bluuebunny/crossref_metadata_2025 huggingface dataset:
Frictional melt and seismic slip
Authors: Nielsen S. , Di Toro G. , Hirose T. , Shimamoto T.
Published: January 2008

Frictional melt is implied in a variety of processes such as seismic slip, ice skating, and meteorite combustion. A steady state can be reached when melt is continuously produced and extruded from the sliding interface, as shown recently in a number of laboratory rock friction experiments. A thin, low‐viscosity, high‐temperature melt layer is formed resulting in low shear resistance. A theoretical solution describing the coupling of shear heating, thermal diffusion, and extrusion is obtained, without imposing a priori the melt thickness. The steady state shear traction can be approximated at high slip rates by the theoretical form τss = σn1/4 (A/) under a normal stress σn, slip rate V, radius of contact area R (A is a dimensional normalizing factor and W is a characteristic rate). Although the model offers a rather simplified view of a complex process, the predictions are compatible with experimental observations. In particular, we consider laboratory simulations of seismic slip on earthquake faults. A series of high‐velocity rotary shear experiments on rocks, performed for σn in the range 1–20 MPa and slip rates in the range 0.5–2 m s−1, is confronted to the theoretical model. The behavior is reasonably well reproduced, though the effect of radiation loss taking place in the experiment somewhat alters the data. The scaling of friction with σn, R, and V in the presence of melt suggests that extrapolation of laboratory measures to real Earth is a highly nonlinear, nontrivial exercise.
Variability of fCO2 in the Eastern Tropical Atlantic from a moored buoy
Authors: Lefèvre Nathalie , Guillot Antoine , Beaumont Laurence , Danguy Théo
Published: January 2008

A fCO2 sensor, based on a colorimetric method used for the CARIOCA buoys, has been installed on a Pilot Research Moored Array in the Tropical Atlantic (PIRATA) mooring at 6°S, 10°W, in the gulf of Guinea, in June 2006 during the EGEE 3 cruise. Hourly fCO2 data recorded from June to December 2006 are presented. An alkalinity‐salinity relationship has been determined using data from different cruises, which allows the calculation of dissolved inorganic carbon. Although the tropical Atlantic is an important source of CO2, an unexpected area of low CO2 concentrations is observed in the South Equatorial Counter Current with fCO2 values close to equilibrium conditions or even slightly undersaturated with respect to the atmospheric fCO2 value of 367.7 μatm measured during the cruise. At the end of June, an increase of seawater fCO2 to 400 μatm is consistent with the beginning of the upwelling season occurring from July to September. Although the mooring is not located within the upwelling area, the spreading of the cold tongue explains the large CO2 outgassing. The monthly CO2 flux ranges from 1.19 mmol m−2 d−1 in June to a maximum of 8.37 mmol m−2 d−1 in October, when high fCO2 values above 420 μatm are maintained by the warming of surface water. Most of the fCO2 distribution can be explained by physical processes and a strong relationship between fCO2 and SST is determined for the upwelling season. From mid‐September, diurnal cycles can be detected. Using a dissolved inorganic carbon budget, periods where net community production or diurnal warming and cooling dominates are observed.
Polar stratospheric clouds over Antarctica from the CALIPSO spaceborne lidar
Authors: Noel Vincent , Hertzog Albert , Chepfer Hélène , Winker David M.
Published: January 2008

This paper presents statistics of polar stratospheric clouds (PSCs) above Antarctica from June to October 2006 using observations from the Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) spaceborne lidar, part of the CALIPSO mission. Synoptic‐scale changes in geographic and temporal distribution are documented weekly and correlated with temperature fields. A high spatial and temporal variability tends to contradict the hypothesis that PSCs are mostly created via slow processes mainly governed by large‐scale temperature changes. Linear depolarization ratios reveal strongly typed PSCs with distinct characteristics (implying different microphysics), but unique cloud compositions cannot be singled out. A west/east imbalance is observed in the depolarization distribution, symptomatic of microphysical disparities. A classification based on depolarization and scattering ratios suggests more than 60% of mixed PSCs, followed by more than 20% of STS, and a roughly equal concentration of nitric acid trihydrate (NAT)‐based and pure ice PSCs (∼8%). Up to the beginning of August, supercooled ternary solution (STS) PSCs experience a steady decrease in concentration correlated with an increase in ice‐based and mixed PSCs; this tendency gets reversed after the first week of August, hinting at the existence of a large‐scale seasonal cycle in PSC population.
Experiments Probe the Wavefunction of Electron Pairs in High-Tc Superconductors
Authors: Levi Barbara Goss
Published: January 2008

One issue that has driven many of the experiments on high-temperature superconductivity is the nature of the wavefunction of the electron pairs that carry the supercurrent (see PHYSICS TODAY, May 1993, page 17). For conventional superconductors, this wavefunction is isotropic, having an angular momentum l = 0, that is, s-wave. But for the anisotropic copper-oxide materials that exhibit critical temperatures above 40 K, the wave-function may not be so simple.
Theorists Take Steps Toward Understanding Ladder Compounds
Authors: Levi Barbara Goss
Published: January 2008

Whatever causes superconductivity above 40 K, it seems to involve the layers containing copper and oxygen atoms, which are common to all compounds exhibiting a high critical temperature, Tc. Hoping to gain insight into the mechanism for superconductivity in these complex structures, some theorists have looked at systems as simple as one-dimensional chains of copper and oxygen atoms. More recently they have turned to copper oxide ladders—that is, pairs of copper oxide chains linked by additional oxygen atoms between the coppers. So far, the theorists have predicted a number of physical properties of such ladders, including the possibility that the ladders can go superconducting.
Stars and Stripes in Manganese Oxide
Authors: Lubkin Gloria B.
Published: January 2008

Charge ordering in a La0.33Ca0.67MnO3 lattice can be seen as regularly spaced stripes in this low-temperature high-resolution (0.5-nm) electron micrograph made by Cheng Hsuan Chen and Sang-Wook Cheong of Bell Laboratories, Lucent Technologies. In the strategically placed inset at the upper left, charge ordering is evidenced by the smallest stars in the electron diffraction pattern.
Thermonuclear Milestones: (1) The American Effort
Authors: Goncharov German A.
Published: January 2008

The history of thermonuclear research traces its roots back to the year 1941. in a lecture delivered in May 1941, Tokutaro Hagiwara, a physicist at the University of Kyoto, postulated that a thermonuclear fusion reaction between hydrogen atoms could be triggered by the explosive fission chain reaction of uranium-235. in September 1941, Enrico Fermi at Columbia University proposed a similar idea to Edward Teller. Discussions between Fermi and Teller ultimately suggested the feasibility of using an atomic explosion to initiate thermonuclear reactions in a deuterium medium. the conversations with Fermi sparked in Teller a decade-long messianic obsession with the notion of building a thermonuclear superbomb.
In the Jupiter-Comet Clash of 1994, Astronomers Are the Big Winners
Authors: Benka Stephen G.
Published: January 2008

Jupiter has now emerged from the Sun's glare into the dark sky of pre-dawn. The giant planet still shows scars—stretched by Jovian winds for more than six months—from its battle with Comet Shoemaker-Levy 9. The comet lost the battle.
Cavity Lases When Occupied, on Average, by Less Than One Atom
Authors: Levi Barbara Gobs
Published: January 2008

A conventional laser cavity contains many excited atoms, each contributing to the radiation field through stimulated emission. Stripped to its bare essentials a laser would be a single atom interacting with a single mode of the electromagnetic field. A group at MIT recently built a laser that was very close to this minimal configuration.
Supersymmetric QCD Sheds Light on Quark Confinement and the Topology of 4-Manifolds
Authors: Collins Graham P.
Published: January 2008

Perturbation theory has long been one of the most successful tools in quantum field theory. The technique produces a series of terms, and ideally most of the “physics” is captured by the more readily computed early teims in the expansion, with later terms supplying progressively more negligible corrections. In contrast to such approximations, exact computations in four-dimensional quantum field theory have been few and far between. As Stephen Shenker (Rutgers University) puts it, “The conventional wisdom was that exact results in four-dimensional quantum field theory were essentially unobtainable.” This lack of analytic results has been particularly problematic for the theory of quark confinement, which involves the strong force in a thoroughly nonperturbative regime.
Applications of High-Temperature Superconductors Approach the Marketplace
Authors: Lubkin Gloria B.
Published: January 2008

When high-temperature superconductivity reached temperatures above that of liquid nitrogen eight years ago, thousands of researchers jumped in, lots of funding followed, and the most enthusiastic people talked of magnetically levitated trains, computers and motors all soon to be operating above 77 K. When reality set in a couple of years later, the field settled into a large, active one, but with only the simplest of products being proposed for the following few years. (See the June 1991 special issue of PHYSICS TODAY on high-temperature superconductivity.)
Measured Steps Advance the Understanding of Molecular Motors
Authors: Levi Barbara Goss
Published: January 2008

Our bodies contain myriad motors to produce mechanical work. We're only too aware of the muscles that propel our limbs or pump our hearts; less noticed are the minuscule molecular motors at the cellular level that accomplish such chores as transporting organelles. All these actions involve motor proteins, enzymes that convert the chemical energy stored in the molecule adenosine triphosphate—known as ATP—into mechanical energy. Exactly how they channel the energy into rectified motion remains a compelling mystery.
Where Do You Go When You've Made it to the Top?
Authors: Ladbury Ray
Published: January 2008

What a difference a year can W make—especially when almost a thousand physicists spend that year in single-minded pursuit of a goal. The goal in this case was the discovery of the top quark, which one year ago sat at the threshold of statistical respectability. (See PHYSICS TODAY, June 1994, page 17.) Since then, the Collider Detector Facility group and the DO group at Fermilab's Tevatron have steadily improved their statistics and their analyses until the question of whether they have in fact seen the top quark is no longer a subject of controversy. CDF sees 56 top candidates over a predicted background of 23.1, for a statistical significance of 4.8 standard deviations. DO sees 17 events over a predicted background of 3.8, for a statistical significance of 4.6 standard deviations. Perhaps equally significant, all subsequent analyses of the kinematic, production and decay properties of the top samples are consistent between the two experiments and support the hypothesis that the excess events over background are indeed due to top production. The focus of debate now seems to have passed from whether the top has been discovered to how significant the discovery will be for particle physics. On the subject of significance, there seem to be two main camps.
Inhaling Hyperpolarized Noble Gas Helps Magnetic Resonance Imaging of Lungs
Authors: Schwarzschild Bertram
Published: January 2008

Since the late 1970s, Princeton physicist William Happer had been investigating spin-exchange optical pumping as a means of polarizing nuclei. Along the way, he had given thought to practical things one might do with large collections of nuclei thus polarized, among them the enhancement of fusion in tokamaks, the creation of new kinds of polarized targets for high-energy physics and the improvement of clinical magnetic resonance imaging. “But I was always too busy to think seriously about imaging,” Happer told us. “Then, when I came to Washington in 1991 [to become, for a time, director of energy research at DOE] I ruptured a disk and they did an mri scan of my spine. I was in great pain, and that concentrates the mind wonderfully.”
Sodium Atoms Kicked by Standing Waves Provide a New Probe of Quantum Chaos
Authors: Collins Graham P.
Published: January 2008

The question of how to quantize a system whose classical dynamics is chaotic has long been a puzzle. The hallmark of classical chaos is sensitive dependence on initial conditions: A small perturbation of the system changes its evolution in a way that grows exponentially with time. But in quantum mechanics a small perturbation of the initial state will in general be mapped into a small perturbation of the final state. In reeveral cent years the study of quantum systems whose underlying classical dynamics is chaotic has been an active area of research.
Astronomical Image Processing May Improve Breast Cancer Diagnostics
Authors: Lubkin Gloria B.
Published: January 2008

Over the last few decades, astronomical image processing has become extremely sophisticated, encompassing image reconstruction and restoration, image filtering, object detection and classification. A collaboration from the Space Telescope Science Institute, in Baltimore; Johns Hopkins University; and the Lombardi Cancer Research Center at the Georgetown University Medical Center, in Washington, DC, is hoping to apply some of these methods to detect telltale signs of breast cancer in a digitized mammogram. The project was catalyzed over a year ago by Benjamin Snavely, program director for advanced technologies and instrumentation in the NSF division of astronomical sciences, which recently awarded the collaboration a $50 000 grant from the Small Grants for Exploratory Research Program.
Atom Interferometers Prove Their Worth in Atomic Measurements
Authors: Levi Barbara Goss
Published: January 2008

Thanks to the wave nature of matter, it's possible to build the atomic analog of an optical interferometer, although it's extremely difficult, because the de Broglie wavelengths of atoms are typically 10 000 times shorter than those of light. The feat was accomplished four years ago by a number of groups, each with a different scheme for splitting and recombining the matter waves. (See PHYSICS TODAY, July 1991, page 17.) We are now starting to see the fulfillment of the hopes raised by those first atom interferometers as researchers hone their interferometers to yield ever more precise values of fundamental constants or explore fundamental aspects of quantum mechanics such as Berry's phase. For example, Steven Chu and his colleagues at Stanford University have measured the ratio h/m of Planck's constant to the mass of a cesium atom with a precision of one part in 107, although the systematic errors are at the level of 1 part in 106. An experiment to improve the absolute accuracy of the h/m measurement to a few parts in 109 is underway. This ratio is one factor in the determination of the finestructure constant.
Rochester and Naval Research Lab Start Running New Laser Fusion Facilities for Direct-Drive Experiments
Authors: Lubkin Gloria B.
Published: January 2008

Two new facilities for inertial confinement fusion have recently started operation. The University of Rochester's Laboratory for Laser Energetics is now running Omega Upgrade at energies as high as 45 kilojoules (at greater than 60 terawatts), at a wavelength of 351 nanometers, making it the most powerful ultraviolet laser facility in the world. (Typical operation will be at 40 kJ in 1-nano-second pulses). Omega Upgrade is a frequency-tripled neodymiumglass solid-state laser, as is Lawrence Livermore National Laboratory's Nova, which has produced as much as 45 kJ in a 2.5-ns pulse at 351 nm. (Nova typically operates at 30-35 kJ in 1-ns pulses). The second ICF facility to recently start running is the Naval Research Laboratory's Nike, a KrF gas laser that produces energies at the target of 3 kJ at 248 nm.
Is the Answer Blowing in the Solar Wind?
Authors: Ladbury Ray
Published: January 2008

In trying to understand the surprise, wonder and skepticism that have greeted the paper by David Thomson, Carol Maclennan and Louis Lanzerotti of AT&T Bell Labs in the 13 July issue of Nature, it helps to remember a few salient facts. First, the authors claim that the solar wind has components that oscillate in a coherent, stable manner. This claim contradicts the standard view that the solar wind is turbulent and possesses no long-lived structures, other than those associated with solar rotation and structure.
How Low Can the Violin Go?
Authors: Benka Stephen G.
Published: January 2008

Almost all beginning violinists generate the sound of a small woodshop when practicing low notes. Almost no professional musicians cultivate that scratchy rumble to produce definite tones of lower frequency than the violin is normally capable of. Yet such tones can be generated, and several investigators are trying to make sense of them.
New Gamma Detector Array Finds Evidence of Hyperdeformed Nuclei
Authors: Schwarzschild Bertram
Published: January 2008

How much spin can a nucleus take before centrifugal force pulls it apart? A surprising answer is proffered by one of the first physics results from the Gammasphere facility at the Lawrence Berkeley National Laboratory. Gammasphere is the first of a new generation of nuclear-physics photon detector arrays. In June the Berkeley, Oak Ridge, Washington University collaboration at Gammasphere published evidence that gadolinium-147 nuclei might be reaching angular momenta as high as 90ħ without succumbing to fission. “That's embarrassingly high,” says collaboration leader Demetrios Sarantites, “because the theory says that the barrier against fission should vanish at around 80ħ.”
Nobel Prize in Physics Goes to Frederick Reines for Detection of the Neutrino…
Authors: Lubkin Gloria B.
Published: January 2008

The Royal Swedish Academy of Sciences has awarded the 1995 Nobel Prize in Physics “for pioneering experimental contributions to lepton physics.” The prize will be shared by Frederick Reines of the University of California, Irvine, for the detection of the neutrino and by Martin L. Perl of the Stanford Linear Accelerator Center for the discovery of the tau lepton.
There's Still Some Resistance to Reports of Extra-High Tc Superconductors
Authors: Levi Barbara Goss
Published: January 2008

Ever since the copper oxide superconductors were first discovered seven years ago, any number of experimenters have come across a sample or two that have exhibited traits suggesting superconductivity at temperatures over 200 K. Typically the behavior was fleeting or elusive, disappearing after the material was taken through several thermal cycles or stubbornly refusing to show up in other samples produced in the same way. In most cases the sample consisted of several phases, and the experimenters could not link the high-Tc behavior to a particular phase. Robert Dynes of the University of California, San Diego, refers to these teasers as “leprechauns.” Paul Chu of the University of Houston has described them as “unidentified superconducting objects.”
Complex Organic Molecules Found in Interplanetary Dust Particles
Authors: Zeman Ellen J.
Published: January 2008

Two research groups—one in Missouri that specializes in the identification and analysis of extraterrestrial particles, and one in California that has a long history of using laser technology to solve a range of problems—have combined their expertise to confirm the presence of polycyclic aromatic (benzene-skelton-containing) hydrocarbon molecules on interplanetary dust particles. Frank Stadermann (now at the Technische Hochschule in Darmstadt, Germany), Patrick Swan and Robert Walker, in work done at the McDonnell Center for the Space Sciences at Washington University, Saint Louis, demonstrated that the particles were indeed extraterrestrial and classified them mineralogically. Then the microprobe two-step laser mass spectrometer, an instrument built by graduate students Simon Clemett and Claude Maechling in the laboratory of Richard Zare at Stanford University, provided evidence of the complex organic compounds: aromatic hydrocarbons composed of two or more six-carbon rings, and their alkylated (hydrocarbon-chain-containing) derivatives.
Evidence of Time Dilation Suggests Gamma Bursters are Very Far Away
Authors: Schwarzschild Bertram
Published: January 2008

The 1 April issue of The Astrophysical Journal brings news of a provocative contribution to the continuing debate about gamma-ray bursters. Astronomers have been looking at these celestial outbursts for three decades—ever since the US military convinced itself that they were not clandestine Soviet bomb tests. But we still don't know for sure whether the sources are local (in our Galaxy) or cosmological (billions of light-years away). Therefore our ignorance of their intrinsic luminosities spans at least 13 orders of magnitude. The Astrophysical Journal article is a novel analysis by Jay Norris and colleagues at the NASA Goddard Space Flight Center and the NASA Ames Research Center of data collected by the orbiting Compton Gamma Ray Observatory's Burst and Transient Source Experiment. Though this anlysis brings strong reinforcements to the cosmological camp, no one has as yet declared victory.
The Very Long Baseline Array Opens Its Eyes
Authors: Benka Stephen G.
Published: January 2008

One might expect a compound eye 5000 miles wide to have extraordinary vision, but also to open rather slowly. So it is with the Very Long Baseline Array, a continent-wide interferometer completed last May.
Reaching for the Top
Authors: Ladbury Ray
Published: January 2008

The uninitiated would have had difficulty explaining the excited crowd of physicists filling Fermilab's auditorium on 26 April—especially since the presenters were careful to disclaim an actual discovery. Yet even though skepticism remained the watchword, the excitement of even the most ardent critics was palpable. The multinational 440-member Collider Detector Facility group at Fermilab's Tevatron may have glimpsed the top quark—the longsought partner of the bottom quark—at 174±17 GeV. This would make the t quark the heaviest fundamental particle yet seen. Indeed, because the observed mass is so close to the mass at which electroweak unification becomes manifest, many speculate that the top quark may hold the secrets to the primordial breaking of vacuum symmetry that is thought to have given masses to elementary particles.
Fermilab Tevatron Collider Group Goes Over the Top—Cautiously
Authors: Ladbury Ray
Published: January 2008

The evidence from Fermilab's Collider Detector Facility group for a 174-GeV/c2 top quark (see PHYSICS TODAY, May, page 20) has been received with cautious enthusiasm by the particle physics community. Since most physicists still have not seen the 153-page preprint submitted on 22 April to Physical Review D, the demand for speakers on the result exceeds even CDF's abundant supply of experimenters. Still, it is unlikely that new developments will suppant CDF's present results before we've had time to digest them. the low top-production rate, the large backgrounds—events that mimic top decay—and the complicated topologies of t-quark decays all but ensure that the discovery of the t quark will be a protracted process rather than a single dramatic event. While awaiting new results from CDF and DO (the other detector at Fermilab's 1.8-TeV pp̄ collider), it is appropriate to evaluate CDF's present evidence and to anticipate possible future developments.
MACHO Searches Find Most Candidates in Unexpected Places
Authors: Schwarzschild Bertram
Published: January 2008

For two years now, three groups of astronomers and refugees from particle physics have been measuring millions of stars night after night in a stellar photometry campaign of unprecedented scale. But in fact they're not really looking for stars at all. What they seek are dark “massive compact halo objects” at least as massive as planets but generally less than a tenth the mass of the Sun. These substellar MACHOs, as they are called, would not be massive enough to ignite the fusion that makes stars shine. Heavier bodies, like neutron stars and old white dwarfs, that are dark because their stellar fuel has run out are also included under the MACHO rubric.
Infrared Laser Demonstrates a New Principle of Operation
Authors: Levi Barbara Goss
Published: January 2008

In conventional semiconductor lasers, the photons come from the recombination of electrons and holes. But in a new semiconductor laser recently unveiled by a team at AT&T Bell Labs, the photons are emitted as electrons cascade from one energy level down to another in a staircase-like arrangement of quantum wells. The new laser is thus unipolar, relying only on electrons rather than on both positive and negative charges. While the physics of this “quantum cascade laser” is very exciting, its commercial potential is not. The threshold current is still very high and might limit its practicality. However, the laser could be used as a research tool and should certainly stimulate still more innovations in design.
New Stanford Facility Squeezes High-Energy Electron Beams
Authors: Schwarzschild Bertram
Published: January 2008

Accelerator physicists at the Final Focus Test Beam, a new facility at the Stanford Linear Accelerator Center, have managed to squeeze the vertical width of a high-energy electron beam down to 75 nanometers. That's an order of magnitude smaller than the focal spots of the 50-GeV electron and positron beams of the five-year-old Stanford Linear Collider. And it's almost small enough for the TeV beams envisioned for the next generation of electron-positron colliders. (See the article by Jonathan Wurtele on page 33.) Linear colliders, in which collisions must all happen in a single pass, require much tighter focal spots than do storage rings, where the beams pass through each other again and again. But for beam energies above about 100 GeV, circulating charged particles as light as electrons radiate away so much energy that storage rings would be prohibitively expensive. A TeV electron-positron collider will, perforce, be a pair of face-to-face linacs.
VLT Interferometer to Be Delayed
Authors: Cioffi Denis F.
Published: January 2008

In December the council of the European Southern Observatory approved a revised construction plan for ESO's Very Large Telescope. (See the article “The New Ground-Based Optical Telescopes,” by Buddy Martin, John M. Hill and Roger Angel in PHYSICS TODAY, March 1991, page 27.) Although the four 8.2-meter telescopes will be constructed as scheduled, the VLT Interferometer, its subarray, the Coudé feed and the associated adaptive optics were postponed to reduce the expenditure in the next few years.
The Advanced Neutron Source Knocks at the Door of Congress
Authors: Levi Barbara Goss
Published: January 2008

In the wake of the cancellation of the $11 billion Superconducting Super Collider, another big-ticket physics facility is seeking its fortunes with Congress. The supplicant is the Advanced Neutron Source, a $2.9 billion research reactor designed at Oak Ridge National Laboratory to provide at least five times the neutron flux of any existing facility and support up to 1000 users per year, conducting experiments in material, biological, condensed-matter and chemical physics. The 1994 Nobel Prize in Physics was just awarded for the type of studies that would be done at the ANS: The Royal Swedish Academy of Sciences gave the prize to Bertram N. Brockhouse of McMaster University in Hamilton, Ontario, and Clifford Shull of MIT for “pioneering contributions to the development of neutron-scattering techniques for studies of condensed matter.”
Half-Filled Landau Level Yields Intriguing Data and Theory
Authors: Schwarzschild Bertram
Published: January 2008

Surprises abound when strong magnetic fields are imposed on two-dimensional electron gases at low temperature. When the electron density approaches an integral multiple ν of the density of magnetic flux quanta threading the plane, the resistivity of the electron system begins to vanish and its Hall resistance takes on the quantized value h/ve2 to a spectacular level of precision. That's the integral quantum Hall effect, for whose serendipitous discovery at Grenoble in 1980 Klaus von Klitzing won the 1985 Nobel Prize in Physics.
New Data Strengthen Weak Link in Stellar Nucleosynthesis
Authors: Levi Barbara Goss
Published: January 2008

According to the standard model of the Big Bang, the only elements created during that event were hydrogen, helium and some lithium; the rest were brewed in the stars. Over the past few decades nuclear astrophysicists have sorted out the complex chain of stellar reactions responsible for the observed abundances of the isotopes, with remarkable success in all the most critical reactions but one: C(α,γ)1612. This reaction is a key stepping-stone in the synthesis of heavier elements, linking the stage that burns helium to produce carbon and oxygen with the later stages fueled by those elements. If helium burning yields more or less oxygen or carbon, it is predicted that the later stages will be strongly affected, and so will the ultimate fate of massive stars.
CEBAF Readies Its Electron Beam for Studies of Nucleons and Nuclei
Authors: Mukerjee Madhusree
Published: January 2008

In April 1983 the Southeastern Universities Research Association won an intense competition for construction and operation of a continuouswave electron accelerator in the energy range 0.5–4.0 GeV designed for studies in high-energy nuclear physics. Four years later Hermann Grunder, director of SURA's proposed Continuous Electron Beam Accelerator Facility at Newport News, Virginia, initiated a radical change in the design of the accelerator from one that would stretch a pulsed beam to one that used superconducting cavity technology, just then becoming viable for large-scale applications, to create an intrinsically continuous beam. (See PHYSICS TODAY, July 1983, page 57; February 1986, page 18; and April 1992, page 19.) Construction of CEBAF began in 1987 and is now nearing completion
The complex folding pathways of protein A suggest a multiple-funnelled energy landscape
Authors: St-Pierre Jean-Francois , Mousseau Normand , Derreumaux Philippe
Published: January 2008

Folding proteins into their native states requires the formation of both secondary and tertiary structures. Many questions remain, however, as to whether these form into a precise order, and various pictures have been proposed that place the emphasis on the first or the second level of structure in describing folding. One of the favorite test models for studying this question is the B domain of protein A, which has been characterized by numerous experiments and simulations. Using the activation-relaxation technique coupled with a generic energy model (optimized potential for efficient peptide structure prediction), we generate more than 50 folding trajectories for this 60-residue protein. While the folding pathways to the native state are fully consistent with the funnel-like description of the free energy landscape, we find a wide range of mechanisms in which secondary and tertiary structures form in various orders. Our nonbiased simulations also reveal the presence of a significant number of non-native β and α conformations both on and off pathway, including the visit, for a non-negligible fraction of trajectories, of fully ordered structures resembling the native state of nonhomologous proteins.
Enhancement in counterflow drag reduction by supersonic jet in high enthalpy flows
Authors: Kulkarni Vinayak , Reddy K. P. J.
Published: January 2008

Counterflow drag reduction by supersonic jet for a 60° apex angle blunt cone flying at hypersonic Mach number is investigated for two different flow enthalpies using conventional and free piston driven hypersonic shock tunnels. Enhancement in drag reduction has been observed with increase in freestream stagnation enthalpy. It is shown that the percentage of drag reduction goes up by a factor of 2 when the flow enthalpy increases by a factor of 2.5 for a given ratio of total pressure of supersonic jet and freestream flow.
Ab initio calculations of NMR chemical shifts
Authors: Casabianca Leah B. , de Dios Angel C.
Published: January 2008

The nuclear magnetic resonance chemical shift is one of the most powerful properties available for structure determination at the molecular level. A review of advances made in the ab initio calculation of chemical shielding during the past five years is presented. Specifically, progress in the areas including the effects of an unpaired electron, electron correlation, and relativistic effects into ab initio chemical shielding calculations, the tensor nature of the chemical shift, and intramolecular and intermolecular effects on the chemical shift will be covered.
Study on the maximum accuracy of the pseudopotential density functional method with localized atomic orbitals versus plane-wave basis sets
Authors: Gusso Michele
Published: January 2008

A detailed study on the accuracy attainable with numerical atomic orbitals in the context of pseudopotential first-principles density functional theory is presented. Dimers of first- and second-row elements are analyzed: bond lengths, atomization energies, and Kohn-Sham eigenvalue spectra obtained with localized orbitals and with plane-wave basis sets are compared. For each dimer, the cutoff radius, the shape, and the number of the atomic basis orbitals are varied in order to maximize the accuracy of the calculations. Optimized atomic orbitals are obtained following two routes: (i) maximization of the projection of plane wave results into atomic orbital basis sets and (ii) minimization of the total energy with respect to a set of primitive atomic orbitals as implemented in the OPENMX software package. It is found that by optimizing the numerical basis, chemical accuracy can be obtained even with a small set of orbitals.
The umbrella motion of core-excited CH3 and CD3 methyl radicals
Authors: Ekström U. , Carravetta V. , Alagia M. , Lavollée M. , Richter R. , Bolcato C. , Stranges S.
Published: January 2008

An accurate experimental and theoretical study of the lowest core excitation of CH3 and CD3 methyl radicals is presented. The complex vibrational structure of the lowest band of the x-ray absorption spectrum (XAS) is due to the large variation of the molecular geometry, which is planar in the ground state and pyramidal in the core-excited state. The XAS spectra of the two radicals were recorded at high resolution and assigned by theoretical simulations of the spectra, taking into account the coupling of symmetrical stretching and symmetrical bending (umbrellalike) deformations of the radicals. An excellent agreement between experimental and theoretical spectral profiles allowed us to accurately characterize the vibrational structure of the electronic transition. The similarities, as well as the differences, of the peculiar vibrational progression observed for the two radicals are explained by the strong anharmonicity along the umbrella coordinate and by the isotopic variation, leading to a different probing of the double-well potential energy surface of the core excited state during the nuclear motion.
Charge-transfer photodissociation of adsorbed molecules via electron image states
Authors: Jensen E. T.
Published: January 2008

The 248 and 193nm photodissociations of submonolayer quantities of CH3Br and CH3I adsorbed on thin layers of n-hexane indicate that the dissociation is caused by dissociative electron attachment from subvacuum level photoelectrons created in the copper substrate. The characteristics of this photodissociation-translation energy distributions and coverage dependences show that the dissociation is mediated by an image potential state which temporarily traps the photoelectrons near the n-hexane–vacuum interface, and then the charge transfers from this image state to the affinity level of a coadsorbed halomethane which then dissociates.
Broadband dielectric investigation on poly(vinyl pyrrolidone) and its water mixtures
Authors: Cerveny Silvina , Alegría Ángel , Colmenero Juan
Published: January 2008

Broadband dielectric spectroscopy and differential scanning calorimetry measurements have been performed to study the molecular dynamics poly (vinyl pyrrolidone) and its water solutions in a wide range of concentrations (0wt%<wc<55wt%) and in the temperature range from 140to500K. The dry material was completely characterized showing the presence of two relaxations in the supercooled state. The slow one has the characteristics of a Johari-Goldstein-type relaxation. On the other hand, the low temperature water dynamics in the mixtures evidences a prominent loss peak due to the reorientation of water molecules inside the polymer matrix for all the hydration levels. We show that the relaxation times are almost water concentration (wc) independent from wc>20wt% suggesting that this dynamical process is dominated by water-water interactions. In addition, the temperature dependence of the water relaxation times exhibits a crossover from non-Arrhenius to Arrhenius behavior during cooling throughout the glass transition range, which has been interpreted as due to the constrains imposed by the rigid polymer matrix on the water molecules dynamics.
Hartree-Fock complete basis set limit properties for transition metal diatomics
Authors: Williams T. Gavin , DeYonker Nathan J. , Wilson Angela K.
Published: January 2008

Numerical Hartree-Fock (HF) energies accurate to at least 1μhartree are reported for 27 diatomic transition-metal-containing species. The convergence of HF energies toward this numerical limit upon increasing the basis set size has been investigated, where standard nonrelativistic all-electron correlation consistent basis sets and augmented basis sets, developed by Balabanov and Peterson [J. Chem. Phys. 123, 064107 (2005)], were employed. Several schemes which enable the complete basis set (CBS) limit to be determined have been investigated, and the resulting energies have been compared to the numerical Hartree-Fock energies. When comparing basis set extrapolation schemes, those in the form of exponential functions perform well for our test set, with mean absolute deviations from numerical HF energies of 234 and 153μEh, when the CBS limit has been determined using a two-point fit as proposed by Halkier et al. [Chem. Phys. Lett. 302, 437 (1999)] on calculations of triple- and quadruple-ζ basis set qualities and calculations of quadruple- and quintuple-ζ basis set qualities, respectively. Overall, extrapolation schemes in the form of a power series are not recommended for the extrapolation of transition metal HF energies. The impact of basis set superposition error has also been examined.
Observable effects of mechanical stress induced by sample spinning in solid state nuclear magnetic resonance
Authors: Jochum M. , Werner-Zwanziger U. , Zwanziger J. W.
Published: January 2008

The stress-induced change in chemical shielding induced by sample spinning is measured and interpreted theoretically. By considering the rotating sample as an elastic body in the plane-strain approximation, the internal stress field as a function of sample size, rotation frequency, and elastic constants is determined. This stress field and the dependence of chemical shielding on strain, as determined by first-principles calculations, are combined to predict the shielding dependence on rotation frequency under isothermal conditions in single crystal gallium phosphide. The prediction is in good qualitative agreement with the experiment. Little to no effect is detected in powder samples of both gallium phosphide and copper iodide, and it is argued that this follows from the stress distribution in granular material, as opposed to bulk crystals. Finally, the temperature and pressure dependence of the chemical shielding is estimated from these considerations and found consistently to underestimate the experimental values, indicating the importance of finite-temperature anharmonic effects even in very simple solids.
Invited Review Article: Contemporary instrumentation and application of charge exchange neutral particle diagnostics in magnetic fusion energy experiments
Authors: Medley S. S. , Donné A. J. H. , Kaita R. , Kislyakov A. I. , Petrov M. P. , Roquemore A. L.
Published: January 2008

An overview of the developments postcirca 1980s in the instrumentation and application of charge exchange neutral particle diagnostics on magnetic fusion energy experiments is presented. First, spectrometers that employ only electric fields and hence provide ion energy resolution but not mass resolution are discussed. Next, spectrometers that use various geometrical combinations of both electric and magnetic fields to provide both energy and mass resolutions are reviewed. Finally, neutral particle diagnostics based on utilization of time-of-flight techniques are presented.
Bias stress stability of indium gallium zinc oxide channel based transparent thin film transistors
Authors: Suresh A. , Muth J. F.
Published: January 2008

The effects of bias stress on transistor performance are important when considering nontraditional channel materials for thin film transistors. Applying a gate bias stress to indium gallium zinc oxide transparent thin film transistors was found to induce a parallel threshold voltage shift without changing the field effect mobility or the subthreshold gate voltage swing. The threshold voltage change is logarithmically dependent on the duration of the bias stress implying a charge tunneling mechanism resulting in trapped negative charge screening the applied gate voltage.
Overlap integral factor enhancement using buried electrode structure in polymer Mach-Zehnder modulator
Authors: Song Reem , Steier William H.
Published: January 2008

A buried coplanar waveguide (CPW) electrode for a polymer modulator is introduced in this letter. In this configuration, the electric and the optical fields are aligned and the overlap factor is enhanced. Thus, the poling efficiency can increase and Vπ decreases. Using two electro-optic polymers, Mach-Zehnder (MZ) modulators based on the buried electrode are demonstrated and compared to the in-plane CPW based MZ modulators. The measured Vπ’s of the buried CPW MZ’s are 6.7 and 4.3V for CLD1 and AJL8 devices, respectively, which give the overlap×r33 products of 19 and 30pm∕V. This shows 20% improvement in the overlap×poling efficiency by using the buried electrode.
Effective velocity for transport in heterogeneous compressible flows with mean drift
Authors: Attinger Sabine , Abdulle Assyr
Published: January 2008

Solving transport equations in heterogeneous flows might give rise to scale dependent transport behavior with effective large scale transport parameters differing from those found on smaller scales. For incompressible velocity fields, homogenization methods have proven to be powerful in describing the effective transport parameters. In this paper, we aim at studying the effective drift of transport problems in heterogeneous compressible flows. Such a study was done by Vergassola and Avellaneda in Physica D 106, 148 (1997). There, it was shown that for static compressible flow without mean drift, impacts on the large scale drift do not occur. We will first discuss the impact of a mean drift and show that static compressible flow with mean drift can produce a heterogeneity driven large scale drift (or ballistic transport). For the case of Gaussian stationary random processes, we derive explicit results for the large scale drift. Moreover, we show that the large scale or effective drift depends on the small scale diffusion coefficients and thus on the molecular weights of the particles. This study could be applied to weight-based particle separation. Numerical simulations are presented to illustrate these phenomena.
Low-loss materials for high Q-factor Bragg reflector resonators
Authors: le Floch Jean-Michel , Tobar Michael E. , Cros Dominique , Krupka Jerzy
Published: January 2008

A Bragg resonator uses dielectric plates within a metallic cavity to confine the energy within a central free space region. The importance of the permittivity is shown with a better Q factor possible using higher permittivity materials of larger intrinsic dielectric losses. This is because the electric energy in the reflectors decreases proportionally to the square root of permittivity and the coupling to the metallic losses decrease linearly. In a sapphire resonator with a single reflector pair a Q factor of 2.34×105 is obtained, which may be improved on by up to a factor of 2 using higher permittivity materials.
Efficient methodology for solving an inverse magnetostatic problem by utilizing material sensitivity
Authors: Yang Chang-Seob , Lee Kang Jin , Jung Giwoo , Chung Hyun-Ju , Park Jong-Sik , Kim Dong-Hun
Published: January 2008

This paper presents an original approach for determining the unknown remanent magnetization of a ferromagnetic thin shell by utilizing material sensitivity information. To achieve this, first, the forward problem is formulated in term of the equivalent magnetic charge method for minimizing system unknowns. Then, a material sensitivity formula, which gives the first-order gradient information of an objective function with respect to the magnetic charge, is analytically derived by exploiting the governing integral system equation, augmented objective function, and adjoint variable method. The formula facilitates applying the deterministic approach to searching for an optimal charge distribution and saving computing time for solving an inverse problem in three dimensions as well. The proposed method has been successfully applied to a scale model ship and the results have been validated on a mock-up with real measurements.
Easy axis magnetization reversal in cobalt antidot arrays
Authors: Mengotti E. , Heyderman L. J. , Nolting F. , Craig B. R. , Chapman J. N. , Lopez-Diaz L. , Matelon R. J. , Volkmann U. G. , Kläui M. , Rüdiger U. , Vaz C. A. F. , Bland J. A. C.
Published: January 2008

The magnetization reversal in square lattice cobalt antidot arrays with the applied field at 45° to the antidot rows was investigated using Lorentz electron microscopy in the Fresnel mode. While the hysteresis loops from magneto-optical Kerr effect measurements only reflect the easy axis character of the reversal, several different reversal processes were identified in the Fresnel images depending on the field history. Details of this complex magnetization reversal were elucidated with micromagnetic simulations.