bluuebunny/crossref_metadata_2025 huggingface dataset:
Mild winter and spring 2007 over western Europe led to a widespread early vegetation onset
Authors: Maignan F. , Bréon F. M. , Vermote E. , Ciais P. , Viovy N.
Published: January 2008

Europe has experienced a wide scale warming over the past decades and climate simulations predict further warming and changes in precipitation patterns during the 21st century. The winter of 2006–2007 has been exceptionally mild with averaged temperatures that may become the norm during the second half of the 21st century. Here we report on satellite observations of the vegetation greening that occurred at the subcontinent scale almost 10 days earlier than the average over the past three decades. Even at the relatively coarse resolution of the satellite data, which mixes several vegetation types, there is a strong negative temporal correlation between the February–April mean temperature and the start of growth date. The western Europe mean vegetation onset sensitivity is −3.9 days per degree of temperature, and is mainly driven by crops and grasslands, with a biome‐specific sensitivity of −4.7 days/°C. For forested biomes, onset anomalies are better correlated to the March–May mean temperature, with a sensitivity of −3.6 days/°C. Based upon the satellite data, there is no consistent indication that a lack of cold days in the winter 2006–2007 had any effect in delaying the vegetation onset.
Subduction of South Pacific waters
Authors: Qu Tangdong , Gao Shan , Fukumori Ichiro , Fine Rana A. , Lindstrom Eric J.
Published: January 2008

Using existing high‐resolution CTD observations, complemented by a large amount of recently available Argo floating data, this study provides a detailed description of the subduction of South Pacific waters. With a significantly improved climatological dataset on the mixed layer properties, we obtain an annual subduction rate of 48.8 Sv (1 Sv = 106 m3 s−1) from 10°S to 60°S in the South Pacific. Two peaks stand out in this subduction rate sorted by winter mixed layer density: one corresponds to the formation of eastern Subtropical Mode Water (STMW) and part of the Subtropical Underwater (SUW) between 25.0 and 25.5 σθ, and the other has a density range between 26.6 and 27.1 σθ, representing the formation of Sub‐Antarctic Mode Water and Antarctic Intermediate Water (SAMW/AAIW). The subduction in eastern STMW/SUW range is 12.7 Sv, only slightly smaller than that (14.6 Sv) associated with the SAMW/AAIW. Sandwiched between these two peaks, roughly in the density range between 26.0 and 26.5 σθ, is the southwestern STMW, with a relatively small annual subduction of 5.6 Sv. Uncertainties of these estimates are discussed.
Modeling the longitudinal variation in the post‐sunset far‐ultraviolet OI airglow using the SAMI2 model
Authors: England S. L. , Immel T. J. , Huba J. D.
Published: January 2008

Recent global‐scale observations of the low‐latitude airglow bands associated with the equatorial ionospheric anomaly (EIA) have revealed a longitudinal variation in the brightness and latitude of the peak airglow emission. For vernal equinox conditions, both of these display a wave number‐four pattern when plotted in a constant‐local‐time frame. It has been proposed that variations in the neutral‐wind driven dynamo electric fields from the E‐region are responsible for this pattern. Additionally, measurements of the electric fields in the E‐region have shown a wave number‐four pattern similar to that of the EIA. Here we use the SAMI2 model, that includes a detailed description of ion photochemistry and transport, to demonstrate that the recently observed zonal variations in the E‐region dynamo electric fields are sufficient to explain the observed variation in brightness and latitude of the airglow bands for these conditions. The vertical drifts associated with E‐region dynamo fields in the SAMI2 model are modified to produce simulations that represent the locations of a maximum and minimum in the wave number‐four pattern. The simulated airglow changes such that the brightness of the maximum case is ∼40% higher than the peak in the minimum case and the latitude of the peak brightness in the maximum case is located 3° poleward of the peak in the minimum case. Both of these results compare favorably with, and even exceed, the observed variations. This result adds quantitative support to the above stated mechanism. The effect of changes in the drifts at different local time periods on the nighttime airglow is also assessed. It is seen that changes at all local time periods make a significant contribution to the total change in the airglow, with the most significant being close to local noon and during the late afternoon.
Bulge Formation of a Buoyant River Outflow
Authors: Chant Robert J. , Glenn Scott M. , Hunter Elias , Kohut Josh , Chen Robert F. , Houghton Robert W. , Bosch Jen , Schofield Oscar
Published: January 2008

Observations taken during the Lagrangian Transport and Transformation Experiment (LaTTE) in 2005 indicated that the Hudson's river outflow formed a bulge of recirculating fluid that limits the volume of fresh water that is advected away in a coastal current. Focusing on an event that began with downwelling winds we made estimates of the fresh‐water flux in the coastal current and the fresh water inventory of the bulge. The coastal current was characterized by a surface advected plume in thermal wind balance. However, the freshwater transport in the coastal current was less than 1/2 of the total freshwater outflow. The bulge extended 30 km from the coast and 40 km in the along‐shore direction and was evident in ocean color imagery. Recirculation in the bulge region was also apparent in daily averaged surface current radar data, but this flow pattern was obscured in the hourly data by tidal and wind‐forcing even in the diurnal band. Nevertheless, many aspects of the Hudson's outflow are consistent with recent laboratory experiments and numerical simulations of buoyant discharges. The growing bulge transports the river's outflow to the head of the Hudson shelf valley where it crosses the 50 m isobath. Previous work in this region indicates that frontal features reside along this isobath. We observed fresh water being transported along this isobath and is suggestive of a rapid cross‐shelf transport pathway for fresh water. Both the bulge formation and cross‐shelf transport have significant biogeochemical implications.
Air pollution radiative forcing from specific emissions sectors at 2030
Authors: Unger Nadine , Shindell Drew T. , Koch Dorothy M. , Streets David G.
Published: January 2008

Reduction of short‐lived air pollutants can contribute to mitigate global warming in the near‐term with ancillary benefits to human health. However, the radiative forcings of short‐lived air pollutants depend on the location and source type of the precursor emissions. We apply the Goddard Institute for Space Studies atmospheric composition‐climate model to quantify near‐future (2030 A1B) global annual mean radiative forcing by ozone (O3) and sulfate from six emissions sectors in seven geographic regions. At 2030 the net forcings from O3, sulfate, black and organic carbon, and indirect CH4 effects for each emission sector are (in mWm−2) biomass burning, +95; domestic, +68; transportation, +67; industry, −131; and power, −224. Biomass burning emissions in East Asia and central and southern Africa, domestic biofuel emissions in East Asia, south Asia, and central and southern Africa, and transportation emissions in Europe and North America have large net positive forcings and are therefore attractive targets to counter global warming. Power and industry emissions from East Asia, south Asia, and north Africa and the Middle East have large net negative forcings. Therefore air quality control measures that affect these regional sectors require offsetting climate measures to avoid a warming impact. Linear relationships exist between O3 forcing and biomass burning and domestic biofuel CO precursor emissions independent of region with sensitivity of +0.2 mWm−2/TgCO. Similarly, linear relationships exist between sulfate forcing and SO2 precursor emissions that depend upon region but are independent of sector with sensitivities ranging from −3 to −12 mWm−2/TgS.
Impact of the summer 2004 Alaska fires on top of the atmosphere clear‐sky radiation fluxes
Authors: Pfister G. G. , Hess P. G. , Emmons L. K. , Rasch P. J. , Vitt F. M.
Published: January 2008

In this study we estimate the radiative impact of wildfires in Alaska during the record wildfire season of 2004 by integrating model simulations and satellite observations of the top of the atmosphere (TOA) radiative fluxes and aerosol optical depth. We compare results for the summer of 2004 to results for the summer of 2000 when fire activity in the boreal zone was low. Both observations and model show a decrease in TOA clear‐sky fluxes over the Alaska fire region during summer 2004 of −7 ± 6 W m−2and −10 ± 4 W m−2, respectively. About two thirds of the change occurs in the longwave, and one third in the shortwave, spectral range. On the bases of detailed model analysis we estimate that the changes in the longwave flux are predominantly explained by a higher surface temperature in summer 2004 compared to 2000. The change in the shortwave flux is largely caused by scattering of solar radiation on organic carbon aerosols emitted from the 2004 fires. This cooling is somewhat mitigated by the warming effect due to absorbing black carbon aerosols emitted from the fires and to a lesser extent by ozone and other greenhouse gases produced and released from the fires. Sensitivity studies with varying aerosol emission scenarios indicate that the ratio of black to organic carbon aerosol emissions of the boreal fires used in this study needs to be increased considerably to match both observations of aerosol optical depth and TOA radiation fluxes, or the biomass burning aerosols must be considerably more absorbing than parameterized in the model. While this study cannot resolve the cause of this discrepancy, it presents a powerful methodology to constrain aerosol emissions. This methodology will benefit from future improvements in measurements and modeling techniques.
Sulfur dioxide emission flux measurements from point sources using airborne near ultraviolet spectroscopy during the New England Air Quality Study 2004
Authors: Melamed M. L. , Langford A. O. , Daniel J. S. , Portmann R. W. , Miller H. L. , Eubank C. S. , Schofield R. , Holloway J. , Solomon S.
Published: January 2008

This work presents measurements of sulfur dioxide (SO2) emission fluxes from point sources using airborne near‐ultraviolet (UV) spectroscopy. A Czerny‐Turner spectrograph has been optimized to measure SO2 and the oxygen collision complex (O4) in the wavelength region of 286–408 nm from an aircraft platform. The spectrograph was deployed aboard the NOAA WP‐3D Orion aircraft during the New England Air Quality Study during the summer of 2004. The spectrograph has zenith and nadir field of views, allowing for measurements of pollution plumes when the aircraft is in or above the planetary boundary layer. The near‐UV spectra are analyzed using the differential optical absorption spectroscopy (DOAS) method to retrieve SO2 and O4 differential slant column densities (DSCDs). The SO2 DSCDs are used to identify point source plumes and are converted to vertical column densities (VCDs), which are needed to calculate emissions of SO2 from point sources. The conversion to VCDs requires knowledge of the photon optical path length or the air mass factor (AMF). We present a novel approach to calculate the AMF using observations of the absorption of solar radiation by O4. The SO2 VCDs, wind speed and direction, and aircraft speed are then used to obtain emission fluxes from power plants. The measured SO2 power plant emission fluxes are compared to the reported emissions from the power plants. The measured and reported SO2 emission fluxes are in good agreement.
Characterization of watershed model behavior across a hydroclimatic gradient
Authors: van Werkhoven Kathryn , Wagener Thorsten , Reed Patrick , Tang Yong
Published: January 2008

A fundamental tradeoff exists in watershed modeling between a model's flexibility for representing watersheds with different characteristics versus its potential for overparameterization. This study uses global sensitivity analysis to investigate how a commonly used intermediate‐complexity model, the Sacramento Soil Moisture Accounting Model (SAC‐SMA), represents a wide range of watersheds with diverse physical and hydroclimatic characteristics. The analysis aims to establish a detailed understanding of model behavior across watersheds and time periods with the ultimate objective to guide model calibration and evaluation studies. Sobol's sensitivity analysis is used to evaluate the SAC‐SMA in 12 Model Parameter Estimation Experiment (MOPEX) watersheds in the US. The watersheds span a wide hydroclimatic gradient from arid to humid systems. Four evaluation metrics reflecting base flows, midrange flows, peak flows, and long‐term water balance were used to comprehensively characterize trends in sensitivity and model behavior. Results show significant variation in parameter sensitivities that are correlated with the hydroclimatic characteristics of the watersheds and time periods analyzed. The sensitivity patterns are consistent with the expected dominant processes and demonstrate the need for moderate model complexity to represent different hydroclimatic regimes. The analysis reveals that the primary model controls for some aspects of the simulated hydrograph are different from those typically assumed for the SAC‐SMA. Results also show that between 6 and 10 parameters are regularly identifiable from daily hydrologic data, which is about twice the range that is often assumed (i.e., 3 to 5). Synthesized results provide comprehensive SAC‐SMA calibration guidance, demonstrate the flexibility of the model for representing multiple hydroclimatic regimes, and highlight the great difficulty in generalizing model behavior across watersheds.
An exact solution for ideal dam‐break floods on steep slopes
Authors: Ancey C. , Iverson R. M. , Rentschler M. , Denlinger R. P.
Published: January 2008

The shallow‐water equations are used to model the flow resulting from the sudden release of a finite volume of frictionless, incompressible fluid down a uniform slope of arbitrary inclination. The hodograph transformation and Riemann's method make it possible to transform the governing equations into a linear system and then deduce an exact analytical solution expressed in terms of readily evaluated integrals. Although the solution treats an idealized case never strictly realized in nature, it is uniquely well‐suited for testing the robustness and accuracy of numerical models used to model shallow‐water flows on steep slopes.
Geodynamo Turns Toward a Stable Magnetic Field
Authors: Ladbury Ray
Published: January 2008

Earth's magnetic field has been a subject of curiosity for at least 3000 years and of quantitative study for more than 400 years. Geologic samples now extend our knowledge of geomagnetism back billions of years, and satellites and observatories log the tiniest changes in the strength and direction of the geomagnetic field. these investigations have revealed many intriguing characteristics of geomagnetism that any successful model of the phenomenon must explain—such as the stability of the geomagnetic field on time scales of 105 years, the field's predominantly dipole nature, the offset of its dipole axis from Earth's rotational axis, its temporal variability and so on. All these geomagnetic observations, however, have done little to suggest what a model of geomagnetism should look like.
Experimenters Cool Helium Below Single-Photon Recoil Limit in Three Dimensions
Authors: Lubkin Gloria B.
Published: January 2008

A group at the Ecole Normale Superieure in Paris has placed a helium atom in a state that is going in six directions at once—a very confusing state for an atom. In such a superposition state, also called a dark state, the atom can't absorb light because the absorption amplitudes cancel out by destructive interference. The temperature corresponding to the velocity spread of each of the six wavepackets is a factor of 20 below the so-called recoil limit for laser-cooled atoms. The three-dimensional subrecoil experiment in helium was reported in the 4 December issue of Physical Review Letters by John La wall (now at the National Institute of Standards and Technology in Gaithersburg, Maryland), Si-mone Kulin, Bruno Saubamea, Nick Bigelow, Michèle Leduc and Claude Cohen-Tannoudji.
Oak Ridge Builds Facility for Radioactive Ion Beams
Authors: Lubkin Gloria B.
Published: January 2008

Oak Ridge National Laboratory has recently completed the construction of a radioactive ion beam facility to create nuclei at the limits of stability. The facility is now being commissioned. Created by reconfiguring two existing accelerators that have been used as a heavy-ion facility, the new Holifield Radioactive Ion Beam Facility is expected to be ready for experiments next summer. Because of the decreasing ratio of protons to neutrons for heavy stable nuclei, says Jerry Garrett, scientific director of the facility, the most efficient technique for producing proton-rich nuclei for nuclear structure studies involves heavy-ion induced fusion-evaporation reactions using proton-rich projectiles and targets with nearly equal masses. “This new facility should be well suited for such studies of proton-rich nuclei,” Garrett says.
US and Japan Plan to Create Cross-Country Neutrino Beams
Authors: Schwarzschild Bertram
Published: January 2008

Deep inside the Kamioka zinc mine, in the mountains west of Tokyo, the just-completed Super Kamiokande proton-decay and neutrino detector is now being filled with 50 000 tons of highly purified water. (Fifty thousand tons is the displacement of a large battleship.) Super Kamiokande is a Japan-US collaboration. In a few months this newest, and largest by far, of the water-Cerenkov detectors will be recording neutrinos from the Sun and from collisions of cosmic rays in the atmosphere. It will also be keeping its 11 200 photomultiplier eyes peeled for neutrino bursts from distant supernova explosions and for evidence that one of the detector's 1034 nucleons has decayed. (See the photograph on this page).
Galileo Reveals the Light and Dark Sides of Ganymede
Authors: Ladbury Ray
Published: January 2008

Following the excitement generated by the Galileo spacecraft's first Ganymede flyby on 27 June, researchers are hoping that data from the 6 September encore performance—which skimmed within 262 km of that Jovian moon—will help them understand why Ganymede is more tectonically and magnetically active than expected. The data trickling back to Earth during the next two months is expected to include the best images yet obtained (some stereoscopic) of Jupiter's and the Solar System's largest moon.
Yohkoh Reveals Site of Solar Flare Energy Release
Authors: Benka Stephen G.
Published: January 2008

It may look like a Volkswagen Beetle on a hill, but the surprising observation1 shown here—specifically the uppermost set of white contours—has solar physicists cruising toward one of their primary goals: unraveling the mysteries of solar flares. This image of a flare was made by a team of Japanese researchers using the Yohkoh satellite (see PHYSICS TODAY, May 1992, page 19), and the emission represented by those contours is closely related to the origin of the flare's energy.
Convincing Evidence Seen for Isotopes of Element 110
Authors: Collins Graham P.
Published: January 2008

On 9 November a group of researchers led by Peter Annbruster and Sigurd Hofmann at the Gesell-schaft für Schwerionenforschung (GSI) in Darmstadt, Germany, observed a sequence of four alpha particles emitted from selected reaction products of a thin piece Pb208 under bombardment from a beam of 311-MeV Ni62 ions. The alphas clearly represented a decay chain, being essentially coincident in position and following one another at intervals ranging from a few hundred microseconds to a few hundred milliseconds. Indeed, the last three alphas had energies and delay times that clearly matched the decay chain
Chromium Surrogate Sun Confirms that Solar Neutrinos Really Are Missing
Authors: Schwarzschild Bertram
Published: January 2008

There may still be some who doubt that the missing-solar-neutrino problem is real. But an impressive experiment with a radioactive solar surrogate recently completed at the Gallex solar neutrino detector in Italy makes it difficult to sustain such skepsis.’ The new experiment supports the reports over the last several years that Gallex and Russia's SAGE, the other large gallium detector, see only about 60% of the solar neutrino signal confidently predicted by astro-physical models. (See PHYSICS TODAY, August 1992, page 17.) Thus it does much to bring the observational features of the solar neutrino puzzle into clearer focus.
‘Asteroseismology’ Offers a New Probe of Stellar Interiors
Authors: Schwarzschild Bertram
Published: January 2008

Just as the study of seismic waves lets us look into the bowels of the Earth, helioseismology has for more than 20 years been a rich source of information about the interior of the Sun. Apparently driven by internal convective motion, the Sun rings like a great spherical bell. Many thousands of resonant pressure-wave modes with periods on the order of 5 minutes have been painstakingly decoded from complex surface motions of dauntingly small amplitude. From the pattern of resonant frequencies one learns much about the composition, density and structure of the Sun. We now know, for example, that the Sun's convection zone is twice as deep as solar modelers believed before these oscillation modes were measured.
Clouds Cast a Shadow of Doubt on Models of Earth's Climate
Authors: Levi Barbara Goss
Published: January 2008

Clouds have always bedeviled those trying to model Earth's climate, because they are such complex systems, involving parameters whose size and time scales range over many orders of magnitudes. (See the article by Jeffrey Kiehl in PHYSICS TODAY, November 1994, page 36.) While struggling to represent clouds realistically in their models, atmospheric researchers have at least felt that they understood the basic physics of clouds. But now they are not so sure.
X Rays Illuminate Dynamics on Near-Atomic Length Scales
Authors: Ladbury Ray
Published: January 2008

When a beam from a laser or some other coherent light source scatters off a random distribution of matter, it produces an interference pattern of light and dark “peckles” that is uniquely determined by the instantaneous matter distribution at the scale of the light's wavelength. And if changes occur in the sample as a function of time, the speckle pattern evolves to reflect those changes. Physicists exploit this phenomenon when they investigate the diffusion of micron-sized particles in liquids, for example, with techniques like photoncorrelation spectroscopy and dynamical light scattering.
In a Two-Dimensional Electron System, the Skyrmion's the Limit
Authors: Ladbury Ray
Published: January 2008

Skyrmions—which can be thought of, loosely, as topological twists or kinks in a spin space—are one of those concepts that seem to jump restlessly from field to field. Technically, in a nonlinear field theory a Skyrmion is defined as a soliton with spin and statistics different from those of the underlying fields. Introduced by Tony H. R. Skyrme in 1958 as a way of representing the nucleon (a fermion) as a topological soliton of bosonic pion fields, skyrmions have sojourned in nuclear physics, particle physics and condensed matter physics.
Gaseous Bose–Einstein Condensate Finally Observed
Authors: Collins Graham P.
Published: January 2008

About seven decades ago Satyendra Nath Bose and Albert Einstein predicted that a gas of noninteracting integer-spin particles would condense into a macroscopic quantum state when cooled below a critical temperature. Of course Bose–Einstein condensation (BEC) has long since been seen in superfluid He4 and superconductors, but the condensing systems in these examples are far from being noninteracting gases; relatively strong interactions between the condensing particles greatly complicate the theoretical analysis and the experimental behavior. For more than 15 years groups have been cooling and compressing clouds of atoms on a quest to produce and observe a Bose–Einstein condensate in a nearideal gas. They pushed their devices to the limit, seeking to traverse 15 orders of magnitude of phase-space density, and as each technique proved insufficient they developed ingenious variations to create ever colder and denser states.
Space-Based Telescopes See Primordial Helium in Spectra of Distant Quasars
Authors: Levi Barbara Goss
Published: January 2008

According to the standard model, shortly after the Big Bang, primordial hydrogen and helium were spread nearly uniformly throughout space. After matter began to coalesce into galaxies and other structures, nuclear cooking within massive stars produced heavier elements. But researchers still expect to see traces of the primordial elements if they look far out in intergalactic space, at distances corresponding to the earliest times. In 1971 they saw signs of hydrogen clumped in clouds located billions of light-years away. Now, thanks to the availability of satelliteborne telescopes sensitive to ultravio let radiation, they have found hydrogen's primordial companion.
Princeton Tokamak Begins Experiments with Tritium–Deuterium Plasmas
Authors: Schwarzschild Bertram
Published: January 2008

Tokamak research entered a new and important phase last month. Shortly before midnight on 9 December, the 11-year-old Tokamak Fusion Test Reactor at the Princeton Plasma Physics Laboratory was fired for the first time with a 50:50 mixture of deuterium and tritium. That's the mixture of hydrogen isotopes envisioned for the first generation of fusion reactors, because DT fusion can be harnessed at much lower temperatures than DD or D3He fusion, and the cross section for the proton-proton reaction that powers the Sun is hopelessly small. But until now most tokamak research has been done, for practical reasons, with pure deuterium plasmas.
New Synchrotron Light Sources Turn on Around the World
Authors: Levi Barbara Goss
Published: January 2008

“First light” shone forth this fall from three new synchrotron radiation facilities around the world: the Advanced Light Source, at the Lawrence Berkeley Laboratory in California; Elettra, at the Sincrotrone Trieste in Italy; and the Synchrotron Radiation Research Center, in Hsinshu, Taiwan. With electron energies ranging from 1 to 2 GeV, they are designed to produce high-brightness beams ranging from the ultraviolet to soft x rays, corresponding to photon energies from 10 eV to several keV. Hard x rays, with energies between 1 and 250 keV, have been produced since the summer of 1992 by the 6-GeV European Synchrotron Radiation Facility in Grenoble, France. The first of three synchrotron facilities designed to supply hard x rays from undulators, the ESRF is preparing to welcome external users to its site adjacent to the Laue-Langevin Institute in September 1994.
Distance to Virgo Kicks Off HST Assault on the Hubble Constant
Authors: Schwarzschild Bertram
Published: January 2008

One of the “Key Projects” intended for the Hubble Space Telescope from the start was to pin down the distance to the Virgo cluster by measuring Cepheid variable stars in that massive assemblage of galaxies some 50-million light-years away. Cepheids are the primary yardsticks for astronomical distances beyond the reach of parallax measurement. The inability to determine cosmologically relevant distances with confidence has been the principal source of confusion in our knowledge of Ho, the Hubble constant. The HST's measurement of Cepheids in the Virgo cluster was to be the opening salvo in an assault that would soon pin down this fundamental parameter of cosmology to within 10%, once and for all. Estimates of the Hubble constant in recent years have differed by as much as a factor of two, and the stakes are high. The reciprocal of Ho is, after all, the first approximation to the age of the universe.
Florida Dedicates National High Magnetic Field Laboratory
Authors: Lubkin Gloria B.
Published: January 2008

On 1 October, four years after the National Science Foundation chose to build a new high-magnetic-field laboratory at Florida State University rather than upgrade the existing Francis Bitter National Magnet Laboratory at MIT (see PHYSICS TODAY, January 1991, page 53), the new laboratory was dedicated in Tallahassee.
Earthquake Yields First Real Evidence of Remote Triggering
Authors: Schwarzschild Bertram
Published: January 2008

Just before dawn on 28 June last year, the Mojave Desert 200 km east of Los Angeles was struck by a magnitude-7.3 earthquake. Although the Landers earthquake, named for the small town nearest the epicenter, was stronger than the earthquake that had rocked San Francisco three years earlier, it attracted little public attention, because the region is so sparsely populated. But it made seismological history. The dense and extensive network of seismographic stations built up in the Far West during the 1980s has provided convincing evidence that the Landers quake triggered secondary tremors as far away as Yellowstone National Park, 1250 km to the northeast.
Mechanical properties of alkanethiol monolayers studied by force spectroscopy
Authors: Oncins Gerard , Vericat Carolina , Sanz Fausto
Published: January 2008

The mechanical properties of alkanethiol monolayers on Au(111) in KOH solution have been studied by force spectroscopy. The analysis of the vertical force versus penetration curves showed that monolayer penetration is a stepped process that combines elastic regions with sudden penetration events. The structural meaning of these events can be explained both by the creation of gauche defects on the hydrocarbon chains and by a cooperative molecular tilting model proposed by Barrena et al. [J. Chem. Phys. 113, 2413 (2000)]. The validity of these models for alkanethiol monolayers of different compactness and chain length has been discussed. The Young’s modulus (E) of the monolayers has been calculated by using a recently developed model which considers the thickness of the monolayer as a parameter, thus allowing a decoupling of the mechanical properties of the thiol layer from those of the Au(111) substrate. As a result, the calculated E values are in the range of 50–150Pa, which are remarkably lower than those previously reported in the literature.
Polymer chain dynamics at interfaces: Role of boundary conditions at solid interface
Authors: Desai Tapan G. , Keblinski Pawel , Kumar Sanat K.
Published: January 2008

Using classical molecular dynamics simulations, we study the dynamical properties of a single polymer chain dissolved in an explicit solvent and strongly adsorbed at solid-liquid interface. To circumvent a serious challenge posed by finite size effects due to long-range hydrodynamic effects, we developed a correction procedure that substantially limits the finite size effects. Concurrently, we provide an analysis of distinctly different size effects in the directions transverse and normal to the interface. We find that on analytically smooth interfaces, corresponding to the slip boundary condition, the motions of the polymer chain and the surrounding solvent are hydrodynamically coupled. This leads to the chain diffusion coefficient D scaling with the chain degree of polymerization N as D∼N−3∕4, consistent with the Zimm dynamics for strongly adsorbed chains. Introduction of transverse forces at the interface results in loss of correlation between the motion of the polymer chain and the solvent. Consequently, D∼N−1, which is a characteristic of the Rouse dynamics.
Two-photon Lee-Goldburg nuclear magnetic resonance: Simultaneous homonuclear decoupling and signal acquisition
Authors: Michal Carl A. , Hastings Simon P. , Lee Lik Hang
Published: January 2008

We present NMR signals from a strongly coupled homonuclear spin system, H1 nuclei in adamantane, acquired with simultaneous two-photon excitation under conditions of the Lee-Goldburg experiment. Small coils, having inside diameters of 0.36mm, are used to achieve two-photon nutation frequencies of ∼20kHz. The very large rf field strengths required give rise to large Bloch-Siegert shifts that cannot be neglected. These experiments are found to be extremely sensitive to inhomogeneity of the applied rf field, and due to the Bloch-Siegert shift, exhibit a large asymmetry in response between the upper and lower Lee-Goldburg offsets. Two-photon excitation has the potential to enhance both the sensitivity and performance of homonuclear dipolar decoupling, but is made challenging by the high rf power required and the difficulties introduced by the inhomogeneous Bloch-Siegert shift. We briefly discuss a variation of the frequency-switched Lee-Goldburg technique, called four-quadrant Lee-Goldburg (4QLG) that produces net precession in the x-y plane, with a reduced chemical shift scaling factor of 1∕3.
Mechanism of the flexural resonance frequency shift of a piezoelectric microcantilever sensor in a dc bias electric field
Authors: Zhu Qing , Shih Wan Y. , Shih Wei-Heng
Published: January 2008

The flexural resonance frequency of a lead magnesium niobate–lead titanate (PMN-PT)/tin piezoelectric microcantilever sensor (PEMS) was shown to vary in a dc bias electric field, similar to the behavior of width-mode resonance frequency of the PEMS. Both the flexural and the width-mode resonance frequency shifts were attributed to Young’s modulus change in the PMN-PT layer as confirmed by Young’s modulus measurements on a separate PMN-PT strip. Young’s modulus change of the PMN-PT layer in an electric field was a result of the non-180° polarization domain switching as evidenced by the dielectric constant change with the field.
Metal-cluster growth with asymmetric diffusion and reversible aggregation on Si(111)7×7
Authors: Wang Daimu , Zhu Hui , Wang Liping
Published: January 2008

The nucleation and growth behavior of metal nanoclusters on a Si(111)7×7 reconstructed surface is studied by using a kinetic Monte Carlo simulation. The main atomistic processes included in the model are diffusion of adatoms between half-unit cells of the 7×7 surface, and aggregation and decay of clusters occurred inside the half-unit cells. Considering the structural difference between faulted and unfaulted half-cells, the model introduces an asymmetric adatom aggregation mechanism by setting different effective binding energies of clusters in the two different types of half-cells, which gives rise to preferential occupation of clusters for the faulted half-cells. The simulation of the model produces growth mode transitions from hit and stick regime at low temperatures to self-organized growth regime at high temperatures. A comparison with relevant growth experiments demonstrates good qualitative agreement.
Intermittency in the heat and particle transports in the SINP tokamak scrape-off layer
Authors: Saha S. K. , Chowdhury S.
Published: January 2008

The intermittent heat and particle transports have been studied in the scrape-off layer of the SINP tokamak. Properties of the plasma structures, responsible for the intermittency, have been measured by the conditional averaging technique. The probability distribution functions of the fluctuations, including temperature fluctuations, obey non-Gaussian statistics. Wavelet analysis has shown that the cross-correlation between two probes is also intermittent in time and is connected to the passage of plasma structures. The structures decay in density as they move radially outward but their temperature is found to decay more rapidly.
Theory and simulations of principle of minimum dissipation rate
Authors: Shaikh Dastgeer , Dasgupta B. , Zank G. P. , Hu Q.
Published: January 2008

We perform a self-consistent, time-dependent numerical simulations of dissipative turbulent plasmas at a higher Lundquist number, typically up to O(106), using full three-dimensional compressible magnetohydrodynamics code with a numerical resolution of 1283. Our simulations follow the time variation of global helicity, magnetic energy, and the dissipation rate and show that the global helicity remains approximately constant, while magnetic energy is decaying faster and the dissipation rate is decaying even faster than the magnetic energy. This establishes that the principle of minimum dissipation rate under the constraint of (approximate) conservation of global helicity is a viable approach for plasma relaxation.
Direct comparison of magnetization reversal process in rectangular and elliptical ring nanomagnets
Authors: Jain S. , Wang C. C. , Adeyeye A. O.
Published: January 2008

We present a comparative study of the magnetization reversal process in rectangular and elliptical ferromagnetic ring structures using various contact configurations. We observed that the magnetic and transport properties are strongly dependent on the ring shape and contact geometry. Magnetoresistance measurements in combination with micromagnetic simulations have been used to understand the magnetization reversal mechanism in both the ring geometries.
Kerr-effect based Sixtus-Tonks experiment for measuring the single domain wall dynamics
Authors: Chizhik A. , Varga R. , Zhukov A. , Gonzalez J. , Blanco J. M.
Published: January 2008

Here we present the Kerr-effect-based Sixtus-Tonks experiments to study the single domain wall dynamics. It combines the advantage of the classical Sixtus-Tonks experiments with the quick optical method to register the domain wall propagation. Instead of the pick-up coil, the reflection of the broken laser beam from the microwire surface is used. The change of the reflected signal has much smaller relaxation time than that of the pickup coil, that allows us to study the propagation of small and fast domain walls. We compare the measurement obtained by the classical and Kerr-effect-based Sixtus-Tonks experiment.
Magnetic properties, phase evolution, and microstructure of melt-spun (Sm1−xPrx)Co7−yHfyCz (x=0–1; y=0.1–0.3; z=0–0.14) ribbons
Authors: Chang H. W. , Chen I. W. , Chang C. W. , Fang Y. K. , Hsieh C. C. , Chang W. C. , Sun A. C. , Yao Y. D.
Published: January 2008

Magnetic properties, phase evolution, and microstructure of melt-spun (Sm1−xPrx)Co7−yHfyCz ribbons have been investigated. For (Sm1−xPrx)Co7−yHfy (x=0–1; y=0.1–0.3) ribbons, the magnetization increases, but coercivity decreases, with increasing Pr content. However, the phase constitution and microstructure have not been influenced by modifying Pr content x. The attractive magnetic properties of Br=6.5kG, Hci=6.1kOe, and (BH)max=7.2MGOe could be obtained for (Sm0.8Pr0.2)Co6.9Hf0.1 ribbons. A slight addition of C in (Sm0.8Pr0.2)Co6.9Hf0.1Cz (z=0.06–0.14) ribbon not only refines the microstructure, but also induces nanoscale fcc-Co precipitation in the matrix, leading to the stronger exchange-coupling effect between magnetic grains and the enhancement of remanence and magnetic energy product. As a result, the improved magnetic properties of Br=7.1kOe, Hci=8.5kOe, and (BH)max=11.2MGOe can be achieved for (Sm0.8Pr0.2)Co6.9Hf0.1C0.12 ribbons.
Magnetic bead micromixer: Influence of magnetic element geometry and field amplitude
Authors: Lund-Olesen Torsten , Buus Bjarke B. , Howalt Jakob G. , Hansen Mikkel F.
Published: January 2008

A scheme for the silicon microfabrication of lab-on-a-chip systems with mixing based on dynamic plugs of magnetic beads is presented. The systems consist of a microfluidic channel integrated with a number of soft magnetic elements by the sides of the channel. The elements are magnetized by a homogeneous external ac magnetic field. The systems are scalable with respect to the number of magnetic bead plugs and number of parallel channels, and thus they have high potential for use in biological separation using functionalized magnetic beads. The mixing efficiency is characterized for two different geometries of the soft magnetic structures and found to be highly sensitive to the geometry and position of the structures.
Drift-tearing magnetic islands in tokamak plasmas
Authors: Fitzpatrick R. , Waelbroeck F. L.
Published: January 2008

A systematic fluid theory of nonlinear magnetic island dynamics in conventional low-β, large aspect-ratio, circular cross-section tokamak plasmas is developed using an extended magnetohydrodynamics model that incorporates diamagnetic flows, ion gyroviscosity, fast parallel electron heat transport, the ion sound wave, the drift wave, and average magnetic field-line curvature. The model excludes the compressible Alfvén wave, geodesic field-line curvature, neoclassical effects, and ion Landau damping. A collisional closure is used for plasma dynamics parallel to the magnetic field. Two distinct branches of island solutions are found, namely the “sonic” and “hypersonic” branches. Both branches are investigated analytically, using suitable ordering schemes, and in each case the problem is reduced to a relatively simple set of nonlinear differential equations that can be solved numerically via iteration. The solution determines the island phase velocity, relative to the plasma, and the effect of local currents on the island stability. Sonic islands are relatively wide, flatten both the temperature and density profiles, and tend to propagate close to the local ion fluid velocity. Hypersonic islands, on the other hand, are relatively narrow, only flatten the temperature profile, radiate drift-acoustic waves, and tend to propagate close to the local electron fluid velocity. The hypersonic solution branch ceases to exist above a critical island width. Under normal circumstances, both types of island are stabilized by local ion polarization currents.
Effects of ferromagnetic ordering and phase transition on the resistivity of spin current
Authors: Akabli K. , Diep H. T.
Published: January 2008

It has been shown experimentally a long time ago that the magnetic ordering causes an anomalous behavior of the electron resistivity in ferromagnetic crystals. Phenomenological explanations based on the interaction between itinerant electron spins and lattice spins have been suggested to explain these observations. We show by extensive Monte Carlo simulation that this behavior is also observed for the resistivity of the spin current calculated as a function of temperature (T) from low-T ordered phase to high-T paramagnetic phase in a ferromagnet. We show, in particular, that across the critical region, the spin resistivity undergoes a huge peak. The origin of this peak is shown to stem from the formation of magnetic domains near the phase transition. The behavior of the resistivity obtained here is compared to experiments and theories. A good agreement is observed.
Anisotropy-graded media: Magnetic characterization
Authors: Lu Zhihong , Visscher P. B. , Harrell J. W.
Published: January 2008

The concept of exchange-coupled media (each grain having a soft end whose exchange field helps to switch a hard end) has recently been generalized to allow a continuous gradation of anisotropy from soft to hard. We have recently shown that the “figure of merit” for such media ξ=2Eb∕μ0MsHsw, proportional to the ratio of the energy barrier Eb to the switching field Hsw, cannot exceed 4 for any anisotropy profile K(r). In the thin-wall limit (exchange constant A⪡KL2), it can be made to approach 4 by choosing a graded anisotropy K(z)∝z2. In developing such a medium, it is important to be able to experimentally probe the anisotropy distribution. In this paper, we study one method for doing this, the hard axis loop. In the absence of exchange, the second derivative of this loop gives the distribution directly; we show that even in the presence of realistic exchange, this remains approximately true and the anisotropy distribution can be extracted from the hard axis loop.
Unit speed stationary points of the acceleration
Authors: Arroyo J. , Garay O. J. , Mencía J. J.
Published: January 2008

We consider the variational problem associated to the L2 norm of the angular acceleration for variations with constant length. This is a classical problem related to recent models for the spinning relativistic particles (both, massive and massless) and to Bernoulli’s elastica problem. Its interest arises also in computer graphics and in trajectory planning problems for rigid body motion. Explicit integration of the Euler-Lagrange equations is considerably difficult, in general. We show how one can use a geometric integration procedure to achieve this goal for unit speed critical points in real space forms. From the Euler-Lagrange equations, we obtain the geometric integration: critical curves are a family of ordinary helices in a real space form of dimension 3 at most. Moreover, ordinary helices in real space forms can be characterized in turn as geodesics in certain constant mean curvature flat “cylinders.” This will allow us to obtain the analytical integration of the Euler-Lagrange equations. We also determine the closed unit speed critical curves for this energy.
Ferromagnetic fraction and exchange anisotropy in SUS 316LN austenitic stainless steel due to strain-induced deformation
Authors: Manjanna J. , Kamada Y. , Kobayashi S. , Takahashi S. , Kikuchi H.
Published: January 2008

The exchange bias behavior was observed between strain-induced ferromagnetic (FM) α′ martensite and antiferromagentic (AFM) austenitic phase in SUS 316LN stainless steel at <30K. Based on the magnetic hysteresis loops, the exchange contact was evident when a small amount of FM was formed, typically about <1.0vol%. The exchange bias field (Hex) was reported here as a function of FM volume fraction at different temperatures. The decreasing Hex with FM is attributed to the increasing size of FM crystallites in the AFM matrix.
On the degree conjecture for separability of multipartite quantum states
Authors: Hassan Ali Saif M. , Joag Pramod S.
Published: January 2008

We settle the so-called degree conjecture for the separability of multipartite quantum states, which are normalized graph Laplacians, first given by Braunstein et al. [Phys. Rev. A 73, 012320 (2006)]. The conjecture states that a multipartite quantum state is separable if and only if the degree matrix of the graph associated with the state is equal to the degree matrix of the partial transpose of this graph. We call this statement to be the strong form of the conjecture. In its weak version, the conjecture requires only the necessity, that is, if the state is separable, the corresponding degree matrices match. We prove the strong form of the conjecture for pure multipartite quantum states using the modified tensor product of graphs defined by Hassan and Joag [J. Phys. A 40, 10251 (2007)], as both necessary and sufficient condition for separability. Based on this proof, we give a polynomial-time algorithm for completely factorizing any pure multipartite quantum state. By polynomial-time algorithm, we mean that the execution time of this algorithm increases as a polynomial in m, where m is the number of parts of the quantum system. We give a counterexample to show that the conjecture fails, in general, even in its weak form, for multipartite mixed states. Finally, we prove this conjecture, in its weak form, for a class of multipartite mixed states, giving only a necessary condition for separability.