in the survival of over 80% of the neurons for 30 hours, with little or no retrograde transport of iodine-125–labeled NGF; whereas application of free iodine-125–labeled NGF (0.5 nanograms per milliliter) produced 20-fold more retrograde transport, but only 29% of the neurons survived. Thus, in contrast to widely accepted theory, a neuronal survival signal can reach the cell bodies unaccompanied by the NGF that initiated it.
Modulation of NMDA Receptor- Dependent Calcium Influx and Gene Expression Through EphB Receptors
Authors: Takasu Mari A. , Dalva Matthew B. , Zigmond Richard E. , Greenberg Michael E.
Published: July 2002

Protein-protein interactions and calcium entry through the
N
-methyl-
d
-aspartate (NMDA)–type glutamate receptor regulate synaptic development and plasticity in the central nervous system. The EphB receptor tyrosine kinases are localized at excitatory synapses where they cluster and associate with NMDA receptors. We identified a mechanism whereby EphBs modulate NMDA receptor function. EphrinB2 activation of EphB in primary cortical neurons potentiates NMDA receptor–dependent influx of calcium. Treatment of cells with ephrinB2 led to NMDA receptor tyrosine phosphorylation through activation of the Src family of tyrosine kinases. These ephrinB2-dependent events result in enhanced NMDA receptor–dependent gene expression. Our findings indicate that ephrinB2 stimulation of EphB modulates the functional consequences of NMDA receptor activation and suggest a mechanism whereby activity-independent and activity-dependent signals converge to regulate the development and remodeling of synaptic connections.
Observational Evidence for an Active Surface Reservoir of Solid Carbon Dioxide on Mars
Authors: Malin Michael C. , Caplinger Michael A. , Davis Scott D.
Published: July 2002

High-resolution images of the south polar residual cap of Mars acquired in 1999 and 2001 show changes in the configuration of pits, intervening ridges, and isolated mounds. Escarpments have retreated 1 to 3 meters in 1 martian year, changes that are an order of magnitude larger than can be explained by the sublimation of water ice, but close to what is expected for sublimation of carbon dioxide ice. These observations support a 35-year-old conjecture that Mars has a large surface reservoir of solid carbon dioxide. The erosion implies that this reservoir is not in equilibrium with the present environment and that global climate change is occurring on Mars.
RNA Helicase MUT-14-Dependent Gene Silencing Triggered in
C. elegans
by Short Antisense RNAs
Authors: Tijsterman Marcel , Ketting René F. , Okihara Kristy L. , Sijen Titia , Plasterk Ronald H. A.
Published: July 2002

Posttranscriptional gene silencing in
Caenorhabditis elegans
results from exposure to double-stranded RNA (dsRNA), a phenomenon designated as RNA interference (RNAi), or from co-suppression, in which transgenic DNA leads to silencing of both the transgene and the endogenous gene. Here we show that single-stranded RNA oligomers of antisense polarity can also be potent inducers of gene silencing. As is the case for co-suppression, antisense RNAs act independently of the RNAi genes
rde-1
and
rde-4
but require the mutator/RNAi gene
mut-7
and a putative DEAD box RNA helicase,
mut-14
. Our data favor the hypothesis that gene silencing is accomplished by RNA primer extension using the mRNA as template, leading to dsRNA that is subsequently degraded.
Divided by Cytochrome Oxidase: A Map of the Projections from V1 to V2 in Macaques
Authors: Sincich Lawrence C. , Horton Jonathan C.
Published: July 2002

Current models partition the primate visual system into dorsal (magno) and ventral (parvo, konio) streams. Perhaps the strongest evidence for this idea has come from the pattern of projections between the primary visual area (V1) and the second visual area (V2). Prior studies describe three distinct pathways: magno to thick stripes, parvo to pale stripes, and konio to thin stripes. We now demonstrate that V1 output arises from just two sources: patch columns and interpatch columns. Patch columns project to thin stripes and interpatch columns project to pale and thick stripes. Projection of interpatches to common V2 stripe types (pale and thick) merges parvo and magno inputs, making it likely that these functional channels are distributed strongly to both dorsal and ventral streams.
A Possible Tektite Strewn Field in the Argentinian Pampa
Authors: Bland P. A. , Filho C. R. de Souza , Jull A. J. T. , Kelley S. P. , Hough R. M. , Artemieva N. A. , Pierazzo E. , Coniglio J. , Pinotti L. , Evers V. , Kearsley A. T.
Published: July 2002

Impact glass associated with 11 elongate depressions in the Pampean Plain of Argentina, north of the city of Rı́o Cuarto, was suggested to be proximal ejecta related to a highly oblique impact event. We have identified about 400 additional elongate features in the area that indicate an aeolian, rather than an impact, origin. We have also dated fragments of glass found at the Rı́o Cuarto depressions; the age is similar to that of glass recovered 800 kilometers to the southeast. This material may be tektite glass from an impact event around 0.48 million years ago, representing a new tektite strewn field.
Planetary Biology--Paleontological, Geological, and Molecular Histories of Life
Authors: Benner Steven A. , Caraco M. Daniel , Thomson J. Michael , Gaucher Eric A.
Published: July 2002

The history of life on Earth is chronicled in the geological strata, the fossil record, and the genomes of contemporary organisms. When examined together, these records help identify metabolic and regulatory pathways, annotate protein sequences, and identify animal models to develop new drugs, among other features of scientific and biomedical interest. Together, planetary analysis of genome and proteome databases is providing an enhanced understanding of how life interacts with the biosphere and adapts to global change.
Microscopic View of Structural Phase Transitions Induced by Shock Waves
Authors: Kadau Kai , Germann Timothy C. , Lomdahl Peter S. , Holian Brad Lee
Published: July 2002

Multimillion-atom molecular-dynamics simulations are used to investigate the shock-induced phase transformation of solid iron. Above a critical shock strength, many small close-packed grains nucleate in the shock-compressed body-centered cubic crystal growing on a picosecond time scale to form larger, energetically favored grains. A split two-wave shock structure is observed immediately above this threshold, with an elastic precursor ahead of the lagging transformation wave. For even higher shock strengths, a single, overdriven wave is obtained. The dynamics and orientation of the developing close-packed grains depend on the shock strength and especially on the crystallographic shock direction. Orientational relations between the unshocked and shocked regions are similar to those found for the temperature-driven martensitic transformation in iron and its alloys.
Explaining the Latitudinal Distribution of Sunspots with Deep Meridional Flow
Authors: Nandy Dibyendu , Choudhuri Arnab Rai
Published: July 2002

Sunspots, dark magnetic regions occurring at low latitudes on the Sun's surface, are tracers of the magnetic field generated by the dynamo mechanism. Recent solar dynamo models, which use the helioseismically determined solar rotation, indicate that sunspots should form at high latitudes, contrary to observations. We present a dynamo model with the correct latitudinal distribution of sunspots and demonstrate that this requires a meridional flow of material that penetrates deeper than hitherto believed, into the stable layers below the convection zone. Such a deep material flow may have important implications for turbulent convection and elemental abundance in the Sun and similar stars.
Climatic Control of the High-Latitude Vegetation Greening Trend and Pinatubo Effect
Authors: Lucht Wolfgang , Prentice I. Colin , Myneni Ranga B. , Sitch Stephen , Friedlingstein Pierre , Cramer Wolfgang , Bousquet Philippe , Buermann Wolfgang , Smith Benjamin
Published: July 2002

A biogeochemical model of vegetation using observed climate data predicts the high northern latitude greening trend over the past two decades observed by satellites and a marked setback in this trend after the Mount Pinatubo volcano eruption in 1991. The observed trend toward earlier spring budburst and increased maximum leaf area is produced by the model as a consequence of biogeochemical vegetation responses mainly to changes in temperature. The post-Pinatubo decline in vegetation in 1992–1993 is apparent as the effect of temporary cooling caused by the eruption. High-latitude CO2uptake during these years is predicted as a consequence of the differential response of heterotrophic respiration and net primary production.
Kobe's Lesson: Dial 711 for “Open” Emergency Communications
Authors: Noam Eli M. , Sato Harumasa
Published: July 2002

The 1995 earthquake that devastated Kobe, Japan, showed that conventional top-down emergency communications sytems—even those with satellite connections—can easily fail. Immediately after the quake, however, citizens began to contribute information to a computer bulletin board system, which quickly became a trustworthy alternative to official information. The Kobe experience suggests that an open-access emergency communctions system could be designed that would allow such citizen input not only from computer terminals but also by telephone.
DPY-26, a Link Between Dosage Compensation and Meiotic Chromosome Segregation in the Nematode
Authors: Lieb Jason D. , Capowski Elizabeth E. , Meneely Philip , Meyer Barbara J.
Published: July 2002

The DPY-26 protein is required in the nematode
Caenorhabditis elegans
for X-chromosome dosage compensation as well as for proper meiotic chromosome segregation. DPY-26 was shown to mediate both processes through its association with chromosomes. In somatic cells, DPY-26 associates specifically with hermaphrodite X chromosomes to reduce their transcript levels. In germ cells, DPY-26 associates with all meiotic chromosomes to mediate its role in chromosome segregation. The X-specific localization of DPY-26 requires two dosage compensation proteins (DPY-27 and DPY-30) and two proteins that coordinately control both sex determination and dosage compensation (SDC-2 and SDC-3).
Scientists Probe Feelings Behind Decision-Making
Authors: Vogel Gretchen
Published: July 2002

Intuition may deserve more respect that it typically gets these days. Although it's often dismissed along with emotion as clouding clear, rational thought, a new study reported on page
1293
suggests that it plays a crucial role in humans' ability to make smart decisions.
Language Acquisition and Use: Learning and Applying Probabilistic Constraints
Authors: Seidenberg Mark S.
Published: July 2002

What kinds of knowledge underlie the use of language and how is this knowledge acquired? Linguists equate knowing a language with knowing a grammar. Classic “poverty of the stimulus” arguments suggest that grammar identification is an intractable inductive problem and that acquisition is possible only because children possess innate knowledge of grammatical structure. An alternative view is emerging from studies of statistical and probabilistic aspects of language, connectionist models, and the learning capacities of infants. This approach emphasizes continuity between how language is acquired and how it is used. It retains the idea that innate capacities constrain language learning, but calls into question whether they include knowledge of grammatical structure.
Stardust Memories
Authors: Cruikshank Dale P.
Published: July 2002

Comet Hale-Bopp is an unusually large and bright comet that will reach perihelion (closest approach to the Sun) in April 1997. In his Perspective, Cruikshank discusses observational results described in seven research reports in this issue of
Science
(see pages
1900
,
1904
,
1907
, 1909,
1913
,
1915
, and 1918). Images from the Hubble Space Telescope indicate a large cometary nucleus, and measurements from the Infrared Satellite Observatory craft reveal the composition of the dust grains being discharged by Hale-Bopp. Radio telescope measurements have tracked the release of carbon, oxygen, hydrogen, nitrogen, and sulfur in various molecular species. Photometry shows that gas production is 20 to 100 times greater than any previously observed comet. Such observations may help unravel the puzzle of comet formation and the origins of organic matter in the universe.
Maintenance of Acetylcholine Receptor Number by Neuregulins at the Neuromuscular Junction in Vivo
Authors: Sandrock Alfred W. , Dryer Stuart E. , Rosen Kenneth M. , Gozani Shai N. , Kramer Rainer , Theill Lars E. , Fischbach Gerald D.
Published: July 2002

ARIA (for acetylcholine receptor–inducing activity), a protein purified on the basis of its ability to stimulate acetylcholine receptor (AChR) synthesis in cultured myotubes, is a member of the neuregulin family and is present at motor endplates. This suggests an important role for neuregulins in mediating the nerve-dependent accumulation of AChRs in the postsynaptic membrane. Nerve-muscle synapses have now been analyzed in neuregulin-deficient animals. Mice that are heterozygous for the deletion of neuregulin isoforms containing an immunoglobulin-like domain are myasthenic. Postsynaptic AChR density is significantly reduced, as judged by the decrease in the mean amplitude of spontaneous miniature endplate potentials and bungarotoxin binding. On the other hand, the mean amplitude of evoked endplate potentials was not decreased, due to an increase in the number of quanta released per impulse, a compensation that has been observed in other myasthenic states. Thus, the density of AChRs in the postsynaptic membrane depends on immunoglobulin-containing neuregulin isoforms throughout the life of the animal.
Atmospheric Science and Public Policy
Authors: Zillman John W.
Published: July 2002

The role of atmospheric science in the provision of public services and the formulation of public policy is going through a period of reappraisal. One of the most fundamental issues at stake is the continued free and unrestricted international exchange of meteorological data in support of operational meteorology and research. The contribution of atmospheric research to the resolution of complex issues associated with the international response to the threat of global climate change has become a matter of vigorous debate in the meteorological community but has also contributed to increasingly close cooperation between meteorology and the rest of the earth science community.
Large Molecular Third-Order Optical Nonlinearities in Polarized Carotenoids
Authors: Marder Seth R. , Torruellas William E. , Blanchard-Desce Mirelle , Ricci Vincent , Stegeman George I. , Gilmour Sharon , Brédas Jean-Luc , Li Jun , Bublitz Greg U. , Boxer Steve G.
Published: July 2002

Garito and co-workers have suggested a mechanism to dramatically increase the second hyperpolarizability, γ, in linear π-electron–conjugated molecules. Polarization is introduced that leads to a difference between the dipole moments of the molecule’s ground state and excited state. Here a series of carotenoids was examined that had increasing intramolecular charge transfer (ICT) from the polyenic chain to the acceptor moiety in the ground state, and γ was measured for these compounds as a function of wavelength by third-harmonic generation. The compound with the greatest ICT exhibited a 35-fold enhancement of γmax(the γ measured at the peak of the three-photon resonance) relative to the symmetric molecule β-carotene, which itself has one of the largest third-order nonlinearities known. Stark spectroscopic measurements revealed the existence of a large difference dipole moment, Δμ, between the ground and excited state. Quantum-chemical calculations underline the importance of interactions involving states with large Δμ.
X-ray Imaging of Stress and Strain of Diamond, Iron, and Tungsten at Megabar Pressures
Authors: Hemley Russell J. , Mao Ho-kwang , Shen Guoyin , Badro James , Gillet Philippe , Hanfland Michael , Häusermann Daniel
Published: July 2002

Synchrotron x-ray imaging and stress measurements of diamond-anvil cell gaskets revealed large elastic strains at the diamond tip at a pressure of 300 gigapascals. The diamond, generally considered a rigid body, bent 16 degrees over a distance of 300 micrometers without failure. To complement these measurements, a technique was developed that permits x-ray diffraction to be measured through a beryllium gasket. Measurements on tungsten and iron revealed the strain anisotropy, deviatoric stress, and texture and showed that the yield strengths of these materials increase by up to two orders of magnitude at confining pressures of 200 to 300 gigapascals. The results allow identification of the maximum amount of strain accommodated by the anvil tips before failure. Further development of ultrahigh pressure techniques requires relieving stress concentrations associated with this large elastic deformation.
Nucleosynthesis in Stars: Recent Developments
Authors: Arnett David , Bazan Grant
Published: July 2002

The development of new observational, experimental, and computational technologies is changing our understanding of the origins of the elements by thermonuclear burning in stars. Gamma-ray lines from newly made radioactive nuclei have been identified using instruments onboard low-Earth orbiting satellites. Grains in meteorites have isotopic anomalies which suggest that the grains were put together in a stellar explosion such as a supernova. Computer simulations allow such anomalies to be used to probe how these events happen. The simulations are being independently tested by experiments with high-energy density lasers. These developments are beginning to provide a quantitative diagnostic of galactic evolution, and of the epoch of formation of the first stars and galaxies.
25-Hydroxyvitamin D
3
1α-Hydroxylase and Vitamin D Synthesis
Authors: Takeyama Ken-ichi , Kitanaka Sachiko , Sato Takashi , Kobori Masato , Yanagisawa Junn , Kato Shigeaki
Published: July 2002

Renal 25–hydroxyvitamin D
3
1α-hydroxylase [1α(OH)ase] catalyzes metabolic activation of 25–hydroxyvitamin D
3
into 1α,25–dihydroxyvitamin D
3
[1α,25(OH)
2
D
3
], an active form of vitamin D, and is inhibited by 1α,25(OH)
2
D
3
. 1α(OH)ase, which was cloned from the kidney of mice lacking the vitamin D receptor (VDR
−
/
−
mice), is a member of the P450 family of enzymes (P450
VD1
α
). Expression of 1α(OH)ase was suppressed by 1α,25(OH)
2
D
3
in VDR
+/+
and VDR
+/
−
mice but not in VDR
−
/
−
mice. These results indicate that the negative feedback regulation of active vitamin D synthesis is mediated by 1α(OH)ase through liganded VDR.
Psychoactive Drug Use in Evolutionary Perspective
Authors: Nesse Randolph M. , Berridge Kent C.
Published: July 2002

Pure psychoactive drugs and direct routes of administration are evolutionarily novel features of our environment. They are inherently pathogenic because they bypass adaptive information processing systems and act directly on ancient brain mechanisms that control emotion and behavior. Drugs that induce positive emotions give a false signal of a fitness benefit. This signal hijacks incentive mechanisms of “liking” and “wanting,” and can result in continued use of drugs that no longer bring pleasure. Drugs that block negative emotions can impair useful defenses, although there are several reasons why their use is often safe nonetheless. A deeper understanding of the evolutionary origins and functions of the emotions and their neural mechanisms is needed as a basis for decisions about the use of psychoactive drugs.
Evolution of Magnetic and Superconducting Fluctuations with Doping of High-TcSuperconductors
Authors: Blumberg G. , Kang Moonsoo , Klein M. V. , Kadowaki K. , Kendziora C.
Published: July 2002

Electronic Raman scattering from high- and low-energy excitations was studied as a function of temperature, extent of hole doping, and energy of the incident photons in Bi2Sr2CaCu2O8±δsuperconductors. For underdoped superconductors, short-range antiferromagnetic (AF) correlations were found to persist with hole doping, and doped single holes were found to be incoherent in the AF environment. Above the superconducting (SC) transition temperatureTc, the system exhibited a sharp Raman resonance ofB1gsymmetry and energy of 75 millielectron-volts and a pseudogap for electron-hole excitations below 75 millielectron-volts, a manifestation of a partially coherent state forming from doped incoherent quasi particles. The occupancy of the coherent state increases with cooling until phase ordering atTcproduces a global SC state.
Connectivity and Management of Caribbean Coral Reefs
Authors: Roberts Callum M.
Published: July 2002

Surface current patterns were used to map dispersal routes of pelagic larvae from 18 coral reef sites in the Caribbean. The sites varied, both as sources and recipients of larvae, by an order of magnitude. It is likely that sites supplied copiously from “upstream” reef areas will be more resilient to recruitment overfishing, less susceptible to species loss, and less reliant on local management than places with little upstream reef. The mapping of connectivity patterns will enable the identification of beneficial management partnerships among nations and the design of networks of interdependent reserves.
Differential Use of CREB Binding Protein-Coactivator Complexes
Authors: Kurokawa Riki , Kalafus Daniel , Ogliastro Marie-Hélène , Kioussi Chrissa , Xu Lan , Torchia Joseph , Rosenfeld Michael G. , Glass Christopher K.
Published: July 2002

CREB binding protein (CBP) functions as an essential coactivator of transcription factors that are inhibited by the adenovirus early gene product E1A. Transcriptional activation by the signal transducer and activator of transcription–1 (STAT1) protein requires the C/H3 domain in CBP, which is the primary target of E1A inhibition. Here it was found that the C/H3 domain is not required for retinoic acid receptor (RAR) function, nor is it involved in E1A inhibition. Instead, E1A inhibits RAR function by preventing the assembly of CBP–nuclear receptor coactivator complexes, revealing differences in required CBP domains for transcriptional activation by RAR and STAT1.