bluuebunny/crossref_metadata_2025 huggingface dataset:
An Array of Cherenkov Telescopes Yields the First Resolved Image of a Celestial Gamma-Ray Source
Authors: Schwarzschild Bertram
Published: March 2007

Imaging with TeV photons provides the most direct evidence yet that cosmic rays originate in the shock fronts of supernova remnants.
New Study Correlates Mantle Melting to Mid-Ocean Ridge Segments
Authors: Wilson Mark
Published: March 2007

Variations found in the composition along a stretch of exposed mantle on the Arabian peninsula offer fresh evidence for the link between the length scale of ridge segments and Earth’s convection process.
R&D Budget Brings Modest Increases to Most Civilian R&D; NSF Takes a Hit
Authors: Dawson Jim
Published: March 2007

As in the past three US budgets, defense and homeland security received most of the federal R&D dollars. With the federal deficit at $413 billion and climbing, most budget experts expect science funding to get worse before it gets better.
God’s Rays
Authors: DeWitt Bryce
Published: March 2007

The physicist’s quest for understanding is not the only way to raise the level of our existence and give our lives meaning.
Two Groups Observe the Spin Hall Effect in Semiconductors
Authors: Day Charles
Published: March 2007

Spins of opposite sign accumulate on opposite sides of a semiconductor in response to an external electric field.
Rereading Einstein on Radiation
Authors: Kleppner Daniel
Published: March 2007

The concepts of spontaneous and stimulated emission are well known from Einstein’s 1917 paper on radiation, but his theory of radiation comprises many other concepts—the paper is a treasure trove of physics.
A New Look at Electrons in Water Clusters Solves a Longstanding Riddle
Authors: Blau Steven K.
Published: March 2007

Influenced by cluster size and temperature, an electron can attach to the surface or be trapped internally.
What’s Making Earth Hum?
Authors: Levi Barbara Goss
Published: March 2007

New evidence suggests that winter storms over the oceans provide the energy that sets Earth freely oscillating at its fundamental frequencies.
Missing Magazines Highlight Staff Distrust of Los Alamos Management
Authors: Guinnessy Paul
Published: March 2007

After an annus horribilis at Los Alamos, the atmosphere at the lab is still uneasy.
Extreme Nonlinear Optics: Coherent X rays from Lasers
Authors: Kapteyn Henry C. , Murnane Margaret M. , Christov Ivan P.
Published: March 2007

Ultrashort laser pulses can generate even shorter bursts of coherent soft x rays. The technology now makes it possible to manipulate atoms on attosecond time scales to create designer wavefunctions.
Teaching Biological Physics
Authors: Goldstein Raymond E. , Nelson Philip C. , Powers Thomas R.
Published: March 2007

Biological physics courses can serve a variety of students, among them life-sciences students who need to understand the role of physical principles in the world of biology.
Optically Pumped Silicon Lases in the Near-Infrared
Authors: Wilson Mark
Published: March 2007

Two groups exploit the Raman effect in silicon to shift the wavelength of incident light and amplify the scattered light in an optical cavity.
First Experiment at National Ignition Facility Focuses on Hydrodynamics of Plasma Jets
Authors: Schwarzschild Bertram
Published: March 2007

Supersonic jets generated by shock waves are common, on vastly different scales, to high-energy astrophysics, inertial-confinement-fusion targets, and nuclear weapons.
Accelerator Labs Regroup as Photon Science Surges
Authors: Feder Toni
Published: March 2007

X-ray free-electron lasers signal a culture change at two accelerator labs— and offer them a new lease on life.
Nuclear Power Needs Government Incentives, Says Task Force
Authors: Dawson Jim
Published: March 2007

A high-profile US government task force says it is in the national interest to use nuclear power as a clean and increasingly economical way to meet the growing demand for electricity.
Physics, Philosophy, and Scientific Progress
Authors: Einstein Albert
Published: March 2007

In this 1950 speech to the International Congress of Surgeons in Cleveland, Ohio, Einstein argued that the 19th-century physicists’ simplistic view of nature, illusory as it was, gave biologists the confidence to treat life as a purely physical phenomenon.
Images of Vortices Reveal Superfluidity in a Fermi Gas
Authors: Levi Barbara Goss
Published: March 2007

The superfluid formed in a low-density atomic gas is a clean and controllable system for exploring strong interactions among particles.
Putting Mechanics into Quantum Mechanics
Authors: Schwab Keith C. , Roukes Michael L.
Published: March 2007

Nanoelectromechanical structures are starting to approach the ultimate quantum mechanical limits for detecting and exciting motion at the nanoscale. Nonclassical states of a mechanical resonator are also on the horizon.
Physics and the Real World
Authors: Ellis George F. R.
Published: March 2007

No current physics experiment or theory explains the nature—or even the existence—of emotions, money, fine art, football games, or people. What can physics say about such things?
Black-Hole Physics in an Electromagnetic Waveguide
Authors: Blau Steven K.
Published: March 2007

By rapidly changing the dielectric constant in part of the guide, one can create mode-trapping horizons and an analogue of Hawking radiation.
Supernova Spectral Feature Addresses the Connection with Gamma-Ray Bursts
Authors: Schwarzschild Bertram
Published: March 2007

Evidence is growing that gamma-ray bursts are generated by relativistic jets of material ejected from atypically massive and energetic supernovae flattened by high spin.
Polarized Gamma-Ray Beams
Authors: Schaerf Carlo
Published: March 2007

Polarized high-energy photons are excellent probes of protons, neutrons, and nuclei. Nowadays they are readily made by shining laser light at a high-energy electron beam.
Radioisotope Tracers Reveal Extensive Melting in Earth’s Distant Past
Authors: Wilson Mark
Published: March 2007

New isotope-ratio measurements from primitive meteorites provide evidence that Earth’s mantle divided into separate, chemically distinct reservoirs.
Novel Medical Imaging Method Shows Promise
Authors: Day Charles
Published: March 2007

When subjected to a varying magnetic field, ferromagnetic nanoparticles produce harmonics that can reveal their location.
Small Programs Survive by Pooling Students
Authors: Feder Toni
Published: March 2007

Physics classes via interactive television can be successful, but both teaching and learning require more work than in a traditional setting.
Obliterating Myths About Minority Institutions
Authors: Sakimoto Philip J. , Rosendhal Jeffrey D.
Published: March 2007

A multiyear NASA initiative for developing research partnerships in space science demonstrates that such programs can have great success in attracting minorities to science.
NSF Cancels Brookhaven’s Last Remaining Particle-Physics Experiment
Authors: Schwarzschild Bertram
Published: March 2007

The planned RSVP search for extremely rare particle metamorphoses has been felled by escalating cost estimates.
Proposed Export Restrictions Alarm US Researchers
Authors: Dawson Jim
Published: March 2007

Despite hundreds of complaints, the US Department of Commerce is considering export rule changes that many say could hinder research.
Stellar Energy Generation and Solar Neutrinos
Authors: Bahcall John N. , Salpeter Edwin E.
Published: March 2007

Half a century after he explained how the Sun shines, Hans Bethe addressed the problem of why we see too few of its neutrinos.
Hans in War and Peace
Authors: Garwin Richard L. , Gottfried Kurt
Published: March 2007

Hans Bethe spent a lifetime enhancing the security of his adopted homeland—initially designing its nuclear bombs, but ultimately warning presidents and the public to guard against the hazards of such bombs.
Hans Bethe and the Theory of Nuclear Matter
Authors: Negele John W.
Published: March 2007

Post-war research helped take nuclear physics from the domain of phenomenology and models to that of many-body theory based on the underlying nucleon—nucleon interaction.
Hans Bethe and Astrophysical Theory
Authors: Brown Gerald E.
Published: March 2007

A scientific colleague and friend reminisces about a nearly 30-year collaboration working on supernovae, collapsing binaries, and California condominiums with nice bathtubs.
First Identification of Host Galaxies for Short Gamma-Ray Bursts
Authors: Schwarzschild Bertram
Published: March 2007

The long search for observational evidence of the sources of a particularly puzzling class of gamma-ray bursts has at last borne fruit. The new evidence suggests that short-duration GRBs are caused by mergers of neutron stars and black holes.
Einstein’s Mistakes
Authors: Weinberg Steven
Published: March 2007

Science sets itself apart from other paths to truth by recognizing that even its greate practitioners sometimes err.
Peace Prize Goes to ElBaradei and Nuclear Watchdog Agency
Authors: Guinnessy Paul
Published: March 2007

Combating nuclear proliferation and improving reactor safety are highlighted by this year’s Nobel Peace Prize.
Albert Einstein as a Philosopher of Science
Authors: Howard Don A.
Published: March 2007

Einstein’s philosophical habit of mind, cultivated by undergraduate training and lifelong dialogue, had a profound effect on the way he did physics.
Why Is Ice Slippery?
Authors: Rosenberg Robert
Published: March 2007

In 1859 Michael Faraday postulated that a thin film of liquid covers the surface of ice—even at temperatures well below freezing. Neglected for nearly a century, the dynamics of ice surfaces has now grown into an active research topic.
Densely packed positronium atoms interact chemically
Authors: Blau Steven K.
Published: March 2007

For the first time, experimenters have seen atoms made from an electron and a positron exchange spins and perhaps form diatomic molecules.
Evolution wins in Pennsylvania, loses in Kansas
Authors: Dawson Jim
Published: March 2007

A slate of “real-world” candidates swept the intelligent design majority off the Dover, Pennsylvania, school board, while in Kansas antievolutionists not only weakened science standards, but redefined science itself.
Turbulent liquid-sodium flow induces magnetic dipole in a laboratory analogue of the geodynamo
Authors: Schwarzschild Bertram
Published: March 2007

Supercomputers still can’t simulate the self-excitation of planetary dynamos. So experimenters have taken up the challenge.
Materials institutes weave global networks
Authors: Feder Toni
Published: March 2007

Six NSF-funded materials science institutes are working to train a new generation of internationally minded US researchers and to stimulate collaborations around the world.
Stronger future for nuclear power
Authors: Guinnessy Paul
Published: March 2007

Nuclear reactor builders are jostling for business as energy utilities take another look at nuclear power.
Enormous magnetic reconnection event washes over three spacecraft millions of kilometers apart
Authors: Schwarzschild Bertram
Published: March 2007

How large can reconnection events in plasmas get? Theory provides no easy answer. But the solar wind beyond Earth’s cramped magnetosphere is a good place to look.
Outlook for French science still uncertain, even as CNRS crisis subsides
Authors: Feder Toni
Published: March 2007

Two years after their massive street protests, French researchers are worried that science remains underfunded and that the government is grabbing increasing control over the directions of research.
Macromolecular phasing
Authors: Shen Qun , Hao Quan , Gruner Sol M.
Published: March 2007

Several techniques, well established and newly developed, allow crystallographers to reconstruct large molecular structures after recovering the phases that are lost in x-ray diffraction patterns.
Lessons from hydrodynamic turbulence
Authors: Falkovich Gregory , Sreenivasan Katepalli R.
Published: March 2007

Turbulent flows, with their irregular behavior, confound any simple attempts to understand them. But physicists have succeeded in identifying some universal properties of turbulence and relating them to broken symmetries.
Erskine Williamson, extreme conditions, and the birth of mineral physics
Authors: Hemley Russell J.
Published: March 2007

A series of papers published between 1916 and 1923 broke new ground in materials research and laid the foundation for modern studies of planetary interiors.
New Cosmic Microwave Background Results Strengthen the Case for Inflationary Big Bang Cosmology
Authors: Schwarzschild Bertram
Published: March 2007

With three years of data, the Wilkinson Microwave Anisotropy Probe has finally produced the first all-sky polarization maps of the CMB, despite polarization levels of only a few parts per 107 and obscuring galactic foreground.
Science board warns of global S&T competition as US K–12 education lags
Authors: Dawson Jim
Published: March 2007

China and other East Asian nations are “recasting the international S&T scene” through heavy investments in science, engineering, and mathematics.
The Biological Frontier of Physics
Authors: Phillips Rob , Quake Stephen R.
Published: March 2007

Problems at the interface between biology and physics offer unique opportunities for physicists to make quantitative contributions to biology. Equally important, they enrich the discipline of physics by challenging its practitioners to think in new ways.
Diversity in physics
Authors: Malcom Shirley M.
Published: March 2007

The underrepresentation of women and minorities among recipients of physics degrees is a manifestation of systemic educational problems. Corrective measures must begin in the primary grades and continue through university schooling and beyond.
Is string theory phenomenologically viable?
Authors: Gates S. James
Published: March 2007

String theory is entering an era in which its theoretical constructs will be confronted by experimental data. Some cherished ideas just might fail to pass the test.
What makes for smooth, creamy chocolate?
Authors: Windhab Erich J.
Published: March 2007

Chocolatiers adjust properties of chocolate’s ingredients to confect a treat that feels as good as it tastes.
Improving students’ understanding of quantum mechanics
Authors: Singh Chandralekha , Belloni Mario , Christian Wolfgang
Published: March 2007

To address the misconceptions that students typically hold concerning quantum mechanics, instructors should couple computer-based visualizations with research-based pedagogical strategies.
DARPA joins industry, academia to build better prosthetic arms
Authors: Guinnessy Paul
Published: March 2007

Can a prosthetic arm react and behave like a biological arm? A crash program to help disabled US soldiers may give an answer within the next two years.
NASA saves SOFIA by slashing planet-finding funds
Authors: Feder Toni
Published: March 2007

A US-German telescope will fly high to observe the infrared sky.
Portugal builds on renewables effort with $78M solar plant
Authors: Kaplan Karen H.
Published: March 2007

A sun-powered system, part of a push to develop more sources of renewable energy, could cut greenhouse gas emissions by 30 000 tons annually.
Science-based cleanup of Rocky Flats
Authors: Clark David L. , Janecky David R. , Lane Leonard J.
Published: March 2007

The chemical and physical interactions of radioactive compounds are key to understanding how they can contaminate the environment and, more importantly, how best to remove them.
Good music unfolds in small steps
Authors: Blau Steven K.
Published: March 2007

Geometry and topology allow music theorists to quantitatively analyze a technique that composers have intuitively practiced for centuries.
Tectonic-plate flexure may explain newly found volcanoes
Authors: Wilson Mark
Published: March 2007

The form and chemical composition of their lavas add to our growing understanding of where on Earth volcanoes can form.
Boehlert brought science to the forefront on Capitol Hill
Authors: Dawson Jim
Published: March 2007

After serving 24 years as a Republican congressman, the last 6 as chair of the House Committee on Science, Sherwood Boehlert is retiring. He says he’s leaving with a smile on his face, but science advocates in Washington aren’t smiling about the loss of one of their strongest supporters.
Manipulating the flow of light with photonic crystals
Authors: Vukusic Peter
Published: March 2007

Structures with periodically varying indices of refraction allow light to be controlled for a variety of purposes. That’s a trick Nature has used for 500 million years.
Observation of room temperature magnetoelectric coupling in Pb1−xBax(Fe0.5Ti0.5)O3 system
Authors: Kumar Manoj , Yadav K. L.
Published: March 2007

Magnetoelectric coupling at room temperature in Pb1−xBax(Fe0.5Ti0.5)O3 (x=0.2 and 0.4) ceramics prepared by conventional solid state reaction method was observed. The Ba substitution for Pb reduces the lattice distortion, i.e., c∕a ratio, and hence the ferroelectric transition temperature reduces on increasing Ba content. High temperature magnetization measurement shows that the ferromagnetic Curie temperature (TC) decreases with increasing Ba in Pb1−xBax(Fe0.5Ti0.5)O3. An anomaly in both the dielectric constant (ε) and dielectric loss (tanδ) was observed in the vicinity of ferromagnetic transition temperature. The saturated magnetic and ferroelectric hysteresis loops in Pb1−xBax(Fe0.5Ti0.5)O3 system exhibit the coexistence of ferromagnetic and ferroelectric orderings at room temperature. An enhancement of 13–15% in magnetization was observed after poling the samples in dc electric field at 10kV∕cm. The magnetoelectric coupling between the electric dipoles and magnetic dipoles at room temperature was demonstrated by measuring the effect of magnetic and dc electric polings on ferroelectric and magnetic hysteresis loops, respectively.
Lights out at LEP
Authors: Schwarzschild Bertram
Published: March 2007

As we were going to press, the CERN directorate announced that the 2 November shutdown of LEP was indeed the e+e− collider's final termination. The 27-km LEP tunnel will now be made ready for the Large Hadron Collider, which should be providing 14-TeV proton-proton collisions by 2005. LEP had already enjoyed a one-month reprieve in response to recent tantalizing hints of a possible Higgs boson with a mass of about 115 GeV/c2. But, after heated discussion, it was finally decided that the extra October run did not uncover sufficient confirmatory evidence to justify a costly further extension of LEP's 11-year life.
Challenging recovery for New Orleans universities after hurricane devastation
Authors: Feder Toni
Published: March 2007

More than a year after Hurricane Katrina, New Orleans campuses are faced with smaller enrollments, reduced faculty sizes, pinched budgets–and disgruntlement about how they’re handling the recovery.
Build astronomical observatories on the Moon?
Authors: Lowman Paul D. , Lester Daniel F.
Published: March 2007

Technological challenges, expense, and the problem of dust are among the issues to confront in any discussion of astronomy performed away from Earth. Two scientists debate the relative merits of Moon-based telescopes compared with ones in free space.
The pros and cons of leap seconds
Authors: Luzum Brian
Published: March 2007

Time as determined by Earth’s rotation and time as determined by atomic clocks slowly but steadily diverge. Should they be forced to agree?
New angles to refraction and reflection in chiral liquids
Authors: Levi Barbara Goss
Published: March 2007

A new experiment demonstrates one exception to a well-known rule: The angle of incidence is not always equal to the angle of reflection.
Geological hazards are focus of Chinese research initiatives
Authors: Feder Toni
Published: March 2007

Investments by China could shift the mode of international research collaborations in geoscience.
China’s 15-year science and technology plan
Authors: Cao Cong , Suttmeier Richard P. , Simon Denis Fred
Published: March 2007

As China implements its plan to improve scientific innovation, it will need to solve such political and economic problems as finding the proper balance between indigenous efforts and engagement with the global community.
Exploring the nanoworld with atomic force microscopy
Authors: Giessibl Franz J. , Quate Calvin F.
Published: March 2007

Over its 20-year history, the atomic force microscope has gradually evolved into an instrument whose spatial resolution is now fine enough to image subatomic features on the scale of picometers.
The entangled dance of physics
Authors: Benka Stephen G.
Published: March 2007

Physics so permeates today’s world that we often can’t even see it.
Reprocessing spent nuclear fuel
Authors: Bodansky David
Published: March 2007

New methods being developed for future extraction of plutonium and uranium from nuclear waste incorporate safeguards against weapons proliferation.
Coupling of spin-transfer torque to microwave magnetic field: A micromagnetic modal analysis
Authors: Torres L. , Lopez-Diaz L. , Martinez E. , Finocchio G. , Carpentieri M. , Azzerboni B.
Published: March 2007

Micromagnetic computational spectral mapping technique is applied to analyze the magnetic oscillation modes excited by either a microwave circularly polarized magnetic field or a spin polarized current flowing through Permalloy (Py) spin valves. A complete study has been carried out on multilayers Py(10 nm)/Cu(5 nm)/Py(2.5 nm) with rectangular cross section (60×20 nm2). The magnetic normal modes obtained agree with recent analytical spin wave models in patterned nanostructures. When both excitations, microwave field and spin polarized current, are applied at the same time a complex coupling process is observed. The detailed micromagnetic analysis of the coupling shows three different stages: (i) The initial stage in which the magnetic normal modes are dominant, (ii) an intermediate stage showing an incoherent behavior, and (iii) the final stage where a persistent domain wall oscillation is present. Micromagnetic spectral mapping technique is shown to be an adequate tool for describing the temporal evolution of the magnetization spatial patterns in nanostructures.
Identification of characteristic protein folding channels in a coarse-grained hydrophobic-polar peptide model
Authors: Schnabel Stefan , Bachmann Michael , Janke Wolfhard
Published: March 2007

Folding channels and free-energy landscapes of hydrophobic-polar heteropolymers are discussed on the basis of a minimalistic off-lattice coarse-grained model. We investigate how rearrangements of hydrophobic and polar monomers in a heteropolymer sequence lead to completely different folding behaviors. Studying three exemplified sequences with the same content of hydrophobic and polar residues, we can reproduce within this simple model two-state folding, folding through intermediates, as well as metastability.
The effect of Si doping on the electrical properties of B12As2 thin films on (0001) 6H-SiC substrates
Authors: Xu Zhou , Edgar J. H. , Look D. C. , Baumann S. , Bleiler R. J. , Wang S. H. , Mohney S. E.
Published: March 2007

The ability to control the resistivity of the wide band gap semiconductor B12As2 by doping with silicon was verified. The electrical properties of nominally undoped and Si-doped rhombohedral B12As2 thin films on semi-insulating 6H-SiC (0001) substrates prepared by chemical vapor deposition were subjected to Hall effect measurements. Varying the Si concentration in the B12As2 thin films from 7×1018to7×1021at.∕cm3 (as measured by secondary ion mass spectrometry) decreased the resistivities of the p-type B12As2 films from 2×105to10Ωcm. The resistivities of the B12As2 films were decreased by one to two orders of magnitude after rapid thermal annealing for 30s in argon. The spatial distribution of the hydrogen concentration was measured before and after annealing. No changes were detected, casting doubt on hydrogen as being the cause for the change in the resistivities of the B12As2 films with annealing.
Comparative study of normal and branched alkane monolayer films adsorbed on a solid surface. II. Dynamics
Authors: Enevoldsen A. D. , Hansen F. Y. , Diama A. , Taub H. , Dimeo R. M. , Neumann D. A. , Copley J. R. D.
Published: March 2007

The dynamics of monolayer films of the n-alkane tetracosane (n-C24H52) and the branched alkane squalane (C30H62) adsorbed on graphite have been studied by quasielastic and inelastic neutron scattering and molecular dynamics (MD) simulations. Both molecules have 24 carbon atoms along their carbon backbone, and squalane has an additional six methyl side groups symmetrically placed along its length. The authors’ principal objective has been to determine the influence of the side groups on the dynamics of the squalane monolayer and thereby assess its potential as a nanoscale lubricant. To investigate the dynamics of these monolayers they used both the disk chopper spectrometer (DCS) and the high flux backscattering spectrometer (HFBS) at the National Institute of Standards and Technology. These instruments made it possible to study dynamical processes such as molecular diffusive motions and vibrations on very different time scales: 1–40ps (DCS) and 0.1–4ns (HFBS). The MD simulations were done on corresponding time scales and were used to interpret the neutron spectra. The authors found that the dynamics of the two monolayers are qualitatively similar on the respective time scales and that there are only small quantitative differences that can be understood in terms of the different masses and moments of inertia of the two molecules. In the course of this study, the authors developed a procedure to separate out the low-frequency vibrational modes in the spectra, thereby facilitating an analysis of the quasielastic scattering. They conclude that there are no major differences in the monolayer dynamics caused by intramolecular branching. It remains to be seen whether this similarity in monolayer dynamics also holds for the lubricating properties of these molecules in confined geometries.
Two characteristic photoluminescence states and their metastability in hydrogenated amorphous silicon and its alloys
Authors: Oheda Hidetoshi
Published: March 2007

Photoluminescence (PL) lifetime distributions, G(τ), in hydrogenated amorphous silicon (a-Si:H) and its alloys are studied by frequency resolved spectroscopy. Two lifetime components, which are characterized with respective specific peak lifetimes of about 1ms and 10μs, are commonly observed in G(τ) throughout the a-Si:H based materials irrespective of their difference in a localized tail-state distribution. The ratio of magnitudes of the two lifetime components greatly changes in the low temperature range where the steady-state PL intensity is almost constant. This is called lifetime conversion and is a kind of metastability characterizing disordered materials. However, a system-dependent difference is observed in its temperature dependence. By careful examination of the temperature variation in G(τ), we have confirmed that the PL corresponding to those lifetime components is caused by a certain localized PL center that will be localized at some special structural unit around the Si–H bond. Another measurement of the modulated IR absorption that can detect structural instability in the vicinity of the Si–H bond is used for examining the self-trapping nature of the localized PL center. Based on these results, it is inferred that the PL center takes on two different states in accordance with the two lifetime components. The PL state associated with the 1ms lifetime component exhibits a much stronger self-trapping nature as compared to the other state associated with the 10μs lifetime component. The lifetime conversion is a change between these two PL states with different degrees of self-trapping. Those characteristic features of the PL in a-Si:H based materials are discussed in relation to the distinctive nature of the disordered network structure, i.e., the local strain.
Enhancement of electron-longitudinal optical phonon coupling in highly strained InGaN/GaN quantum well structures
Authors: Chen Dong , Luo Yi , Wang Lai , Li Hongtao , Xi Guangyi , Jiang Yang , Hao Zhibiao , Sun Changzheng , Han Yanjun
Published: March 2007

An anomalously strong phonon replica is observed in the photoluminescence (PL) spectra of In0.23Ga0.77N/GaN multiple quantum well (MQW) samples at 10 K. The Huang-Rhys factor for the two-well sample reaches 0.95, in contrast to the value of 0.45 for the five-well sample obtained with the same growth conditions. Narrow linewidth and clearly resolved emission peak splitting are also exhibited in the two-well structure. The strong enhancement of electron-longitudinal optical (LO) phonon coupling is attributed to the interface features and the high residual strain in the quantum well structure. A model is proposed to account for the experimental results that takes the interface optical phonon mode as well as the impact of the residual strain into consideration.
Depolarization of a piezoelectric film under an alternating current field
Authors: Kwok K. W. , Cheung M. K. , Chan H. L. W. , Choy C. L.
Published: March 2007

In this article, we demonstrate that a sol-gel-derived niobium-doped lead zirconate titanate film can be depolarized by the application of alternating current (ac) fields of diminishing amplitude and we explain the phenomenon based on the concept of the Preisach model. The amplitude of the ac fields is decreased from 20 to 2 MV/m in ten steps. The observed piezoelectric coefficient of the film decreases after each ac field step. Depending on the initial polarization and the direction of the ac fields, the piezoelectric coefficient can decrease to a very small value indicating the complete depolarization of the film. Our results reveal the existence of a distribution of the switching fields in the microdomains (Preisach dipolar units), and that because of mutual interactions the magnitudes of the switch-up and switch-down fields for each microdomain are not necessarily the same. Our results also suggest that the sputter deposition of the top electrode can induce more “down-state” microdomains, thus giving rise to an initial polarization in the film. Because of interactions with other microdomains or other effects, part of these microdomains exhibit very high switching fields.
Collective diffusion in charge-stabilized suspensions: Concentration and salt effects
Authors: Gapinski J. , Patkowski A. , Banchio A. J. , Holmqvist P. , Meier G. , Lettinga M. P. , Nägele G.
Published: March 2007

The authors present a joint experimental-theoretical study of collective diffusion properties in aqueous suspensions of charge-stabilized fluorinated latex spheres. Small-angle x-ray scattering and x-ray photon correlation spectroscopy have been used to explore the concentration and ionic-strength dependence of the static and short-time dynamic properties including the hydrodynamic function H(q), the wave-number-dependent collective diffusion coefficient D(q), and the intermediate scattering function over the entire accessible range. They show that all experimental data can be quantitatively described and explained by means of a recently developed accelerated Stokesian dynamics simulation method, in combination with a modified hydrodynamic many-body theory. In particular, the behavior of H(q) for de-ionized and dense suspensions can be attributed to the influence of many-body hydrodynamics, without any need for postulating hydrodynamic screening to be present, as it was done in earlier work. Upper and lower boundaries are provided for the peak height of the hydrodynamic function and for the short-time self-diffusion coefficient over the entire range of added salt concentrations.
Comparing calorimetric and dielectric polarization modes in viscous 2-ethyl-1-hexanol
Authors: Huth Heiko , Wang Li-Min , Schick Christoph , Richert Ranko
Published: March 2007

Dielectric relaxation and dynamic heat capacity measurements are compared for 2-ethyl-1-hexanol near its glass transition temperature Tg in order to further clarify the origin of the prominent Debye-type loss peak observed in many monohydroxy alcohols and other hydrogen-bonding liquids. While the dielectric spectrum ε″ displays two distinct polarization processes that are separated by a factor of 2000 in terms of the peak frequency, the heat capacity cp″ shows only a single peak. The dielectric process with lower amplitude and higher peak frequency coincides with the calorimetric signal, whereas the large dielectric Debye signal is not associated with calorimetric modes. The authors conclude that the Debye process corresponds to a transition among states which differ in energy only in the case of an external electric field.
Characterization of a low-pressure argon plasma using optical emission spectroscopy and a global model
Authors: Palmero A. , van Hattum E. D. , Rudolph H. , Habraken F. H. P. M.
Published: March 2007

The excitation mechanisms of the lower lying excited levels in a low-ionized, low-pressure, argon plasma are modeled and studied in order to characterize the plasma through optical emission spectroscopy. It is found that the lower lying excited states of argon play an important role in the excitation and that they must be taken explicitly into account for an accurate determination of the excitation rates. The possible influence of radiative cascades from upper argon excited states, which are not included in the model, has been studied by introducing an effective level in the description and studying its influence on the results. The model has been used to calculate the electron density and electron temperature in an argon magnetron sputtering plasma produced at different electromagnetic powers and gas pressures, as a function of the intensity of the optical emission lines λ1=750.38nm and λ2=696.54nm. The results obtained from the model have been compared with Langmuir probe measurements, showing a good agreement. It is hereby demonstrated that optical emission spectroscopy can be used to monitor the plasma properties during the deposition process when applying a nonlinear model.
Effects of thermal and electrical histories on hard piezoelectrics: A comparison of internal dipolar fields and external dc bias
Authors: Gao Yongkang , Uchino Kenji , Viehland Dwight
Published: March 2007

Rare earth modified Pb(Zr,Ti)O3–Pb(Sb,Mn)O3 piezoelectric ceramics have been studied for various thermal and electrical histories. In both “freshly” poled and unpoled conditions, thermal quenching was found to increase the remnant polarization (Pr) and induced strain of hard piezoelectrics, relative to that of annealed condition. A “pinched” double-loop P-E response was found in the aged unpoled condition, whereas a single P-E loop was observed after the quenching near Curie temperature. Investigations of the effect of an applied dc bias on the P-E and ε-E responses of hard piezoelectrics were also performed. In the unpoled and quenched condition, dc bias resulted in asymmetric P-E responses and a shift of the response along the E axis. Systematic investigations revealed that internal dipolar fields and applied positive dc biases have the same effect on domain dynamics. Large internal dipolar field is essential for high power performance. A fabrication method of quenching hard piezoelectrics near the Curie temperature before poling is proposed to enhance the induced polarization and strain levels.
Thin film solid-state reactions forming carbides as contact materials for carbon-containing semiconductors
Authors: Leroy W. P. , Detavernier C. , Van Meirhaeghe R. L. , Lavoie C.
Published: March 2007

Metal carbides are good candidates to contact carbon-based semiconductors (SiC, diamond, and carbon nanotubes). Here, we report on an in situ study of carbide formation during the solid-state reaction between thin films. The solid-state reaction was examined between 11 transition metals (W, Mo, Fe, Cr, V, Nb, Mn, Ti, Ta, Zr, and Hf) and an amorphous carbon layer. Capping layers (C or TiN) of different thicknesses were applied to prevent oxidation. Carbide formation is evidenced for nine metals and the phases formed have been identified (for a temperature ranging from 100to1100°C). W first forms W2C and then WC; Mo forms Mo2C; Fe forms Fe3C; Cr first forms metastable phases Cr2C and Cr3C2−x, and finally forms Cr3C2; V forms VCx; Nb transforms into Nb2C followed by NbC; Ti forms TiC; Ta first forms Ta2C and then TaC; and Hf transforms into HfC. The activation energy for the formation of the various carbide phases has been obtained by in situ x-ray diffraction.
Single molecule photon emission statistics of driven three-level systems
Authors: Peng Yonggang , Zheng Yujun , Brown Frank L. H.
Published: March 2007

We study the statistics of photon emission from three-level single molecule systems. The generating function method [Y. Zheng and F. L. H. Brown, Phys. Rev. Lett. 90, 238305 (2003)] is used to calculate steady state absorption line shapes and Mandel’s Q parameter as a function of excitation frequency, as well as the time dependence associated with approach to the steady state. The line shape calculations confirm known results derived via other methods, while the Q parameter results display complex frequency dependences not amenable to simple interpretation. This study confirms the applicability of the generating function formalism to multilevel quantum systems, including the proper modeling of quantum coherence effects.
SASS: A symmetry adapted stochastic search algorithm exploiting site symmetry
Authors: Wheeler Steven E. , Schleyer Paul v. R. , Schaefer Henry F.
Published: March 2007

A simple symmetry adapted search algorithm (SASS) exploiting point group symmetry increases the efficiency of systematic explorations of complex quantum mechanical potential energy surfaces. In contrast to previously described stochastic approaches, which do not employ symmetry, candidate structures are generated within simple point groups, such as C2, Cs, and C2v. This facilitates efficient sampling of the 3N-6 Pople's dimensional configuration space and increases the speed and effectiveness of quantum chemical geometry optimizations. Pople's concept of framework groups [J. Am. Chem. Soc. 102, 4615 (1980)] is used to partition the configuration space into structures spanning all possible distributions of sets of symmetry equivalent atoms. This provides an efficient means of computing all structures of a given symmetry with minimum redundancy. This approach also is advantageous for generating initial structures for global optimizations via genetic algorithm and other stochastic global search techniques. Application of the SASS method is illustrated by locating 14 low-lying stationary points on the cc-pwCVDZ ROCCSD(T) potential energy surface of Li5H2. The global minimum structure is identified, along with many unique, nonintuitive, energetically favorable isomers.
Improved diffusion Monte Carlo for bosonic systems using time-step extrapolation “on the fly”
Authors: Håkansson P. , Mella M.
Published: March 2007

A diffusion Monte Carlo algorithm employing “on the fly” extrapolation with respect to the time step is implemented and demonstrated simulating realistic systems. Significant advantages are obtained when using on the fly extrapolation, leading to reduced systematic and statistical errors. The sound theoretical basis of extrapolation on the fly is discussed and compared to justifications for the a posteriori extrapolation.
Mathematical adventures in biology
Authors: Deem Michael W.
Published: March 2007

A short personal tour of biological systems reveals the flavor and variety of biological questions amenable to illumination by mathematical analysis.
The physics of climate modeling
Authors: Schmidt Gavin A.
Published: March 2007

Climate is a large-scale phenomenon that emerges from complicated interactions among small-scale physical systems. Yet despite the phenomenon’s complexity, climate models have demonstrated some impressive successes.
Analysis of a modular generator for high-voltage, high-frequency pulsed applications, using low voltage semiconductors (<1kV) and series connected step-up (1:10) transformers
Authors: Redondo L. M. , Fernando Silva J. , Margato E.
Published: March 2007

This article discusses the operation of a modular generator topology, which has been developed for high-frequency (kHz), high-voltage (kV) pulsed applications. The proposed generator uses individual modules, each one consisting of a pulse circuit based on a modified forward converter, which takes advantage of the required low duty cycle to operate with a low voltage clamp reset circuit for the step-up transformer. This reduces the maximum voltage on the semiconductor devices of both primary and secondary transformer sides. The secondary winding of each step-up transformer is series connected, delivering a fraction of the total voltage. Each individual pulsed module is supplied via an isolation transformer. The assembled modular laboratorial prototype, with three 5kV modules, 800V semiconductor switches, and 1:10 step-up transformers, has 80% efficiency, and is capable of delivering, into resistive loads, −15kV∕1A pulses with 5μs width, 10kHz repetition rate, with less than 1μs pulse rise time. Experimental results for resistive loads are presented and discussed.
Mechanism of leakage current reduction of tantalum oxide capacitors by titanium doping
Authors: Lau W. S. , Tan T. S. , Babu Premila , Sandler Nathan P.
Published: March 2007

In this letter, the authors will point out that defect states related to oxygen vacancies in tantalum oxide capacitors can be suppressed by titanium doping, resulting in significant leakage current reduction. The theory is that titanium forms an acceptor which can move at high temperature and neutralize other donors. However, defect states which cannot be suppressed by titanium doping were detected. These are explained by H2O-related contamination occurring at low temperature (<400°C) during the cooling down period.
A study of swirling turbulent pipe and jet flows
Authors: Facciolo Luca , Tillmark Nils , Talamelli Alessandro , Alfredsson P. Henrik
Published: March 2007

Axially rotating turbulent pipe flow is an example in which the rotation strongly affects the turbulence, which then also influences the mean flow properties. For instance, in the fully developed flow as well, the fluid is not in solid body rotation due to the influence of the cross-stream Reynolds stress. The present paper reports new measurements from a rotating pipe flow and the streamwise mean velocity distribution is compared with recent scaling ideas of Oberlack [J. Fluid Mech. 379, 1 (1999)] and good agreement is found. A second part of the paper deals with the initial stages when the flow leaves the pipe and forms a swirling jet. The measurements in the jet show that at some distance downstream (approximately five jet diameters) the central part of the jet actually rotates in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross-stream Reynolds shear stress.
Simple formulas for the trapping rate by nonspherical absorber and capacitance of nonspherical conductor
Authors: Berezhkovskii A. M. , Barzykin A. V.
Published: March 2007

Simple formulas are suggested for the capacitance of a nonspherical conductor and for the rate coefficient, k(t), that determines the survival probability of an immobile nonspherical absorber which annihilates at the first contact with one of the freely diffusing particles. Comparison with known analytical and numerical results, as well as numerical results obtained by the authors, shows that the formulas provide reasonable approximations for k(t) and the capacitance.
Future of US nuclear weapons a tangle of visions, science, and money
Authors: Dawson Jim
Published: March 2007

As National Nuclear Security Administration officials push for a new nuclear bomb, some scientists and arms control experts are asking what’s wrong with the old ones.
Bell Labs parent merges into communications giant
Authors: Feder Toni
Published: March 2007

With fundamental research in industrial settings long on the decline, researchers inside and outside of Bell Labs wonder how it will fare as part of a new, bigger company.
Casimir forces: Still surprising after 60 years
Authors: Lamoreaux Steve K.
Published: March 2007

The once startling idea of a connection between quantum fluctuations and forces has by now been applied throughout physics. Nonetheless, experimentalists and theorists alike still find challenges in the Casimir force.
Memories of Feynman
Authors: Welton Theodore A.
Published: March 2007

In this memoir, written in 1983, a contemporary and close friend of Richard Feynman’s recalls the blossoming of Feynman’s genius with vignettes from college, Los Alamos, and afterward.
Your wetting day
Authors: Courbin Laurent , Stone Howard A.
Published: March 2007

Why does water fall off a duck’s back but form bridges between grains of sand at the beach? The answer lies in the physics of surface wetting.
Finite size effect in ZnO nanowires
Authors: Chang Pai-Chun , Chien Chung-Jen , Stichtenoth Daniel , Ronning Carsten , Lu Jia Grace
Published: March 2007

To clarify the size effect in semiconductor nanowires with decreasing diameters but not yet reaching the quantum confinement region, single crystalline zinc oxide nanowires with diameters around 10nm are synthesized. Electrical transport measurements of these thin nanowires show significant increase in conductivity accompanied by diminished gate modulation and reduced mobility. This phenomenon is a result of the enrichment of surface states owing to the increased surface-to-volume ratio. The enhanced surface effect is confirmed by the temperature dependent photoluminescence measurements and contributes to the “anomalous” blueshift. This study shows that surface states play a dominant role in the electrical and optical properties of quasi-one-dimensional materials.
High-efficiency switchable flat diffractive ophthalmic lens with three-layer electrode pattern and two-layer via structures
Authors: Li Guoqiang , Valley Pouria , Äyräs Pekka , Mathine David L. , Honkanen Seppo , Peyghambarian N.
Published: March 2007

A general design for switchable, flat, liquid crystal diffractive lens with three-layer electrode pattern and two-layer via structures is reported for near-, intermediate-, and distance-vision corrections of presbyopic eyes. The microfabricated transparent concentric ring electrodes are distributed in two layers and different voltages are applied to each electrode through bus lines in another layer. Connection between the electrodes and the bus lines is achieved by vias (conducting holes for vertical interconnections) in the third dimension. For demonstration, a lens is first tuned with a focal length of 1m (1 diopter add power), eight-level phase modulation, and diffraction efficiency above 92% and then reconfigured to operate as a 2 diopter four-level lens with a diffraction efficiency of 78%. The lens operates with low voltages and a rise time of 150ms. This design methodology allows the aperture needed for practical use and a power-failure-safe configuration.
Nanoelectromechanical switches based on carbon nanotubes for microwave and millimeter waves
Authors: Dragoman M. , Takacs A. , Muller A. A. , Hartnagel H. , Plana R. , Grenier K. , Dubuc D.
Published: March 2007

In this letter the authors analyze two configurations of nanoelectromechanical switches, which are working in the microwave and millimeter wave frequency ranges. Metallic nanotubes vertically or horizontally positioned on a propagating high frequency planar waveguide disable or enable the electromagnetic field propagation, whether they are actuated or not by a dc electric field of a few volts. Although the dimensions of metallic nanotubes are very small in comparison with the planar waveguide dimensions and the operating wavelengths, these switches are very efficient and agile.