bluuebunny/crossref_metadata_2025 huggingface dataset:
Adsorption and decomposition of diethyldiethoxysilane on silicon surfaces: New possibilities for SiO2 deposition
Authors: Wise M. L. , Sneh O. , Okada L. A. , George S. M.
Published: July 2002

Diethyldiethoxysilane is an organosilicate that may offer new possibilities for SiO2 deposition. In this study, the adsorption and decomposition of diethyldiethoxysilane (DEDEOS) was examined on Si(100)2×1 and porous silicon surfaces using laser-induced thermal desorption (LITD), temperature-programmed desorption (TPD), Fourier-transform infrared (FTIR), and Auger electron spectroscopy techniques. The FTIR studies revealed that DEDEOS dissociatively adsorbs on porous silicon and deposits ethyl and ethoxy species. These species are observed to decompose via a β-hydride elimination mechanism at ∼700 K. In agreement with this mechanism, TPD studies on Si(100)2×1 observed ethylene (C2H4) at ∼700 K and H2 desorption at ∼800 K. Additionally, the controlled deposition of SiO2 was achieved on Si(100)2×1 using repetitive cycles of DEDEOS adsorption at 300 K followed by thermal annealing at 820 K for 300 s. After the rapid deposition of an oxygen coverage of θO∼2.4 ML, the oxygen deposition rate decreased and reached a constant deposition rate of ∼0.04 ML oxygen per cycle after ten cycles. The constant reactivity of the growing SiO2 film was attributed to dangling bond reactive sites resulting from β-hydride elimination of the ethyl groups (SiCH2CH3→SiH+CH2=CH2) and subsequent H2 desorption (2SiH→2Si+H2). DEDEOS was also utilized to grow SiO2 films under high-pressure chemical vapor deposition conditions. The rate of SiO2 deposition displayed Arrhenius behavior with an activation barrier of Eact=49±6 kcal/mol. This activation barrier is similar to activation barriers measured earlier for SiO2 growth using tetraethoxysilane (TEOS). The deposition rate of SiO2 by DEDEOS was approximately 60 times slower than the deposition rate by TEOS at 943 K and 0.5 Torr.
Time-resolved luminescence measurements on GaAs homostructures using pulse excitation of a scanning tunneling microscope
Authors: Stehle M. , Bischoff M. , Pagnia H. , Horn J. , Marx N. , Weiss B. L. , Hartnagel H. L.
Published: July 2002

We report the first time-resolved luminescence measurements using the tunneling current of a scanning tunneling microscope for excitation with its high spatial resolution. This new technique enabled us to measure the minority-carrier lifetime and the doping concentration in the intrinsic region of a specially tailored n+-n-i-n+-GaAs homostructure. Further, we show that the minority-carrier lifetime depends, as expected, strongly on surface recombination which demonstrates that this technique may be used to determine the local surface state density.
Initial results from an extreme ultraviolet interferometer operating with a compact laser plasma source
Authors: Ray-Chaudhuri A. K. , Krenz K. D. , Nissen R. P. , Haney S. J. , Fields C. H. , Sweatt W. C. , MacDowell A. A.
Published: July 2002

When characterizing an extreme ultraviolet (EUV) lithographic optical system, visible light interferometry is limited to measuring wave front aberration caused by surface figure error while failing to measure wave front errors induced by the multilayer coatings. This necessitates the development of interferometric techniques at an EUV camera’s operational wavelength (at-wavelength testing), which is typically around 13 nm. While a laser plasma source (LPS) is being developed as a lithography production source, it has generally been considered that only an undulator located at a synchrotron facility can provide the necessary laserlike point source brightness for EUV interferometry. Although an undulator-based approach has been successfully demonstrated, it would be advantageous to test a camera in its operational configuration with an LPS. We are developing the latter approach by utilizing extended source size schemes to provide usable flux throughput. A slit mounted at the source plane can provide the necessary spatial coherence for lateral shearing interferometry. Initial results from an EUV lateral shear interferometer based on the Ronchi test are presented.
Realization of atomic layer etching of silicon
Authors: Athavale Satish D. , Economou Demetre J.
Published: July 2002

An experimental system and methodology were developed to realize dry etching of single crystal silicon with monolayer accuracy. Atomic layer etching of silicon is a cyclic process composed of four consecutive steps: reactant adsorption, excess reactant evacuation, ion irradiation, and product evacuation. When successful, completion of one cycle results in removal of one monolayer of silicon. The process was self-limiting with respect to both reactant and ion dose. Control of the ion energy was the most important factor in realizing etching of one monolayer per cycle.
Reliability of ultimate ultrathin silicon oxide films produced by the continuous ultradry process
Authors: Yamada Hiroshi
Published: July 2002

Time-dependent dielectric breakdown lifetime of 3.0–3.5-nm-thick silicon oxide films was investigated using metal–oxide semiconductors (MOSs) produced by our recently proposed continuous ultradry process. This process prepares an ideal MOS gate oxide with few hydrogen-related defects and precise thickness distribution that drastically affect the lifetime of such ultimate ultrathin oxide films. The lifetime of 3.0-nm-thick oxide films was almost equivalent to that of 3.5-nm-thick films, although their lifetime was apparently different from the lifetime of 5.0-nm-thick films. Therefore, the stress-induced charge trapping mechanism in such ultimate ultrathin films that mainly determines the lifetime is probably identifiable. This lifetime saturation also suggests that their lifetime may be close to the intrinsic maximum limitation for ultrathin silicon oxide films.
Microwave tunneling current from the resonant interaction of an amplitude modulated laser with a scanning tunneling microscope
Authors: Hagmann Mark J.
Published: July 2002

Simulations of the interaction of light with tunneling electrons in a scanning tunneling microscope suggest that there is a resonance in which the tunneling current is markedly increased. If a laser operating near the resonance is amplitude modulated, then the tunneling current will have components at the frequency of modulation and its harmonics, as well as at optical frequencies and dc. It is suggested that this effect should be considered as a new mode for laser-assisted scanning tunneling microscopy. The simulations for the examples were made by solving the one-dimensional time-dependent Schrödinger equation by the numerical procedure of shooting methods, using a planar–planar scanning tunneling microscope model with the exact multiple image potential.
Modeling of thermal effects in silicon field emitters
Authors: Ancona M. G.
Published: July 2002

Thermal effects in silicon field emitters are analyzed and an attempt is made to identify ‘‘intrinsic’’ phenomena that can occur under normal operating conditions which might trigger catastrophic breakdown. For this analysis, an approximate quasiequilibrium treatment is employed that is presumably most accurate at low current levels. Unlike for metal field emitters, where Nottingham heating dominates, for silicon field emitters it is found that the heating is predominantly ohmic in origin except at low currents. The temperatures produced by this heating are much larger than those in metals for a given current and geometry. And although the calculations indicate that for typical designs these temperatures are still small enough so as not to represent a direct reliability threat, their size coupled with their sensitivity to changes in current (ohmic heating increases as the square of the current) suggests caution both in design and in trusting the results of this relatively crude analysis. With respect to the former, designs for which the heat sinking is weak, e.g., atomically sharp tips or tips on high aspect-ratio posts, may be susceptible to thermal failure. Similarly, additional ohmic heating due to ac currents can push a design into thermal jeopardy. Finally, it is found that increasing the doping does not appear to lessen the ohmic heating significantly (because of band bending and scattering effects), nor does the Nottingham effect, which under some circumstances seems to actually provide cooling, appear sufficient to offset the ohmic effects.
Interpretation of surface-induced optical anisotropy of clean, hydrogenated, and oxidized vicinal silicon surfaces investigated by reflectance-difference spectroscopy
Authors: Rossow U. , Mantese L. , Aspnes D. E.
Published: July 2002

Using reflectance-difference spectroscopy, we determine surface-induced optical anisotropy (SIOA) spectra of clean, hydrogenated, and oxidized (113) and vicinal (001) Si surfaces to obtain a better understanding of the origin of the optical response of surfaces and interfaces. Hydrogenation was performed either by etching in dilute HF or by exposing clean surfaces to atomic hydrogen. Hydrogenated and oxidized vicinal surfaces show energy-derivativelike spectra that roughly scale with offcut angle, indicating step-induced behavior, and exhibiting features near 3.4 and 4.2 eV, the threshold energies of the (E1, E0′) and E2 interband critical points of bulk Si, respectively. The appearance of derivativelike line shapes indicates that bulk threshold energies become dichroic near the surface due to the surface-induced modification of the potential, as supported by model calculations. However, direct integration yields dielectric functions somewhat different from bulk values, indicating that the surface affects the near-surface dielectric function. SIOA spectra for clean surfaces are qualitatively different, exhibiting dielectric-functionlike line shapes that appear to originate primarily from terraces, although indications of smaller derivativelike contributions are also found.
Neural network correction of nonlinearities in scanning probe microscope images
Authors: Hadjiiski L. , Münster S. , Oesterschulze E. , Kassing R.
Published: July 2002

A neural network approach has been applied to three-dimensional correction of the geometrical nonlinearities in scanning probe microscope images. Creep and hysteresis of the piezo scanner cause nonlinear trajectory of the sensor movements over the sample which results in geometrically distorted scanning probe microscope images. A calibration sample with regular structure has been used for collection of data for the geometrical distortions. The inverse neural network model has been trained with these data. A single hidden layer network was sufficient to obtain satisfactory accuracy of the corrections. Using the same training data set polynomial approximation models have been implemented also. The accuracy of the neural network approximations shows to be better than the accuracy of the polynomial one. Additionally neural network corrected images reveal no boundary distortions typical for the linear piecewise approximation. Using the neural network approach surface maps of different scanning probe microscope techniques scanning tunneling microscopy, atomic force microscopy, and scanning thermal microscope have been successfully corrected.
Subnanometer stability of nanostage supports
Authors: van der Wulp H. , Pistecky P. V. , Heerens W. Chr.
Published: July 2002

In order to obtain atomic resolution in a transmission electron microscope, a piezo-driven nanostage with subnanometer stability is under development. Inertial sliding motion is used to move the nanostage table in the plane perpendicular to gravity. One of the most important parameters on the way to such a nanostage is the design of the nanostage table supports. Through several experiments, the stability of two different nanostage table supports is studied: a kinematic and a nonkinematic support. In order to explain the submicrometer and nanometer stage table drift measured in the direction parallel to gravity, a subnanometer contact theory is presented. This theory explains the stage table drift by the following parameters: the size of the apparent contact area of the support, the gravity forces working on the support, the multimolecular layer of adsorbed water molecules on all contact surfaces, creep in the contact points and settling of the contact through microsliding at the contact points. For the initial placement of a nonkinematic support in ambient air with an apparent contact area size of 15×15 mm2, a stage table drift of 100 nm over 30 min was measured, which almost exactly followed a logarithmic curve. This drift reduced to about 45 nm when the support was placed in a low vacuum, where the number of layers of adsorbed water molecules on the support surfaces is reduced to one or two. Filling the vacuum chamber with nitrogen gas resulted in an even lower stage table drift (25 nm). Stage table drift after single inertial sliding steps is about 25% of the initial amount of drift. In case of a kinematic support, the apparent contact area reduces significantly and stage table drifts after initial placement of 1 up to 3 nm were found. The drift after single inertial sliding steps is on the same order of magnitude. These drifts are attributed to contact creep, which was minimized by optimization of the material selection of both contact surfaces. A combination of two hard surfaces showed almost no creep at the subnanometer level. Therefore, this highly stable kinematic support suits the nanostage application very well.
Microtips and resistive sheet: A theoretical description of the emissive properties of this system
Authors: Baptist R. , Bachelet F. , Constancias C.
Published: July 2002

The purpose of this article is to present a model which describes completely in a very simple way the physics of field-effect emission for an ensemble of tips located on a two-dimensional lateral resistive sheet. Given the well-known a and b parameters of each tips, the geometry of the mesh and the square resistance of the resistive sheet, we have determined exactly the voltage on each tip as a function of its position in the mesh and then calculated each current effectively emitted. Thanks to this theory some simple questions, as the following, could be answered: (1) When a specified number of tips is needed, and the mesh geometry fixed, where do we have to put the tips in order to get the most uniform emission or the maximum of current? (2) When a tip has a short with the grid, how many tips are affected by the increase of potential in the region near the tip? (3) Is it «better» to have some big meshes with a great number of tips or numerous small ones with a few tips? Is it «better» to have lines of tips or compact arrays of tips distributed on a resistive sheet? (4) What is the «equivalent» resistance of this ensemble? What is its exact meaning?
Atom probe determination of the multicomponent material thermo-field microprotrusion parameters
Authors: Kontorovich E. L. , Loginov M. V. , Shrednik V. N.
Published: July 2002

A time of flight atom probe has been used for investigation of some alloy thermo-field microprotrusions. The microprotrusions from HfMo (7 at. % Hf), HfW (3 at. % Hf), and IrRh (26 at. % Ir) alloys have been formed on the tip surface by simultaneous action of high temperature T and high electric field F. Conditions of forming, reproducibility of growth, location, emission properties, and chemical composition of the microprotrusions have been carefully studied. The peculiarities of field evaporation of the microprotrusions both at room temperature and at elevated temperature have also been investigated. Appreciable segregation of Rh and Hf on the surface of the alloys has been discovered. The thermo-field microprotrusions consist mainly of these surface components. Point ion and electron sources controlled by changing of T and F can be created on the basis of these microprotrusions.
Characterization of low-energy (100 eV–10 keV) boron ion implantation
Authors: Collart E. J. H. , Weemers K. , Gravesteijn D. J. , van Berkum J. G. M.
Published: July 2002

Low-energy boron implantations between 100 eV and 10 keV are characterized using secondary ion mass spectrometry and electrical measurements of sheet carrier concentrations and sheet resistance. Factors that may limit the use of ion implantation for future generations of semiconductor devices are discussed. At 1 keV various tilt angles show identical channeling behavior, and only a slight difference with an amorphous implant. It is found that as the energy is lowered from 1 keV to 100 eV much of the reduction in profile depth is canceled out by transient enhanced diffusion during a rapid thermal anneal. Hall–van der Pauw measurements show that with lower implant energy it becomes more difficult to activate the implanted dose. This is possibly due to increased clustering of boron, but more likely due to the fact that the surface starts to act as a trapping and deactivation center for B.
Field emission energy distribution analysis of wide-band-gap field emitters
Authors: Schlesser R. , McCarson B. L. , McClure M. T. , Sitar Z.
Published: July 2002

Field emission from diamond and cubic boron nitride coated emitters has been investigated by field emission energy distribution (FEED) analysis. In order to study the mechanisms governing the emission process, we developed a modified FEED technique that consisted of measuring electron energy spectra as a function of the extraction voltage applied to the field emitter. This technique has led to a detailed study of band bending effects due to field penetration. Voltage drop across the emitting, wide-band-gap layers (thickness typ. 0.5 μm) was in the order of 1% of the applied extraction voltage. The observed FEED spectra and their dependence on the extraction voltage were explained in terms of a simplified band model. In nominally undoped diamond and cubic boron nitride samples, field emission was found to originate from the conduction band minimum. Injection of electrons from the metallic back contact into the wide-band-gap material has been identified as the essential limiting factor to the emission current. Mo2C interlayers between Mo back contacts and emitting diamond layers were formed by annealing at 500 °C for several hours and led to more stable emission currents, typically enhanced by more than one order of magnitude.
Reflection high-energy electron diffraction during substrate rotation: A new dimension forin situcharacterization
Authors: Braun W. , Möller H. , Zhang Y.-H.
Published: July 2002

We present two methods to access reciprocal space with reflection high-energy electron diffraction (RHEED) during substrate rotation. The extraction of an arbitrary number of still frames from a continuously changing RHEED pattern is realized by triggering the substrate rotation and it allows analysis of quasistatic RHEED patterns that are updated every revolution. At the same time, the intensity along a line parallel to the shadow edge can be used to reconstruct a planar cut through the reciprocal lattice similar to a low-energy electron diffraction pattern. This RHEED pattern directly reveals the symmetry of the surface reconstruction and its changes during the deposition process.
Structural study of YSi1.7 layers formed by channeled ion beam synthesis
Authors: Wu M. F. , Yao Shude , Vantomme A. , Hogg S. , Pattyn H. , Langouche G. , Yang Qinqing , Wang Qiming
Published: July 2002

High quality YSi1.7 layers (χmin of Y is 3.5%) have been formed by 60 keV Y ion implantation in Si (111) substrates to a dose of 1.0×1017/cm2 at 450 °C using channeled ion beam synthesis (CIBS). It shows that, compared to the conventional nonchanneled ion beam synthesis, CIBS is beneficial in forming YSi1.7 layers with better quality due to the lower defect density created in the implanted layer. Rutherford backscattering/channeling and x-ray diffraction have been used to study the structure and the strain of the YSi1.7 layers. The perpendicular and parallel elastic strains of the YSi1.7 epilayer are e⊥ =−0.67%±0.02% and e∥=+1.04%±0.08%. The phenomenon that a nearly zero mismatch of the YSi1.7/Si (111) system results in a nonpseudomorphic epilayer with a rather large parallel strain relative to the Si substrate (ε∥=+1.09%) is explained, and the model is further used to explain the elastic strain of epitaxial ErSi1.7 and GdSi1.7 rare-earth silicides.
Patterning of thin film NiMnSb using inductively coupled plasma etching
Authors: Hong J. , Caballero J. A. , Lambers E. S. , Childress J. R. , Pearton S. J.
Published: July 2002

Several plasma chemistries based on fluorine (SF6, NF3), Cl2/Ar, BI3/Ar, or BBr3/Ar were compared for etching NiMnSb under inductively coupled plasma (ICP) conditions. ICP source power, radio frequency chuck power, and plasma composition were found to have strong effects on the etch rate. SF6/Ar discharges provided the fastest etch rates for NiMnSb (>1 μm min−1) even with small amounts of source power (100 W) addition. On the other hand, NF3/Ar showed net deposition rather than etching at source powers >100 W or at high NF3 percentages. Cl2/Ar showed a similar trend, with net deposition at low dc self-bias (−100 V), but net etching above this threshold. BBr3/Ar discharges produced deposition under all the conditions investigated while relatively high etch rates (⩾3000 Å min−1) were obtained with BI3/Ar at halide percentages ⩾70%. In terms of etched surface morphology, SF6/Ar provided the best surfaces, with root-mean-square roughness of 2.5 nm and vertical etched profiles. Surfaces etched in plasma chemistries other than SF6/Ar revealed Mn enrichment, an indication of involatile Mn etch products. The SF6 chemistry has the further advantage of the absence of the corrosion noted with Cl2-based mixtures.
Ohmic contacts in AlSb/InAs high electron mobility transistors for low-voltage operation
Authors: Boos J. B. , Bennett B. R. , Kruppa W. , Park D. , Mittereder J. , Bass R. , Twigg M. E.
Published: July 2002

We report on the fabrication and characteristics of Pd/Pt/Au ohmic contacts that have been used in AlSb/InAs high electron mobility transistors (HEMTs) with low access resistance. The metalization exhibits minimal in-diffusion and a contact resistance of 0.08 Ω mm after a 175 °C hot-plate heat treatment. By comparison, AuGe/Ni/Pt/Au ohmic contact metalizations formed using a 300 °C rapid thermal anneal exhibit a contact resistance of 0.11 Ω mm, but with considerable Au in-diffusion. Using the Pd/Pt/Au contact, 0.6 μm gate-length AlSb/InAs HEMTs exhibit a low-field source-drain resistance of 0.47 Ω mm and a transconductance above 1 S/mm. After removal of the gate bonding pad capacitance from an equivalent circuit, an fTLg product of 38 GHz μm is obtained at VDS=0.4 V. HEMTs with a 60 nm gate length exhibit a low-field source-drain resistance of 0.35 Ω mm and a measured fT of 90 GHz at a drain voltage of only 100 mV. These fTLg and fT values are the highest reported for any field effect transistor at these drain voltages.
Finite-element calculations of mechanical stresses induced by water adsorption/desorption in silicate glasses
Authors: Hoffmann Thomas , LeDuc Philippe , Senez Vincent
Published: July 2002

The mechanical stresses induced by adsorption or desorption of water from various dielectric films (vitreous silicate glass, phosphosilicate glass, borophosphosilicate glass, and spin on glass) are analyzed in terms of two-dimensional finite-element simulations. For this, a new model that predicts the stress induced by the adsorption or desorption of water in the films (i.e., the extrinsic stress) has been added to a general algorithm dedicated to the calculation of stresses in silicon processes. This model was calibrated for (un)doped silicate glasses and spin on glass. Thanks to this work, a mechanical study and optimization of back-end processes, including all sources of stress, are now available. It is shown that the stresses developed in densified spin on glass can be very high and are likely to degrade the reliability of devices planarized by these techniques.
Influence of the depth calibration procedure on the apparent shift of impurity depth profiles measured under conditions of long-term changes in erosion rate
Authors: Wittmaack K.
Published: July 2002

The long-term shift of impurity depth profiles in silicon, observed by secondary ion mass spectrometry under oblique low-energy O2+ bombardment with oxygen flooding is simulated using a simple model. The erosion rate is assumed to decrease in two steps, a rapid initial falloff due to oxygen incorporation in the sample followed by a less pronounced, but long-term change associated with ripple formation. Two methods of depth calibration are compared, the standard procedure of measuring the crater depth and the recent approach of using shallow delta doping markers with known spacing. The results obtained by the two methods exhibit pronounced differences, which are due to the fact that the delta-spacing approach is based on the use of a depth-dependent local erosion rate whereas the crater-depth method involves a mean erosion rate averaged over the total sputtered depth. The vastly different shifts reported by independent groups for profiling of boron-delta markers in silicon by 1 keV O2+ at about 60° can be reproduced surprisingly well. It is shown that the apparent shifts observed under conditions of ongoing changes in erosion rate are not fixed numbers, as in the case of normally incident O2+ beams, but depend strongly on the details of the depth calibration procedure. Used in combination with shallow markers (<20 nm), the delta-spacing approach yields misleadingly small apparent shifts because the local erosion rate is still significantly higher than the (quasi-) stationary value. Full control of erosion artifacts can only be achieved by profiling to sufficiently large depths (>50 nm). The cumulative shifts for 1 keV O2+ at 60° with flood are about three times larger than at normal incidence in vacuum, and even four times larger for 0.5 and 1 keV at 55° with flood.
Anthropogenic Warming of Earth's Climate System
Authors: Levitus Sydney , Antonov John I. , Wang Julian , Delworth Thomas L. , Dixon Keith W. , Broccoli Anthony J.
Published: July 2002

We compared the temporal variability of the heat content of the world ocean, of the global atmosphere, and of components of Earth's cryosphere during the latter half of the 20th century. Each component has increased its heat content (the atmosphere and the ocean) or exhibited melting (the cryosphere). The estimated increase of observed global ocean heat content (over the depth range from 0 to 3000 meters) between the 1950s and 1990s is at least one order of magnitude larger than the increase in heat content of any other component. Simulation results using an atmosphere-ocean general circulation model that includes estimates of the radiative effects of observed temporal variations in greenhouse gases, sulfate aerosols, solar irradiance, and volcanic aerosols over the past century agree with our observation-based estimate of the increase in ocean heat content. The results we present suggest that the observed increase in ocean heat content may largely be due to the increase of anthropogenic gases in Earth's atmosphere.
Length of the Flagellar Hook and the Capacity of the Type III Export Apparatus
Authors: Makishima Shigeru , Komoriya Kaoru , Yamaguchi Shigeru , Aizawa Shin-Ichi
Published: July 2002

Length determination in biology generally uses molecular rulers. The hook, a part of the flagellum of motile bacteria, has an invariant length. Here, we examined hook length and found that it was determined not by molecular rulers but probably by the amount of subunit protein secreted by the flagellar export apparatus. The export apparatus shares common features with the type III virulence-factor secretion machinery and thus may be used more widely in length determination of structures other than flagella.
A Microscopic Basis for the Global Appearance of Energy Landscapes
Authors: Wales David J.
Published: July 2002

It is shown that the appearance of a multidimensional potential energy surface, or potential energy landscape, can be related to the form of the interatomic or intermolecular potential. Catastrophe theory enables us to describe how the geometry of the surface changes with parameters in the potential, and provides universal scaling relations that explain, for example, the asymmetric reaction profiles observed for systems bound by long-range forces. The principal result is an unexpected connection between barrier heights, path lengths, and vibrational frequencies, with applications to a wide variety of problems in chemical physics, ranging from Hammond's postulate in organic chemistry, to the relaxation dynamics of complex systems such as glasses and biomolecules.
Embryonic Skulls of Titanosaur Sauropod Dinosaurs
Authors: Chiappe Luis M. , Salgado Leonardo , Coria Rodolfo A.
Published: July 2002

Little is known about the cranial anatomy of the taxonomically diverse and geographically widespread titanosaurs, a paucity that has hindered inferences about the genealogical history and evolutionary development of the latest sauropod dinosaurs. Newly discovered fossil eggs containing embryonic remains from the Late Cretaceous of Argentina provide the first articulated skulls of titanosaur dinosaurs. The nearly complete fetal skulls shed light on the evolution of some of the most notable cranial features of sauropod dinosaurs, including the retraction of the external nares, the forward rotation of the braincase, and the abbreviation of the infraorbital region.
Origin of Whales from Early Artiodactyls: Hands and Feet of Eocene Protocetidae from Pakistan
Authors: Gingerich Philip D. , Haq Munir ul , Zalmout Iyad S. , Khan Intizar Hussain , Malkani M. Sadiq
Published: July 2002

Partial skeletons of two new fossil whales,
Artiocetus clavis
and
Rodhocetus balochistanensis
, are among the oldest known protocetid archaeocetes. These came from early Lutetian age (47 million years ago) strata in eastern Balochistan Province, Pakistan. Both have an astragalus and cuboid in the ankle with characteristics diagnostic of artiodactyls;
R. balochistanensis
has virtually complete fore- and hind limbs. The new skeletons are important in augmenting the diversity of early Protocetidae, clarifying that Cetacea evolved from early Artiodactyla rather than Mesonychia and showing how early protocetids swam.
Requirement of Heterochromatin for Cohesion at Centromeres
Authors: Bernard Pascal , Maure Jean-François , Partridge Janet F. , Genier Sylvie , Javerzat Jean-Paul , Allshire Robin C.
Published: July 2002

Centromeres are heterochromatic in many organisms, but the mitotic function of this silent chromatin remains unknown. During cell division, newly replicated sister chromatids must cohere until anaphase when Scc1/Rad21–mediated cohesion is destroyed. In metazoans, chromosome arm cohesins dissociate during prophase, leaving centromeres as the only linkage before anaphase. It is not known what distinguishes centromere cohesion from arm cohesion. Fission yeast Swi6 (a Heterochromatin protein 1 counterpart) is a component of silent heterochromatin. Here we show that this heterochromatin is specifically required for cohesion between sister centromeres. Swi6 is required for association of Rad21-cohesin with centromeres but not along chromosome arms and, thus, acts to distinguish centromere from arm cohesion. Therefore, one function of centromeric heterochromatin is to attract cohesin, thereby ensuring sister centromere cohesion and proper chromosome segregation.
Confinement Effect on Dipole-Dipole Interactions in Nanofluids
Authors: Baugh Jonathan , Kleinhammes Alfred , Han Daxing , Wang Qi , Wu Yue
Published: July 2002

Intermolecular dipole-dipole interactions were once thought to average to zero in gases and liquids as a result of rapid molecular motion that leads to sharp nuclear magnetic resonance lines. Recent papers have shown that small residual couplings survive the motional averaging if the magnetization is nonuniform or nonspherical. Here, we show that a much larger, qualitatively different intermolecular dipolar interaction remains in nanogases and nanoliquids as an effect of confinement. The dipolar coupling that characterizes such interactions is identical for all spin pairs and depends on the shape, orientation (with respect to the external magnetic field), and volume of the gas/liquid container. This nanoscale effect is useful in the determination of nanostructures and could have unique applications in the exploration of quantum space.
A Selective Advantage to Immigrant Genes in a
Daphnia
Metapopulation
Authors: Ebert Dieter , Haag Christoph , Kirkpatrick Mark , Riek Myriam , Hottinger Jürgen W. , Pajunen V. Ilmari
Published: July 2002

Immigrants to habitats occupied by conspecific organisms are usually expected to be competitively inferior, because residents may be locally adapted. If residents are inbred, however, mating between immigrants and residents results in offspring that may enjoy a fitness advantage from hybrid vigor. We demonstrate this effect experimentally in a natural
Daphnia
metapopulation in which genetic bottlenecks and local inbreeding are common. We estimate that in this metapopulation, hybrid vigor amplifies the rate of gene flow several times more than would be predicted from the nominal migration rate. This can affect the persistence of local populations and the entire metapopulation.
Artificial Blood
Authors: Squires Jerry E.
Published: July 2002

Blood substitutes are under development for transfusion in place of donor blood during emergencies and lengthy surgeries. The first generation of blood substitutes is currently in clinical trials.
Should Non-Peer-Reviewed Raw DNA Sequence Data Release Be Forced on the Scientific Community?
Authors: Adams Mark D. , Venter J. Craig
Published: July 2002

The authors argue that the early release of unedited, unfinished genome sequence information into the public domain, as currently done by some sequencing centers on a nightly basis, should not be a policy imposed on all laboratories involved in genome research. Such data are presented without the benefit of either peer review or the researchers' own analysis, and their early release may even subvert the peer-review process. Also, as the number of genome sequencing projects increases, the amount of raw data added to public databases would become unmanageably large. Without rigorous quality control procedures and annotation, genomic sequence data are not necessarily useful.
Regulation of Cell Cycle Synchronization by
decapentaplegic
During
Drosophila
Eye Development
Authors: Penton Andrea , Selleck Scott B. , Hoffmann F. Michael
Published: July 2002

In the developing
Drosophila
eye, differentiation is coordinated with synchronized progression through the cell cycle. Signaling mediated by the transforming growth factor-β-related gene
decapentaplegic
(
dpp
) was required for the synchronization of the cell cycle but not for cell fate specification. DPP may affect cell cycle synchronization by promoting cell cycle progression through the G
2
-M phases. This synchronization is critical for the precise assembly of the eye.
Subterranean Life
Authors: Ghiorse William C.
Published: July 2002

A surprising array of subterranean microorganisms can be found living in a variety of geological formations. In his Perspective, Ghiorse reports on a recent meeting in Davos, Switzerland, on subsurface microbiology. Among the exciting topics being discussed in this field are new hypotheses on origins and long-term survival of microbial life in deep crustal rocks, subsurface ecosystems supported by chemosynthetic primary production, and the possibility of new sources of industrially important organisms.
Numerical Simulation of the Cretaceous Tethys Circumglobal Current
Authors: Bush Andrew B. G.
Published: July 2002

Certain paleobiogeographical reconstructions of ocean currents during the Cretaceous (about 144 to 65 million years ago) suggest that a circumglobal tropical current flowed westward through the continental configuration of that time. Although some numerical climate models failed in initial attempts to simulate this current, simulations with a coupled atmosphere-ocean model with relatively high spatial resolution and a late Cretaceous continental distribution show that a circumglobal current is a robust feature even though local surface currents in the Tethys Seaway reverse during the south Eurasian monsoon months.
Extraterrestrial Handedness?
Authors: Bada Jeffrey L.
Published: July 2002

Biological systems exhibit molecular handedness: During biosynthesis, predominantly L amino acids rather than D amino acids are incorporated into proteins. The origins of this handedness remain puzzling. In his Perspective, Bada discusses results reported by Cronin and Pizzarello in the same issue (p.
951
) showing a slight excess of L amino acids in a well-known extraterrestrial object, the Murchison meteorite. This finding suggests the existence of an enrichment process in cosmochemical environments and could possibly be the source of molecular handedness on Earth.
Lessons from Litigation over Silicone Breast Implants: A Call for Activism by Scientists
Authors: Rosenbaum James T.
Published: July 2002

In ongoing litigation to determine whether silicone breast implants cause autoimmune disease, the question of what constitutes good science is being addressed by courts, and the answer has implications for approximately $50 billion in settlements. A federal court recently used neutral scientific advisers in excluding the admissibility of the “expert” opinion that implants cause systemic disease. The lesson from this litigation paradigm is that scientists should actively ensure that neutral, reliable scientific expertise is available to the court.
Impact Excavation on Asteroid 4 Vesta: Hubble Space Telescope Results
Authors: Thomas Peter C. , Binzel Richard P. , Gaffey Michael J. , Storrs Alex D. , Wells Eddie N. , Zellner Benjamin H.
Published: July 2002

Hubble Space Telescope images of asteroid 4 Vesta obtained during the favorable 1996 apparition show an impact crater 460 kilometers in diameter near the south pole. Color measurements within the 13-kilometer-deep crater are consistent with excavation deep into a high-calcium pyroxene-rich crust or olivine upper mantle. About 1 percent of Vesta was excavated by the crater formation event, a volume sufficient to account for the family of small Vesta-like asteroids that extends to dynamical source regions for meteorites. This crater may be the site of origin for the howardite, eucrite, and diogenite classes of basaltic achondrite meteorites.
Deformation in the Lower Crust of the San Andreas Fault System in Northern California
Authors: Henstock Timothy J. , Levander Alan , Hole John A.
Published: July 2002

A continuous seismic velocity and reflectivity cross-section of the San Andreas fault system in northern California shows offsets in the lower crust and the Mohorovic̆ić Discontinuity near the San Andreas and Maacama strike-slip faults. These faults may cut through the crust to the upper mantle in a zone less than 10 kilometers wide. The northern California continental margin to the eastern edge of the Coast Ranges is underlain by a high-velocity lowermost crustal layer that may have been emplaced within 2 million years following the removal of the Gorda plate slab. The rapid emplacement and structure within this layer are difficult to reconcile with existing tectonic models.
Mapping the Inside of the Ribosome with an RNA Helical Ruler
Authors: Joseph Simpson , Weiser Bryn , Noller Harry F.
Published: July 2002

The structure of ribosomal RNA (rRNA) in the ribosome was probed with hydroxyl radicals generated locally from iron(II) tethered to the 5′ ends of anticodon stem-loop analogs (ASLs) of transfer RNA. The ASLs, ranging in length from 4 to 33 base pairs, bound to the ribosome in a messenger RNA–dependent manner and directed cleavage to specific regions of the 16
S
, 23
S
, and 5
S
rRNA chains. The positions and intensities of cleavage depended on whether the ASLs were bound to the ribosomal A or P site, and on the lengths of their stems. These data predict the three-dimensional locations of the rRNA targets relative to the positions of A- and P- site transfer RNAs inside the ribosome.
Observed Hemispheric Asymmetry in Global Sea Ice Changes
Authors: Cavalieri D. J. , Gloersen P. , Parkinson C. L. , Comiso J. C. , Zwally H. J.
Published: July 2002

From November 1978 through December 1996, the areal extent of sea ice decreased by 2.9 ± 0.4 percent per decade in the Arctic and increased by 1.3 ± 0.2 percent per decade in the Antarctic. The observed hemispheric asymmetry in these trends is consistent with a modeled response to a carbon dioxide–induced climate warming. The interannual variations, which are 2.3 percent of the annual mean in the Arctic, with a predominant period of about 5 years, and 3.4 percent of the annual mean in the Antarctic, with a predominant period of about 3 years, are uncorrelated.
Nearly Singular Magnetic Fluctuations in the Normal State of a High-
T
c
Cuprate Superconductor
Authors: Aeppli G. , Mason T. E. , Hayden S. M. , Mook H. A. , Kulda J.
Published: July 2002

Polarized and unpolarized neutron scattering was used to measure the wave vector– and frequency-dependent magnetic fluctuations in the normal state (from the superconducting transition temperature,
T
c
= 35 kelvin, up to 350 kelvin) of single crystals of La
1.86
Sr
0.14
CuO
4
. The peaks that dominate the fluctuations have amplitudes that decrease as
T
−2
and widths that increase in proportion to the thermal energy,
k
B
T
(where
k
B
is Boltzmann's constant), and energy transfer added in quadrature. The nearly singular fluctuations are consistent with a nearby quantum critical point.
Magnetic Properties Experiments on the Mars Pathfinder Lander: Preliminary Results
Authors: Hviid S. F. , Madsen M. B. , Gunnlaugsson H. P. , Goetz W. , Knudsen J. M. , Hargraves R. B. , Smith P. , Britt D. , Dinesen A. R. , Mogensen C. T. , Olsen M. , Pedersen C. T. , Vistisen L.
Published: July 2002

Many of the particles currently suspended in the martian atmosphere are magnetic, with an average saturation magnetization of about 4 A·m
2
/kg (amperes times square meters per kilogram). The particles appear to consist of claylike aggregates stained or cemented with ferric oxide (Fe
2
O
3
); at least some of the stain and cement is probably maghemite (γ-Fe
2
O
3
). The presence of the γ phase would imply that Fe
2+
ions leached from the bedrock, passing through a state as free Fe
2+
ions dissolved in liquid water. These particles could be a freeze-dried precipitate from ground water poured out on the surface. An alternative is that the magnetic particles are titanomagnetite occurring in palagonite and inherited directly from a basaltic precursor.
Activation of Unimolecular Reactions by Ambient Blackbody Radiation
Authors: Dunbar R. C. , McMahon T. B.
Published: July 2002

The historical “radiation hypothesis” as a mechanism for activating unimolecular thermal dissociation of gas-phase molecules, long discredited on the authority of Langmuir, has been revitalized by the discovery and characterization of the process of thermal dissociation of trapped gas-phase ions by the ambient blackbody radiation field surrounding the ions. This development was made possible by improvements in Fourier transform ion cyclotron instrumentation that allowed long-time trapping of weakly bound cluster ions at extremely low pressures. Binding energies can be derived from measurements of these dissociation rate constants both by detailed kinetic modeling and by simpler Arrhenius temperature-dependence approaches, although the latter require special considerations for small molecules. These approaches have been applied to thermal dissociations of molecules, including cluster ions and large biomolecule ions.
Bragg Diffraction from Crystallized Ion Plasmas
Authors: Itano W. M. , Bollinger J. J. , Tan J. N. , Jelenković B. , Huang X.-P. , Wineland D. J.
Published: July 2002

Single crystals of a one-component plasma were observed by optical Bragg diffraction. The plasmas contained 10
5
to 10
6
single-positive beryllium-9 ions (
9
Be
+
) at particle densities of 10
8
to 10
9
per cubic centimeter. In approximately spherical plasmas, single body-centered cubic (bcc) crystals or, in some cases, two or more bcc crystals having fixed orientations with respect to each other were observed. In some oblate plasmas, a mixture of bcc and face-centered cubic ordering was seen. Knowledge of the properties of one-component plasma crystals is required for models of white dwarfs and neutron stars, which are believed to contain matter in that form.
An Analysis of the Origins of a Cooperative Binding Energy of Dimerization
Authors: Williams Dudley H. , Maguire Alison J. , Tsuzuki Wakako , Westwell Martin S.
Published: July 2002

The cooperativity between binding of cell wall precursor analogs (ligands) to and antibiotic dimerization of the clinically important vancomycin group antibiotics was investigated by nuclear magnetic resonance. When dimerization was weak in the absence of a ligand, the increase in the dimerization constant in the presence of a ligand derived largely from changes associated with tightening of the dimer interface. When dimerization was strong in the absence of a ligand, the increase in the dimerization constant in the presence of a ligand derived largely from changes associated with tightening of the ligand-antibiotic interface. These results illustrate how, when a protein has a loose structure, the binding energy of another molecule to the protein can derive in part from changes occurring within the protein.
Cope's Rule and the Dynamics of Body Mass Evolution in North American Fossil Mammals
Authors: Alroy John
Published: July 2002

Body mass estimates for 1534 North American fossil mammal species show that new species are on average 9.1% larger than older species in the same genera. This within-lineage effect is not a sampling bias. It persists throughout the Cenozoic, accounting for the gradual overall increase in average mass (Cope's rule). The effect is stronger for larger mammals, being near zero for small mammals. This variation partially explains the unwavering lower size limit and the gradually expanding mid-sized gap, but not the sudden large increase in the upper size limit, at the Cretaceous-Tertiary boundary.
A Role for the AKT1 Potassium Channel in Plant Nutrition
Authors: Hirsch Rebecca E. , Lewis Bryan D. , Spalding Edgar P. , Sussman Michael R.
Published: July 2002

In plants, potassium serves an essential role as an osmoticum and charge carrier. Its uptake by roots occurs by poorly defined mechanisms. To determine the role of potassium channels in planta, we performed a reverse genetic screen and identified an
Arabidopsis thaliana
mutant in which the
AKT1
channel gene was disrupted. Roots of this mutant lacked inward-rectifying potassium channels and displayed reduced potassium (rubidium-86) uptake. Compared with wild type, mutant plants grew poorly on media with a potassium concentration of 100 micromolar or less. These results and membrane potential measurements suggest that the AKT1 channel mediates potassium uptake from solutions that contain as little as 10 micromolar potassium.
The Radio-Frequency Single-Electron Transistor (RF-SET): A Fast and Ultrasensitive Electrometer
Authors: Schoelkopf R. J. , Wahlgren P. , Kozhevnikov A. A. , Delsing P. , Prober D. E.
Published: July 2002

A new type of electrometer is described that uses a single-electron transistor (SET) and that allows large operating speeds and extremely high charge sensitivity. The SET readout was accomplished by measuring the damping of a 1.7-gigahertz resonant circuit in which the device is embedded, and in some ways is the electrostatic “dual” of the well-known radio-frequency superconducting quantum interference device. The device is more than two orders of magnitude faster than previous single-electron devices, with a constant gain from dc to greater than 100 megahertz. For a still-unoptimized device, a charge sensitivity of 1.2 × 10
−5
e
/

hertz

was obtained at a frequency of 1.1 megahertz, which is about an order of magnitude better than a typical, 1/
f
-noise-limited SET, and corresponds to an energy sensitivity (in joules per hertz) of about 41 ℏ︀.