bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of the effect of size on carbon fiber surfaces
Authors: Weitzsacker Cara L. , Bellamy Michael , Sherwood Peter M. A.
Published: July 2002

The effect of size on carbon fiber surfaces was examined by x-ray photoelectron spectroscopy and scanning electron and x-ray wavelength dispersive microscopies. The fibers studied had been subjected to various levels of commercial surface treatment before the commercially applied size was added. The removal of the size from the fiber surface was examined both by using solvents and by heating the sized fibers. The surface chemistry of the sized fibers was dominated by the sizing and indicated that more sizing was often found on the fibers that received greater prior surface treatment. The study suggests that some significant interaction occurs between the size and the surface treated fiber surface, and this interaction leads to a greater stability of the size on the fiber surface.
Carbon dimer, C2, as a growth species for diamond films from methane/hydrogen/argon microwave plasmas
Authors: Gruen Dieter M. , Zuiker Chris D. , Krauss Alan R. , Pan Xianzheng
Published: July 2002

It was shown recently that nanocrystalline diamond films can be grown using fullerenes as precursors in an argon microwave plasma without the addition of hydrogen or oxygen. Extensive fragmentation of C60 in the microwave discharge leads to a copious production of the carbon dimer molecule, C2, as evidenced by intense Swan-band emission. Here we have investigated hydrogen–methane–argon plasmas and found that high argon fractions (≳50%) lead to intense C2 emission, indicating significant production of C2 in the plasma. In situ measurements of the substrate reflectivity were used to determine the growth rate. A correlation between the C2 emission intensity and growth rate was observed. These results prompted us to propose a scheme for diamond film growth on the (100)–(2×1): H reconstructed diamond surface with C2 as the growth species. Each surface carbon atom (bonded twice to carbons in the bulk, once to a surface carbon, creating a ‘‘dimer’’ and then forming a five-membered ring) is terminated with hydrogen. With C2 as the growth species, no hydrogen abstraction reactions are required because its very high energy of adsorption (815 kJ/mol C) allows the C2 molecule to insert directly into the dimer bonds. A second C2 can form across the adjoining trough. The added carbons then dimerize, forming a new (2×1) surface on a new layer with dimer rows orthogonal to the original rows.
Electron cyclotron resonance plasma chemical vapor deposited silicon nitride for micromechanical applications
Authors: Leclerc S. , Lecours A. , Caron M. , Richard E. , Turcotte G. , Currie J. F.
Published: July 2002

We have studied the electron cyclotron resonance plasma chemical vapor deposition (ECR PCVD) of silicon nitride films onto unheated substrates to evaluate their potential for use in micromechanical devices. We demonstrate that this low temperature process (<120 °C) is compatible with lift-off patterning of both thin and thick films. We measured the deposition rate, stress and buffered HF (BHF) etch rate for different gas flows of silane, nitrogen and argon. The deposition rate is rate controlled by the silane flow and we obtained rates sufficiently high (0.7–1.4 μm/h) to obtain films for practical use in micromechanics (0.5–2 μm) in about 1 h. We measured stresses ranging from 1000 MPa compressive to 60 MPa tensile, and we can repeatedly obtain films with stress of less than 10 MPa. These films are ideal for both low-stress and stress-compensated device designs. We observed BHF (NH4F:HF 6:1 at 20 °C) etch rates from 5 nm/min to 150 nm/min, which are five times faster than those for films deposited by CVD. ECR films are ideal for fast etched sacrificial layers at 150 nm/min or as slowly etched masking layers at 5 nm/min. We found that stress and the BHF etch rate were strongly correlated: low etch rates are observed for highly compressively stressed films and high etch rates for lower stress. This correlation has a major impact on the design of micromechanical devices and on their fabrication processes using ECR PCVD.
Surface structure and bonding in the strongly correlated metal oxides NiO and UO2
Authors: Castell M. R. , Dudarev S. L. , Muggelberg C. , Sutton A. P. , Briggs G. A. D. , Goddard D. T.
Published: July 2002

We show atomic resolution elevated temperature scanning tunneling microscope images of the NiO (001) and UO2 (111) surfaces. When imaging the empty states structure of NiO, a fourfold symmetric hole state is observed around defects and step edges which influences the second nearest neighbor Ni ions. A similar effect is not observed on the UO2 (111) surface. We explain this difference in terms of the degree of covalent coupling that is involved in the bonding of these oxides. Modeling of the electronic structure through the local spin density approximation incorporating the Hubbard U is used to create charge density maps which show that there is a significant element of nonsphericity of charge distribution around the Ni ions in NiO, but that the distribution is spherical in UO2.
Modeling surface kinetics and morphology during 3C, 2H, 4H, and 6H–SiC (111) step-flow growth
Authors: Stout Phillip J.
Published: July 2002

A Monte Carlo surface kinetics model has been developed to predict growth rate, morphology, and the atomic content of thin films of various SiC polytypes (3C, 2H, 4H, 6H). The model represents the crystal lattice on a structured mesh which retains fixed atom positions and bond partners indicative of a perfect crystal lattice. Specified events occurring at different rates and probabilities change the configuration and the atomic content of the lattice. Events in the model include precursor transport to surface, adatom adsorption, diffusion, and desorption from surface, growth and etching reactions between adatoms and lattice atoms, and evaporation. The initial validation of the model for SiC consists of replicating trends seen when experimentally growing polytypes of SiC (111) on circular mesas and on off-axis vicinal planes. The model can predict the (111) plane faceting seen in β-SiC and the hexagonal structures seen when growing on α-SiC. Also, step flow growth rates which change with flow direction are predicted by the model. Step flow growth on a (111) plane in 〈112̄0〉 directions is faster than growth in 〈11̄00〉 directions. The model is used to investigate step bunching. Step bunching is shown to be due in part to the growth rate variability of 〈11̄00〉 directions and the hexagonal stacking content in the α-SiC polytype. The chemical variation of the step risers in the step bunch could lead to reaction rate limited growth of some steps over others leading to further bunching. The model is used to predict growth rate differences between the Si and C face of SiC. The face growth rate differences are a function of the SiH4 and CH4 surface reaction probabilities.
Hollow cathode sputtering cluster source for low energy deposition: Deposition of Fe small clusters
Authors: Ishii K. , Amano K. , Hamakake H.
Published: July 2002

We have developed a new cluster deposition source, which can be applied to the formation of nanostructured thin films. By our method, clusters are created by a hollow cathode sputtering at high pressure in the range of hundreds of Pa, which are directly deposited onto substrates. A pipe target with small internal diameter of 6 mm is used, and He is injected as a carrier gas while using Ar as a sputtering gas. The cluster size can be controlled between 2 and 6 nm in diameter, and high deposition rates up to 25 nm/min for 6 nm Fe clusters are obtained on a considerably wide substrate. It is also demonstrated that the deposition of 6 nm Fe clusters produces continuous films consisting of very small grains of nearly the same size with depositing clusters.
Platinum etching using a TiO2 hard mask in an O2/Cl2/Ar plasma
Authors: Chung Chee Won , Chung Ilsub
Published: July 2002

The dry etch behavior of Pt films using a TiO2 hard mask was investigated with an O2/Cl2/Ar gas chemistry in an inductively coupled plasma. The variations in etch rates and etch profiles of both the Pt films and the TiO2 masks were examined as a function of Cl2 and O2 concentration. It was found that the ratio of O2 to Cl2 concentration strongly influenced the etch selectivity of the Pt film relative to the TiO2 mask. X-ray photoelectron spectroscopy indicates that the addition of O2 to the gas mix causes a decrease in the TiO2 etch rate as a result of the oxidation of Ti2O3 and TiO species. It is proposed that these oxides arise via the reduction of TiO2 in the presence of a Cl2/Ar plasma. A 1 μm ×1 μm minimum feature size was successfully etched with this TiO2 mask material, yielding a sidewall of approximately 75° in slope that did not exhibit redeposition or residue.
Investigation of titanium nitride gates for tantalum pentoxide and titanium dioxide dielectrics
Authors: Gilmer D. , Hobbs C. , Hegde R. , La L. , Adetutu O. , Conner J. , Tiner M. , Prabhu L. , Bagchi S. , Tobin P.
Published: July 2002

The compatibility of metallic titanium nitride (TiN) as a gate electrode on TiO2 and Ta2O5 gate dielectrics is investigated by rapid thermal annealing (RTA). The electrical and physical properties are characterized using I–V measurements and transmission electron microscopy. TiN/TiO2 capacitors are electrically stable up to at least 800 °C for 90 s. However, the leakage density for TiN/Ta2O5 capacitors is increased by an 850 °C, 60 s RTA. Both TiN/TiO2 and TiN/Ta2O5 capacitors display a large increase in leakage density after a 1025 °C, 20 s RTA.
Metal contacts on semiconductors: The adsorption of Sb, Sn, and Ga on InP(110) cleaved surfaces
Authors: Williams R. H. , McKinley A. , Hughes G. J. , Humphreys T. P. , Maani C.
Published: July 2002

We have studied the adsorption and growth of Ga (group III), Sn (group IV), and Sb (group V) on InP(110) clean cleaved surfaces, using a range of surface sensitive techniques. The adsorption processes differ for the three adsorbate elements ranging from cluster growth for Ga to layer upon layer growth for Sb. The systematics of the adsorption processes as well as the mechanisms driving the chemical reactions and interdiffusion are considered and strong indications of a close relationship between adlayer clustering and the movement of atoms across the interface is reported. The electrical barriers at these interfaces are also briefly discussed as well as some studies at liquid nitrogen temperatures for Sn and Al overlayers.
Chemical reaction and Schottky barrier formation at the Ti/InP(110) and Sn/InP(110) interfaces: Reactive versus nonreactive cases
Authors: Kendelewicz Tom , Mahowald P. H. , McCants C. E. , Bartness K. A. , Lindau I. , Spicer W. E.
Published: July 2002

We present a soft x-ray photoemission study of the in situ grown Ti/InP(110) and Sn/InP(110) interfaces. We find that the Ti overlayer deposited onto the cleaved InP(110) surface readily reacts even at submonolayer coverages. A Ti–In alloy (In dispersed in the Ti overlayer) of changing composition and two Ti–P phases are formed. While In outdiffuses at room temperature (RT) through more than 40 Å of Ti, the P-containing reaction phases are more confined to the interface. In contrast, the Sn overlayers do not react with the InP(110) surface and the overlayer is growing in the Stranski–Krastanow fashion with at least one uniform Sn layer. Independent of large differences in reactivity, the surface Fermi level pinning energies are for both interfaces studied similar (0.3 eV for Ti and 0.4 eV for Sn). This indicates, in agreement with our previous studies of InP interfaces, that the RT pinning level is insensitive to the microscopic chemistry and calls for a simple (in the first approximation) mechanism of the Schottky barrier formation.
Low-temperature growth of silicon dioxide films: A study of chemical bonding by ellipsometry and infrared spectroscopy
Authors: Lucovsky G. , Mantini M. J. , Srivastava J. K. , Irene E. A.
Published: July 2002

This paper presents a spectroscopic study using the techniques of ellipsometry and infrared (IR) absorption spectroscopy of the chemical bonding in silicon dioxide (SiO2) films grown in dry oxygen ambients at temperatures between 550 and 1000 °C. We find that the index of refraction at 632.8 nm increases and the frequency of the dominant IR active bond-stretching vibration at about 1075 cm−1 decreases as the growth temperature is decreased below 1000 °C. Comparing the properties of these films with suboxides (SiOx, x<2) grown by plasma-enhanced chemical vapor deposition, and compacted bulk silica has lead us to conclude: (i) that films grown at temperatures at or below 1000 °C are homogeneous stoichiometric oxides (SiO2); and (ii) that the systematic and correlated variations in the index of refraction and the IR frequency result from increases in the film density with decreasing growth temperature. We present a microscopic model that accounts for (i) the increases in the density and the index of refraction; and (ii) the accompanying decrease in the IR stretching frequency in terms of a decrease in the Si–O–Si bond angle.
Ordered nucleation of Ni and Au islands on Au(111) studied by scanning tunneling microscopy
Authors: Chambliss D. D. , Wilson R. J. , Chiang S.
Published: July 2002

The scanning tunneling microscope reveals that Ni deposited on Au(111) at room temperature forms regular arrays of two-dimensional islands. The islands grow with spacing 73 Å in rows 140 Å apart at sites determined by the Au(111) ‘‘herringbone’’ reconstruction. This nucleation at evenly spaced sites yields islands with a narrow size distribution. The apparent Ni island height (1.9 Å) is bias-independent and agrees with a hard-sphere model of pseudomorphic Ni/Au(111). The behavior of Ni is contrasted with Au deposited on Au(111), for which far fewer islands are formed.
Growth of epitaxial TiN thin films on Si(100) by reactive magnetron sputtering
Authors: Choi C.-H. , Hultman L. , Chiou W.-A. , Barnett S. A.
Published: July 2002

Growth temperature was found to play a crucial role in determining the structure of stoichiometric TiN films deposited on Si(100) by reactive magnetron sputtering. X-ray diffraction, transmission electron microscopy (TEM), and reflection high-energy electron diffraction were used to show that films deposited at substrate temperatures Ts ranging from 700 to 1100 °C were polycrystalline, with either (100) preferred orientation or mixed (100) and (111) orientations. Films deposited using a two-temperature technique were epitaxial with the relationships TiN(100)//Si(100) and TiN[011]//Si[011] but with considerable misorientation. The most highly oriented films were obtained when 45 nm was first deposited at Ts =750 °C, followed by 250 nm at 1000 °C. High-resolution cross-sectional TEM studies showed that increasing the nucleation temperature above 750 °C led to random orientation of TiN lattice fringes relative to the Si lattice. This was explained by the small energy difference between oriented and misoriented TiN nuclei due to the large lattice mismatch (22%).
In situ ellipsometric study of As capping and low temperature molecular-beam epitaxy GaAs growth and implications for the low temperature critical thickness
Authors: Eyink K. G. , Cong Y. S. , Gilbert R. , Capano M. A. , Haas T. W. , Streetman B. G.
Published: July 2002

Low temperature GaAs (LT–GaAs) has been demonstrated to be a useful high resistivity buffer layer for subsequent growth of a variety of epitaxial device layers. It has proven difficult to reproducibly grow these films due to the low (∼250 °C) substrate temperatures involved and the need for tight III/V flux control. In this work in situ ellipsometry was used to reproducibly set the substrate temperature through a study of the As capping process on a GaAs surface. A hysteresis of ∼200 °C occurred in the temperature at which As was deposited and subsequently removed. Subtle changes occur in the ellipsometric response in this temperature window which suggest minimal variation in the adsorbed As behavior on the GaAs surface. A point occurs in the ellipsometric response which characterizes that temperature at which As deposition begins for a given As overpressure. Using this signature, reproducible LT–GaAs films were grown and followed by ellipsometry. A uniform growth spiral was initially obtained indicating the formation of a homogeneous LT–GaAs layer. The ellipsometric response deviates substantially from the uniform growth spiral as the film thickness exceeds a critical value and may correspond to the formation of an amorphous or polycrystalline structure previously reported [Z. Lilental-Weber, W. Swider, K. M. Yu, J. Kortright, F. W. Smith, and A. R. Calawa, Appl. Phys. Lett. 58, 2153 (1991)], [D. J. Eagelsham, L. N. Pfeiffer, K. W. West, and D. R. Dykaar, Appl. Phys. Lett. 58, 65 (1991)]. Subsequent low angle thin film x-ray diffractometry showed that the formation of polycrystalline LT–GaAs at submicron thicknesses was commensurate with the above signature. Finally variations in the index for LT–GaAs were determined from the growth spirals.
Combined method of electron-beam lithography and ion implantation techniques for the fabrication of high-temperature superconductor Josephson junctions
Authors: Hollkott J. , Hu S. , Becker C. , Auge J. , Spangenberg B. , Kurz H.
Published: July 2002

Established semiconductor process technologies are demonstrated to be suitable for the fabrication of high temperature superconductor Josephson junctions. Single YBa2Cu3O7 bridges have been modified by local oxygen ion irradiation through a narrow slit in an implantation mask, which was formed by electron-beam lithography and reactive ion etching. The influence of the slit dimension, the mask thickness, and the irradiation dose have been investigated systematically. The critical current and the normal resistance of the modified microbridges were found to be controllable by these parameters achieving a great variety of different I/V curves, i.e., resistive or superconductor/normal/superconductor (SNS) Josephson junction behavior. Further investigations were performed on SNS junctions. Microwave irradiation of the microbridges exhibits Shapiro steps in the I/V characteristics. In dc superconducting quantum interference devices a voltage modulation as a function of an applied magnetic flux is observed.
Titanium nitride coated tungsten cold field emission sources
Authors: Lo W. K. , Parthasarathy G. , Lo C. W. , Tanenbaum D. M. , Craighead H. G. , Isaacson M. S.
Published: July 2002

Titanium nitride (TiN) thin film coatings were studied by field emission microscopy and spectroscopy. Coated tungsten tips were found to be capable of emitting extremely high currents at low extraction voltages (∼1 mA at 900–1700 V). Current fluctuations for >400 μA total emission from a single tip were 7% rms, measured over ∼ 1 h. Electron energy distributions measured <0.4 eV (full width at half-maximum). Since TiN thin films are commonly used in the microelectronics industry, TiN coatings have the potential for being a relatively simple and widely accessible method for improving the performance of cold field emission sources.
Improving the surface morphology of InSb quantum-well structures on GaAs substrates
Authors: Chung S. J. , Ball M. A. , Lindstrom S. C. , Johnson M. B. , Santos M. B.
Published: July 2002

The electron mobility in AlxIn1−xSb/InSb quantum-well structures grown on GaAs substrates is reduced due to the presence of crystalline defects. In structures grown by molecular beam epitaxy, we observe three kinds of defects: hillocks, oriented abrupt steps, and square mounds. The hillocks and oriented abrupt steps are caused by lattice mismatch. The square mounds arise only when AlxIn1−xSb is grown and probably originate near the AlxIn1−xSb/buffer layer interface. The optimum V/III growth rate ratio for reducing the square-mound density is ≈1.1 at a substrate temperature of ∼440 °C.
Room-Temperature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide
Authors: Matsumoto Yuji , Murakami Makoto , Shono Tomoji , Hasegawa Tetsuya , Fukumura Tomoteru , Kawasaki Masashi , Ahmet Parhat , Chikyow Toyohiro , Koshihara Shin-ya , Koinuma Hideomi
Published: July 2002

Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with the concentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at 2 kelvin.
Costs and Benefits of High Mutation Rates: Adaptive Evolution of Bacteria in the Mouse Gut
Authors: Giraud Antoine , Matic Ivan , Tenaillon Olivier , Clara Antonio , Radman Miroslav , Fons Michel , Taddei François
Published: July 2002

We have shown that bacterial mutation rates change during the experimental colonization of the mouse gut. A high mutation rate was initially beneficial because it allowed faster adaptation, but this benefit disappeared once adaptation was achieved. Mutator bacteria accumulated mutations that, although neutral in the mouse gut, are often deleterious in secondary environments. Consistently, the competitiveness of mutator bacteria is reduced during transmission to and re-colonization of similar hosts. The short-term advantages and long-term disadvantages of mutator bacteria could account for their frequency in nature.
Photosynthesis-Induced Biofilm Calcification and Calcium Concentrations in Phanerozoic Oceans
Authors: Arp Gernot , Reimer Andreas , Reitner Joachim
Published: July 2002

Photosynthetic carbon assimilation is commonly invoked as the cause of calcium carbonate precipitation in cyanobacterial biofilms that results in the formation of calcareous stromatolites. However, biofilm calcification patterns in recent lakes and simulation of photosynthetically induced rise in calcium carbonate supersaturation demonstrate that this mechanism applies only in settings low in dissolved inorganic carbon and high in calcium. Taking into account paleo–partial pressure curves for carbon dioxide, we show that Phanerozoic oceans sustaining calcified cyanobacteria must have had considerably higher calcium concentrations than oceans of today. In turn, the enigmatic lack of calcified cyanobacteria in stromatolite-bearing Precambrian sequences can now be explained as a result of high dissolved inorganic carbon concentrations.
An Iron-Regulated Ferric Reductase Associated with the Absorption of Dietary Iron
Authors: McKie Andrew T. , Barrow Dalna , Latunde-Dada Gladys O. , Rolfs Andreas , Sager Giamal , Mudaly Elida , Mudaly Melitta , Richardson Christopher , Barlow David , Bomford Adrian , Peters Timothy J. , Raja Kishor B. , Shirali Sima , Hediger Matthias A. , Farzaneh Farzin , Simpson Robert J.
Published: July 2002

The ability of intestinal mucosa to absorb dietary ferric iron is attributed to the presence of a brush-border membrane reductase activity that displays adaptive responses to iron status. We have isolated a complementary DNA, Dcytb (for duodenal cytochrome b), which encoded a putative plasma membrane di-heme protein in mouse duodenal mucosa. Dcytb shared between 45 and 50% similarity to the cytochrome b561 family of plasma membrane reductases, was highly expressed in the brush-border membrane of duodenal enterocytes, and induced ferric reductase activity when expressed in
Xenopus
oocytes and cultured cells. Duodenal expression levels of Dcytb messenger RNA and protein were regulated by changes in physiological modulators of iron absorption. Thus, Dcytb provides an important element in the iron absorption pathway.
X-ray Pulses Approaching the Attosecond Frontier
Authors: Drescher Markus , Hentschel Michael , Kienberger Reinhard , Tempea Gabriel , Spielmann Christian , Reider Georg A. , Corkum Paul B. , Krausz Ferenc
Published: July 2002

Single soft-x-ray pulses of ∼90–electron volt (eV) photon energy are produced by high-order harmonic generation with 7-femtosecond (fs), 770-nanometer (1.6 eV) laser pulses and are characterized by photoionizing krypton in the presence of the driver laser pulse. By detecting photoelectrons ejected perpendicularly to the laser polarization, broadening of the photoelectron spectrum due to absorption and emission of laser photons is suppressed, permitting the observation of a laser-induced downshift of the energy spectrum with sub-laser-cycle resolution in a cross correlation measurement. We measure isolated x-ray pulses of 1.8 (+0.7/−1.2) fs in duration, which are shorter than the oscillation cycle of the driving laser light (2.6 fs). Our techniques for generation and measurement offer sub-femtosecond resolution over a wide range of x-ray wavelengths, paving the way to experimental attosecond science. Tracing atomic processes evolving faster than the exciting light field is within reach.
Ancient Geodynamics and Global-Scale Hydrology on Mars
Authors: Phillips Roger J. , Zuber Maria T. , Solomon Sean C. , Golombek Matthew P. , Jakosky Bruce M. , Banerdt W. Bruce , Smith David E. , Williams Rebecca M. E. , Hynek Brian M. , Aharonson Oded , Hauck II Steven A.
Published: July 2002

Loading of the lithosphere of Mars by the Tharsis rise explains much of the global shape and long-wavelength gravity field of the planet, including a ring of negative gravity anomalies and a topographic trough around Tharsis, as well as gravity anomaly and topographic highs centered in Arabia Terra and extending northward toward Utopia. The Tharsis-induced trough and antipodal high were largely in place by the end of the Noachian Epoch and exerted control on the location and orientation of valley networks. The release of carbon dioxide and water accompanying the emplacement of ∼3 × 10
8
cubic kilometers of Tharsis magmas may have sustained a warmer climate than at present, enabling the formation of ancient valley networks and fluvial landscape denudation in and adjacent to the large-scale trough.
Regional Climate Impacts of the Northern Hemisphere Annular Mode
Authors: Thompson David W. J. , Wallace John M.
Published: July 2002

The Northern Hemisphere annular mode (NAM) (also known as the North Atlantic Oscillation) is shown to exert a strong influence on wintertime climate, not only over the Euro-Atlantic half of the hemisphere as documented in previous studies, but over the Pacific half as well. It affects not only the mean conditions, but also the day-to-day variability, modulating the intensity of mid-latitude storms and the frequency of occurrence of high-latitude blocking and cold air outbreaks throughout the hemisphere. The recent trend in the NAM toward its high-index polarity with stronger subpolar westerlies has tended to reduce the severity of winter weather over most middle- and high-latitude Northern Hemisphere continental regions.
Loss of Huntingtin-Mediated BDNF Gene Transcription in Huntington's Disease
Authors: Zuccato Chiara , Ciammola Andrea , Rigamonti Dorotea , Leavitt Blair R. , Goffredo Donato , Conti Luciano , MacDonald Marcy E. , Friedlander Robert M. , Silani Vincenzo , Hayden Michael R. , Timmusk Tõnis , Sipione Simonetta , Cattaneo Elena
Published: July 2002

Huntingtin is a 350-kilodalton protein of unknown function that is mutated in Huntington's disease (HD), a neurodegenerative disorder. The mutant protein is presumed to acquire a toxic gain of function that is detrimental to striatal neurons in the brain. However, loss of a beneficial activity of wild-type huntingtin may also cause the death of striatal neurons. Here we demonstrate that wild-type huntingtin up-regulates transcription of brain-derived neurotrophic factor (BDNF), a pro-survival factor produced by cortical neurons that is necessary for survival of striatal neurons in the brain. We show that this beneficial activity of huntingtin is lost when the protein becomes mutated, resulting in decreased production of cortical BDNF. This leads to insufficient neurotrophic support for striatal neurons, which then die. Restoring wild-type huntingtin activity and increasing BDNF production may be therapeutic approaches for treating HD.
Phenotypic Plasticity in the Interactions and Evolution of Species
Authors: Agrawal Anurag A.
Published: July 2002

When individuals of two species interact, they can adjust their phenotypes in response to their respective partner, be they antagonists or mutualists. The reciprocal phenotypic change between individuals of interacting species can reflect an evolutionary response to spatial and temporal variation in species interactions and ecologically result in the structuring of food chains. The evolution of adaptive phenotypic plasticity has led to the success of organisms in novel habitats, and potentially contributes to genetic differentiation and speciation. Taken together, phenotypic responses in species interactions represent modifications that can lead to reciprocal change in ecological time, altered community patterns, and expanded evolutionary potential of species.
Identification of a Gene Associated with Bt Resistance in
Heliothis virescens
Authors: Gahan Linda J. , Gould Fred , Heckel David G.
Published: July 2002

Transgenic crops producing insecticidal toxins from
Bacillus thuringiensis
(Bt) are widely used for pest control. Bt-resistant insect strains have been studied, but the molecular basis of resistance has remained elusive. Here, we show that disruption of a cadherin-superfamily gene by retrotransposon-mediated insertion was linked to high levels of resistance to the Bt toxin Cry1Ac in the cotton pest
Heliothis virescens
. Monitoring the early phases of Bt resistance evolution in the field has been viewed as crucial but extremely difficult, especially when resistance is recessive. Our findings enable efficient DNA-based screening for resistant heterozygotes by directly detecting the recessive allele.
Control of Octopus Arm Extension by a Peripheral Motor Program
Authors: Sumbre German , Gutfreund Yoram , Fiorito Graziano , Flash Tamar , Hochner Binyamin
Published: July 2002

For goal-directed arm movements, the nervous system generates a sequence of motor commands that bring the arm toward the target. Control of the octopus arm is especially complex because the arm can be moved in any direction, with a virtually infinite number of degrees of freedom. Here we show that arm extensions can be evoked mechanically or electrically in arms whose connection with the brain has been severed. These extensions show kinematic features that are almost identical to normal behavior, suggesting that the basic motor program for voluntary movement is embedded within the neural circuitry of the arm itself. Such peripheral motor programs represent considerable simplification in the motor control of this highly redundant appendage.
Complete Genome Sequence of a Virulent Isolate of
Streptococcus pneumoniae
Authors: Tettelin Hervé , Nelson Karen E. , Paulsen Ian T. , Eisen Jonathan A. , Read Timothy D. , Peterson Scott , Heidelberg John , DeBoy Robert T. , Haft Daniel H. , Dodson Robert J. , Durkin A. Scott , Gwinn Michelle , Kolonay James F. , Nelson William C. , Peterson Jeremy D. , Umayam Lowell A. , White Owen , Salzberg Steven L. , Lewis Matthew R. , Radune Diana , Holtzapple Erik , Khouri Hoda , Wolf Alex M. , Utterback Terry R. , Hansen Cheryl L. , McDonald Lisa A. , Feldblyum Tamara V. , Angiuoli Samuel , Dickinson Tanja , Hickey Erin K. , Holt Ingeborg E. , Loftus Brendan J. , Yang Fan , Smith Hamilton O. , Venter J. Craig , Dougherty Brian A. , Morrison Donald A. , Hollingshead Susan K. , Fraser Claire M.
Published: July 2002

The 2,160,837–base pair genome sequence of an isolate of
Streptococcus pneumoniae
, a Gram-positive pathogen that causes pneumonia, bacteremia, meningitis, and otitis media, contains 2236 predicted coding regions; of these, 1440 (64%) were assigned a biological role. Approximately 5% of the genome is composed of insertion sequences that may contribute to genome rearrangements through uptake of foreign DNA. Extracellular enzyme systems for the metabolism of polysaccharides and hexosamines provide a substantial source of carbon and nitrogen for
S. pneumoniae
and also damage host tissues and facilitate colonization. A motif identified within the signal peptide of proteins is potentially involved in targeting these proteins to the cell surface of low–guanine/cytosine (GC) Gram-positive species. Several surface-exposed proteins that may serve as potential vaccine candidates were identified. Comparative genome hybridization with DNA arrays revealed strain differences in
S. pneumoniae
that could contribute to differences in virulence and antigenicity.
Effects of Size and Temperature on Metabolic Rate
Authors: Gillooly James F. , Brown James H. , West Geoffrey B. , Savage Van M. , Charnov Eric L.
Published: July 2002

We derive a general model, based on principles of biochemical kinetics and allometry, that characterizes the effects of temperature and body mass on metabolic rate. The model fits metabolic rates of microbes, ectotherms, endotherms (including those in hibernation), and plants in temperatures ranging from 0° to 40°C. Mass- and temperature-compensated resting metabolic rates of all organisms are similar: The lowest (for unicellular organisms and plants) is separated from the highest (for endothermic vertebrates) by a factor of about 20. Temperature and body size are primary determinants of biological time and ecological roles.
Loss of Caveolae, Vascular Dysfunction, and Pulmonary Defects in Caveolin-1 Gene-Disrupted Mice
Authors: Drab Marek , Verkade Paul , Elger Marlies , Kasper Michael , Lohn Matthias , Lauterbach Birgit , Menne Jan , Lindschau Carsten , Mende Fanny , Luft Friedrich C. , Schedl Andreas , Haller Hermann , Kurzchalia Teymuras V.
Published: July 2002

Caveolae are plasma membrane invaginations that may play an important role in numerous cellular processes including transport, signaling, and tumor suppression. By targeted disruption of caveolin-1, the main protein component of caveolae, we generated mice that lacked caveolae. The absence of this organelle impaired nitric oxide and calcium signaling in the cardiovascular system, causing aberrations in endothelium-dependent relaxation, contractility, and maintenance of myogenic tone. In addition, the lungs of knockout animals displayed thickening of alveolar septa caused by uncontrolled endothelial cell proliferation and fibrosis, resulting in severe physical limitations in caveolin-1–disrupted mice. Thus, caveolin-1 and caveolae play a fundamental role in organizing multiple signaling pathways in the cell.
Present-Day Crustal Deformation in China Constrained by Global Positioning System Measurements
Authors: Wang Qi , Zhang Pei-Zhen , Freymueller Jeffrey T. , Bilham Roger , Larson Kristine M. , Lai Xi'an , You Xinzhao , Niu Zhijun , Wu Jianchun , Li Yanxin , Liu Jingnan , Yang Zhiqiang , Chen Qizhi
Published: July 2002

Global Positioning System (GPS) measurements in China indicate that crustal shortening accommodates most of India's penetration into Eurasia. Deformation within the Tibetan Plateau and its margins, the Himalaya, the Altyn Tagh, and the Qilian Shan, absorbs more than 90% of the relative motion between the Indian and Eurasian plates. Internal shortening of the Tibetan plateau itself accounts for more than one-third of the total convergence. However, the Tibetan plateau south of the Kunlun and Ganzi-Mani faults is moving eastward relative to both India and Eurasia. This movement is accommodated through rotation of material around the eastern Syntaxis. The North China and South China blocks, east of the Tibetan Plateau, move coherently east-southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year, respectively, with respect to the stable Eurasia.
Regional Magnetic Fields as Navigational Markers for Sea Turtles
Authors: Lohmann Kenneth J. , Cain Shaun D. , Dodge Susan A. , Lohmann Catherine M. F.
Published: July 2002

Young loggerhead sea turtles (
Caretta caretta
) from eastern Florida undertake a transoceanic migration in which they gradually circle the north Atlantic Ocean before returning to the North American coast. Here we report that hatchling loggerheads, when exposed to magnetic fields replicating those found in three widely separated oceanic regions, responded by swimming in directions that would, in each case, help keep turtles within the currents of the North Atlantic gyre and facilitate movement along the migratory pathway. These results imply that young loggerheads have a guidance system in which regional magnetic fields function as navigational markers and elicit changes in swimming direction at crucial geographic boundaries.
Methyltransferase Recruitment and DNA Hypermethylation of Target Promoters by an Oncogenic Transcription Factor
Authors: Di Croce Luciano , Raker Veronica A. , Corsaro Massimo , Fazi Francesco , Fanelli Mirco , Faretta Mario , Fuks Francois , Coco Francesco Lo , Kouzarides Tony , Nervi Clara , Minucci Saverio , Pelicci Pier Giuseppe
Published: July 2002

DNA methylation of tumor suppressor genes is a frequent mechanism of transcriptional silencing in cancer. The molecular mechanisms underlying the specificity of methylation are unknown. We report here that the leukemia-promoting PML-RAR fusion protein induces gene hypermethylation and silencing by recruiting DNA methyltransferases to target promoters and that hypermethylation contributes to its leukemogenic potential. Retinoic acid treatment induces promoter demethylation, gene reexpression, and reversion of the transformed phenotype. These results establish a mechanistic link between genetic and epigenetic changes during transformation and suggest that hypermethylation contributes to the early steps of carcinogenesis.
An Extensive Class of Small RNAs in
Caenorhabditis elegans
Authors: Lee Rosalind C. , Ambros Victor
Published: July 2002

The
lin-4
and
let-7
antisense RNAs are temporal regulators that control the timing of developmental events in
Caenorhabditis elegans
by inhibiting translation of target mRNAs.
let-7
RNA is conserved among bilaterian animals, suggesting that this class of small RNAs [microRNAs (miRNAs)] is evolutionarily ancient. Using bioinformatics and cDNA cloning, we found 15 new miRNA genes in
C. elegans.
Several of these genes express small transcripts that vary in abundance during
C. elegans
larval development, and three of them have apparent homologs in mammals and/or insects. Small noncoding RNAs of the miRNA class appear to be numerous and diverse.
Spintronics: A Spin-Based Electronics Vision for the Future
Authors: Wolf S. A. , Awschalom D. D. , Buhrman R. A. , Daughton J. M. , von Molnár S. , Roukes M. L. , Chtchelkanova A. Y. , Treger D. M.
Published: July 2002

This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.
Partial Dissociation of Water on Ru(0001)
Authors: Feibelman Peter J.
Published: July 2002

Initial water deposition on the moderately reactive precious metal surface Ru(0001) has been thought to produce an ice-like bilayer. However, calculations from first principles show that the wetting layer observed on Ru(0001) cannot be formed of undissociated water molecules. An energetically favorable alternative, consistent with the remarkable observation that the wetting layer's oxygen atoms are nearly coplanar, is a half-dissociated monolayer wherein water molecules and hydroxyl fragments are hydrogen-bonded in a hexagonal structure and hydrogen atoms bind directly to the metal.
Water in Earth's Lower Mantle
Authors: Murakami Motohiko , Hirose Kei , Yurimoto Hisayoshi , Nakashima Satoru , Takafuji Naoto
Published: July 2002

Secondary ion mass spectrometry measurements show that Earth's representative lower mantle minerals synthesized in a natural peridotitic composition can dissolve considerable amounts of hydrogen. Both MgSiO
3
-rich perovskite and magnesiowüstite contain about 0.2 weight percent (wt%) H
2
O, and CaSiO
3
-rich perovskite contains about 0.4 wt% H
2
O. The OH absorption bands in Mg-perovskite and magnesiowüstite were also confirmed with the use of infrared microspectroscopic measurements. Earth's lower mantle may store about five times more H
2
O than the oceans.
Structural Mechanisms of QacR Induction and Multidrug Recognition
Authors: Schumacher Maria A. , Miller Marshall C. , Grkovic Steve , Brown Melissa H. , Skurray Ronald A. , Brennan Richard G.
Published: July 2002

The
Staphylococcus aureus
multidrug binding protein QacR represses transcription of the
qacA
multidrug transporter gene and is induced by structurally diverse cationic lipophilic drugs. Here, we report the crystal structures of six QacR-drug complexes. Compared to the DNA bound structure, drug binding elicits a coil-to-helix transition that causes induction and creates an expansive multidrug-binding pocket, containing four glutamates and multiple aromatic and polar residues. These structures indicate the presence of separate but linked drug-binding sites within a single protein. This multisite drug-binding mechanism is consonant with studies on multidrug resistance transporters.
The RAR1 Interactor SGT1, an Essential Component of
R
Gene-Triggered Disease Resistance
Authors: Azevedo Cristina , Sadanandom Ari , Kitagawa Katsumi , Freialdenhoven Andreas , Shirasu Ken , Schulze-Lefert Paul
Published: July 2002

Plant disease resistance (
R
) genes trigger innate immune responses upon pathogen attack. RAR1 is an early convergence point in a signaling pathway engaged by multiple
R
genes. Here, we show that RAR1 interacts with plant orthologs of the yeast protein SGT1, an essential regulator in the cell cycle. Silencing the barley gene
Sgt1
reveals its role in
R
gene–triggered,
Rar1
-dependent disease resistance. SGT1 associates with SKP1 and CUL1, subunits of the SCF (Skp1-Cullin–F-box) ubiquitin ligase complex. Furthermore, the RAR1-SGT1 complex also interacts with two COP9 signalosome components. The interactions among RAR1, SGT1, SCF, and signalosome subunits indicate a link between disease resistance and ubiquitination.
Regulation of Daily Locomotor Activity and Sleep by Hypothalamic EGF Receptor Signaling
Authors: Kramer Achim , Yang Fu-Chia , Snodgrass Pamela , Li Xiaodong , Scammell Thomas E. , Davis Fred C. , Weitz Charles J.
Published: July 2002

The circadian clock in the suprachiasmatic nucleus (SCN) is thought to drive daily rhythms of behavior by secreting factors that act locally within the hypothalamus. In a systematic screen, we identified transforming growth factor-α (TGF-α) as a likely SCN inhibitor of locomotion. TGF-α is expressed rhythmically in the SCN, and when infused into the third ventricle it reversibly inhibited locomotor activity and disrupted circadian sleep-wake cycles. These actions are mediated by epidermal growth factor (EGF) receptors on neurons in the hypothalamic subparaventricular zone. Mice with a hypomorphic EGF receptor mutation exhibited excessive daytime locomotor activity and failed to suppress activity when exposed to light. These results implicate EGF receptor signaling in the daily control of locomotor activity, and identify a neural circuit in the hypothalamus that likely mediates the regulation of behavior both by the SCN and the retina.
A Role for Lipid Shells in Targeting Proteins to Caveolae, Rafts, and Other Lipid Domains
Authors: Anderson Richard G. W. , Jacobson Ken
Published: July 2002

The surface membrane of cells is studded with morphologically distinct regions, or domains, like microvilli, cell-cell junctions, and coated pits. Each of these domains is specialized for a particular function, such as nutrient absorption, cell-cell communication, and endocytosis. Lipid domains, which include caveolae and rafts, are one of the least understood membrane domains. These domains are high in cholesterol and sphingolipids, have a light buoyant density, and function in both endocytosis and cell signaling. A major mystery, however, is how resident molecules are targeted to lipid domains. Here, we propose that the molecular address for proteins targeted to lipid domains is a lipid shell.
Constraints in the Evolution of Sex Ratio Adjustment
Authors: West Stuart A. , Sheldon Ben C.
Published: July 2002

When the relative fitness of male and female offspring varies with environmental conditions, evolutionary theory predicts that parents should adjust the sex of their offspring accordingly. Qualitative and even quantitative support for this prediction is striking in some taxa but much less convincing in others. Explaining such variation across taxa in the fit of sex ratio theory remains a major challenge. We use meta-analysis to test the role of two constraints in the evolution of sex ratios. Based on analysis of sex ratio skews in birds and wasps, we show that (i) mechanisms of sex determination do not necessarily constrain the evolution of sex ratio adjustment, and (ii) parental ability to predict their offsprings' environment influences the evolution of sex ratio patterns across taxa. More generally, our results show that multiple constraints may determine the precision of adaptation.
Discrete Microdomains with High Concentration of cAMP in Stimulated Rat Neonatal Cardiac Myocytes
Authors: Zaccolo Manuela , Pozzan Tullio
Published: July 2002

The second messenger cyclic adenosine monophosphate (cAMP) is the most important modulator of sympathetic control over cardiac contractility. In cardiac myocytes and many other cell types, however, cAMP transduces the signal generated upon stimulation of various receptors and activates different cellular functions, raising the issue of how specificity can be achieved. In the general field of signal transduction, the view is emerging that specificity is guaranteed by tight localization of signaling events. Here, we show that in neonatal rat cardiac myocytes, β-adrenergic stimulation generates multiple microdomains with increased concentration of cAMP in correspondence with the region of the transverse tubule/junctional sarcoplasmic reticulum membrane. The restricted pools of cAMP show a range of action as small as approximately 1 micrometer, and free diffusion of the second messenger is limited by the activity of phosphodiesterases. Furthermore, we demonstrate that such gradients of cAMP specifically activate a subset of protein kinase A molecules anchored in proximity to the T tubule.
Sharp Sides to the African Superplume
Authors: Ni Sidao , Tan Eh , Gurnis Michael , Helmberger Don
Published: July 2002

Beneath southern Africa is a large structure about 1200 kilometers across and extending obliquely 1500 kilometers upward from the core-mantle boundary with a shear velocity reduction of about 3%. Using a fortuitous set of
SKS
phases that travel along its eastern side, we show that the boundary of the anomaly appears to be sharp, with a width less than 50 kilometers, and is tilted outward from its center. Dynamic models that fit the seismic constraints have a dense chemical layer within an upwardly flowing thermal structure. The tilt suggests that the layer is dynamically unstable on geological time scales.