bluuebunny/crossref_metadata_2025 huggingface dataset:
In Earth's Magnetic Pas de Deux, the Inner Core Leads
Authors: Ladbury Ray
Published: January 2008

The phrase “geologic time” conjures images of continents moving at centimeters per year, or of mountains forming and eroding over hundreds of millions of years. Yet, some geologic processes evolve at a rate that allows significant changes to take place on the scale of a human lifetime. Earth's magnetic field changes on a time scale of decades—presumably mirroring changes in the planet's electrically conducting, fluid outer core, the motion of which is thought to generate and sustain the geomagnetic field. Last year, Gary Glatzmaier (Los Alamos National Laboratory) and Paul Roberts (University of California, Los Angeles) generated the most realistic model yet of this “geodynamo,” and predicted that the rotation rate of Earth's inner core evolves on a scale of centuries rather than eons. Moreover, in their simulation the magnetic field and the flow of the fluid outer core caused the solid inner core nearly always to spin more rapidly than the mantle and crust—the result of magnetic coupling between the inner core and a jet-stream–like layer of rapidly rotating outer-core fluid at the inner-core boundary (See PHYSICS TODAY, January, page 17).
One-Dimensional Systems Show Signs of Interacting Electrons
Authors: Levi Barbara Goss
Published: January 2008

Conventional metals have been very successfully described by the Fermi-liquid theory, but researchers are now studying a variety of more exotic materials for which other approaches might be more appropriate. Among the alternatives is a model of interacting electrons in one dimension. The model is known as the Tomonaga-Luttinger liquid theory, after Joaquin Luttinger, who formulated it in the 1960s, and Sin-itiro Tbmonaga, who did influential one-dimensional studies in the 1950s. It is frequently called simply a Luttinger liquid.
Role of magnetostatic interactions in micromagnetic structure of multiferroics
Authors: Armstrong Jason N. , Sullivan M. R. , Le Romancer Maëlane , Chernenko Volodymyr A. , Chopra Harsh Deep
Published: January 2008

While it is well known that magnetoelastic coupling governs the magnitude of field-induced strain in magnetic shape memory alloys, the present study shows that the zero-field micromagnetic structure and the pathway leading to the field-induced strain is governed by magnetostatic coupling across martensite twins. The micromagnetic investigations reveal a new energy barrier to the motion of domain walls arising from magnetostatic coupling between walls across the twin planes.
The effective interdiffusivity, structure, and magnetic properties of [Fe∕Pt]n multilayer films
Authors: Yao Bo , Coffey Kevin R.
Published: January 2008

This paper reports the influence of the deposition temperature and the wavelength of the periodicity on the effective interdiffusivity, microstructure, and magnetic properties of annealed [Fe∕Pt]n multilayer films (MLs). It was found that both the deposition temperature and periodicity of [Fe∕Pt]n MLs have a significant influence on the effective Fe–Pt interdiffusivity, microstructure, and magnetic properties of the annealed films. It was also observed that the magnitude of the effective interdiffusion coefficient, the L10 grain size, and the long-range order parameter were positively correlated. This result suggests that nonequilibrium reaction kinetics are desirable for the reduced temperature formation of the L10 FePt phase.
Effective field investigation in arrays of polycrystalline ferromagnetic nanowires
Authors: Hernández Eduardo Padrón , Rezende S. M. , Azevedo A.
Published: January 2008

Nanowire arrays have been used as prototypes to investigate basic issues such as size effect, shape anisotropy, and dipolar interaction on the magnetic properties. Under ideal conditions, the nanowires are approximated as perfect long cylinders. Here, coherent rotation as the magnetization reversal mode cannot completely interpret the experimental data. The internal magnetic field value, in nanowire arrays, decreases due to the wire inhomogeneities and the dipolar interaction between the wires. Realistic models must account for the imperfections due to the fabrication process. Instead of it, in this work, a modified ellipsoid-chain array model is proposed to describe magnetization reversal in nanowire arrays. From the angular dependence of the ferromagnetic resonance field presented previously in the literature and from our proper results here, we present experimental confirmations to the model.
Impact of the transient formation of molecular hydrogen on the microcrack nucleation and evolution in H-implanted Si (001)
Authors: Personnic S. , Bourdelle K. K. , Letertre F. , Tauzin A. , Cherkashin N. , Claverie A. , Fortunier R. , Klocker H.
Published: January 2008

We study the implant-induced hydrogenated defects responsible for the Smart Cut™ layer transfer of Si (001) films. Different experimental methods are used to quantify the time dependence of the defect evolution and interactions during isothermal annealings. An optical characterization technique was developed for the statistical analysis of the formation and growth of micrometer size microcracks in the buried implanted layer. We show that the formation of molecular hydrogen is dominated by a transient phenomenon related to the rapid dissociation of the hydrogenated point defects. The impact of the H2 formation kinetics on the microcrack evolution is described and the physical mechanisms involved in their growth are identified. A comprehensive picture of the fracture phenomenon in H implanted Si leading to the full layer transfer is proposed and discussed.
Effects of annealing on the structure and magnetic properties of Fe27Co23Pb50 nanowire arrays
Authors: Wang R. L. , Tang S. L. , Shi Y. G. , Fei X. L. , Nie B. , Du Y. W.
Published: January 2008

Ferromagnetic-nonmagnetic heterogeneous Fe27Co23Pb50 ternary metal nanowire arrays were successfully fabricated by alternating current electrodeposition into anodic alumina oxide (AAO) template. The effects of the different annealing temperatures (100, 200, 300, 400, 500, 600°C) on the structure and magnetic properties have been discussed. X-ray diffraction observations indicated that FeCo and Pb phases coexist for the as-deposited and annealed samples. Magnetic measurements indicate that the nanowire arrays have high perpendicular magnetic anisotropy with their easy axis parallel to the nanowire arrays. The coercivity and remanence ratio increases as the annealing temperature rises, reaches their maximum at 400°C, and then decreases as the annealing temperature rises further. The mechanism of the magnetic properties and magnetic variety should be attributed to the special structure of the nanowires∕AAO.
Structural and magnetoresistive properties of magnetic tunnel junctions with half-metallic Co2MnAl
Authors: Qiu J. J. , Han G. C. , Yeo W. K. , Luo P. , Guo Z. B. , Osipowicz T.
Published: January 2008

A series of polycrystalline full-Heusler Co2MnAl thin films were deposited on Si (100) coated with thermo SiO2 by using different types of seed layers such as Cr, Mg, MgO∕Cr, and MgO. The properties of the Co2MnAl thin films such as the coercivity, atomic composition, and crystalline structure strongly depend on the deposition conditions and seed layers. Very soft Co2MnAl thin films with coercivity of 8.3Oe and small magnetostriction coefficient λS=1.43×10−5 had been obtained when MgO was used as seed layer. Magnetic tunnel junctions with magnetoresistance ratio of 12%–19% by utilizing the Co2MnAl as bottom ferromagnetic electrode have been successfully fabricated.
Reduction of the unbalanced magnetic force of a transverse flux machine by using symmetric multipair cores
Authors: Jang G. H. , Park N. K. , Lee C. I. , Chang J. H. , Jeong S. W. , Kang D. H.
Published: January 2008

This paper investigates the characteristics of the magnetic force and the torque in the conventional rotatory two-phase transverse flux machine (TFM) by using the three-dimensional finite element method. This research shows that the unbalanced magnetic force is one of the dominant excitation forces in this machine, and it proposes a TFM with symmetric multipair cores in which each stator core of phases A and B is divided into two and the divided cores are disposed symmetrically to cancel the unbalanced magnetic force of each phase of a TFM. However, symmetric multipair cores of a TFM may reduce the winding space of coil which results in the reduction of torque and power. This research performs the optimization of teeth-slot configuration of the stator to overcome this shortcoming. It shows that the unbalance magnetic force of a TFM can be effectively eliminated without sacrificing torque or power by introducing symmetric multipair cores.
A model of the temperature dependence of exchange bias in coupled ferromagnetic∕antiferromagnetic bilayers
Authors: Craig B. , Lamberton R. , Johnston A. , Nowak U. , Chantrell R. W. , O’Grady K.
Published: January 2008

A granular level model of the magnetic properties of coupled ferromagnetic∕antiferromagnetic layers is used to calculate the temperature dependence of the exchange bias. The predicted results are in good qualitative agreement with experiment. Agreement with experiment requires the introduction of the temperature dependence of the anisotropy constant of the antiferromagnetic layer.
Measurement of residual stress of magnetic substance by surface acoustic waves
Authors: Osada Hiroshi , Yamaguchi Takumi , Nakamura Satoshi , Hatafuku Hiroshi , Chiba Shigeki , Oka Hideo
Published: January 2008

In magnetic substances under oscillating stress due to the application of acoustic waves (such as ultrasonic waves), an oscillating anisotropic field may be generated by the inverse effect of magnetostriction. If this magnetic field oscillation can be detected, the stress state inside the magnetic substance can be determined. Based upon this principle, we have set up an apparatus for stress measurements, applied it to Ni plates with different residual stresses, and have obtained output signals corresponding to different stresses. This measuring technique makes it possible to know the stress state in structures before the formation of cracks, and hence offers a very promising nondestructive testing device.
Growth and magnetic and electrical-transport properties of NiAs structured Mn1−xGaxAs thin films
Authors: Song J. H. , Cui Y. , Ketterson J. B.
Published: January 2008

We have successfully fabricated epitaxial Mn1−xGaxAs thin films with the NiAs crystal structure on GaAs(001) substrates by molecular-beam epitaxy characterized by their magnetic and electrical-transport properties. For a sample with x=0.17, the magnetic Curie temperature (TC) was ∼340K, which exceeds room temperature, and a high anisotropy in the M-H hysteresis loops was observed. Furthermore, the temperature dependence of the resistance shows a semiconducting behavior with an indication of an interplay between the charge carriers and localized magnetic ions. The magnetoresistance is different from that of a MnAs film, indicating a radical alteration of electronic structure by a partial substitution of Ga for Mn in NiAs structured MnAs.
Magnetic and magnetocaloric properties on the U1−yRyGa2 (R=Er and Dy) compound
Authors: da Silva L. M. , dos Santos A. O. , Gandra F. G. , Cardoso L. P. , Gama S.
Published: January 2008

The magnetic, calorimetric, and magnetocaloric properties of the pseudobinary U1−yRyGa2 (R=Er and Dy) series were studied to determine its potential as a candidate for use in cryogenic magnetic refrigeration. The partial substitution of Dy and Er for U provides a wide range of the ordering temperature and increases the saturation magnetic moment. The results for U1−yDyyGa2 with 0.6<y<0.9 show evidences of a spin-glass-like (SG) behavior, possibly as a consequence of competing anisotropy and exchange interactions within a frustrated hexagonal spin lattice. The isothermal magnetic entropy change (ΔSmag) observed for UGa2 shows a well defined peak centered on TC, which is gradually broadened and shifted to lower temperatures as the Er and Dy content increases. For low concentrations (0.2⩽y⩽0.4) a tablelike profile is observed in the ΔSmag curve.
Electronic transport in La1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 under high electric fields
Authors: Fisher B. , Genossar J. , Knizhnik A. , Patlagan L. , Reisner G. M.
Published: January 2008

We report high-field conductivity measurements of polycrystalline samples of La1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 at various temperatures in the stripe ordered state. Since their discovery, an important issue has been the response of the stripes to high electric fields. The main result of this work is that while the pulsed I−V characteristics, using single short current pulses, are ohmic for Nd1.67Sr0.33NiO4 up to fields of ∼550 V/cm and ohmic or only slightly non-ohmic for La1.67Sr0.33NiO4 in fields of the same order of magnitude, the corresponding dc I−V characteristics are strongly nonlinear. The conditions for four-probe pulsed I−V measurements that avoid self-heating were determined experimentally. The low-field resistivity and the thermopower of our samples are in satisfactory agreement with those reported in the literature for polycrystals and for single crystals of these materials.
SiC and carbon nanotube distinctive effects on the superconducting properties of bulk MgB2
Authors: Serrano G. , Serquis A. , Dou S. X. , Soltanian S. , Civale L. , Maiorov B. , Holesinger T. G. , Balakirev F. , Jaime M.
Published: January 2008

This work describes in detail the simultaneous enhancement of the upper critical field (Hc2) and the critical current density (Jc) of MgB2 bulk samples doped with nano-SiC particles, as well as single-walled and double-walled (dw) carbon nanotubes (CNTs). The magnetization properties were examined in a superconducting quantum interference device magnetometer, and four-probe transport measurements were performed using a 50T pulsed magnet to determine Hc2(T). We found that the Jc enhancement is similar in all doped samples at 5K but nano-SiC addition is more effective to improve the flux pinning in the high temperature range (T⩾20K); this improvement cannot solely be attributed to the C incorporation to the lattice but also to the presence of other types of defects (i.e., several kinds of nanoinclusions). CNTs produce a better C incorporation that is more effective to enhance Hc2 [i.e., dwCNT-doped samples reached a record Hc2(0)∼44T value for bulk MgB2]. All the Hc2(T) curves obtained for different types of doping can be successfully described using a model for a two-gap superconductor in the dirty limit.
Nonlocal continuum theories of beams for the analysis of carbon nanotubes
Authors: Reddy J. N. , Pang S. D.
Published: January 2008

The equations of motion of the Euler–Bernoulli and Timoshenko beam theories are reformulated using the nonlocal differential constitutive relations of Eringen [International Journal of Engineering Science 10, 1–16 (1972)]. The equations of motion are then used to evaluate the static bending, vibration, and buckling responses of beams with various boundary conditions. Numerical results are presented using the nonlocal theories to bring out the effect of the nonlocal behavior on deflections, buckling loads, and natural frequencies of carbon nanotubes.
Duality Demonstrated on D-Branes
Authors: Levi Barbara Goss
Published: January 2008

Juan Maldacena made clear the duality between two very different types of theories by formulating them both in terms of strange topological structures Known as D-branes. D-branes are a subset of a larger class of objects known as p-branes (p is the object's spatial dimension), which are distortions of spacetime geometry, rather like dislocations in a crystal. If p = 2, the p-brane is an ordinary two-dimensional sheet, or membrane; if p = 1, it's a string and if p = 0, a point particle.
Color-converting combinations of nanocrystal emitters for warm-white light generation with high color rendering index
Authors: Nizamoglu Sedat , Zengin Gulis , Demir Hilmi Volkan
Published: January 2008

Warm-white light emitting diodes with high color rendering indices are required for the widespread use of solid state lighting especially indoors. To meet these requirements, we propose and demonstrate warm-white hybrid light sources that incorporate the right color-converting combinations of CdSe∕ZnS core-shell nanocrystals hybridized on InGaN∕GaN LEDs for high color rendering index. Three sets of proof-of-concept devices are developed to generate high-quality warm-white light with (1) tristimulus coordinates (x,y)=(0.37,0.30), luminous efficacy (LE)=307lm∕W, color rending index (CR)=82.4, and correlated color temperature (CCT)=3228K; (2) (x,y)=(0.38,0.31), LE=323lm∕W, CRI=81.0, and CCT=3190K; and (3) (x,y)=(0.37,0.30), LE=303lm∕W, CRI=79.6, and CCT=1982K.
Role of metallic cobalt in room temperature dilute ferromagnetic semiconductor Zn0.95Co0.05O1−δ
Authors: Liu Q. , Gan C. L. , Yuan C. L. , Han G. C.
Published: January 2008

Using two suitably designed thin film systems, the impact of metallic cobalt nanodots on the mechanism of ferromagnetism (FM) in ZnO thin films was studied. At a relatively higher oxygen partial pressure, Co nanodots embedded ZnO thin films no longer show FM, as compared to Co-doped ZnO dilute magnetic semiconductor (DMS). The structural and magnetic properties of these two systems support the hypothesis that (1) the FM of DMS thin films is due to bound magnetic polarons instead of cobalt nanoclusters and (2) the critical defect concentration is the key parameter which controls the FM properties in DMS thin films.
The Physics of Granular Materials
Authors: Jaeger Heinrich M. , Nagel Sidney R. , Behringer Robert P.
Published: January 2008

Victor Hugo suggested the possibility that patterns created by the movement of grains of sand are in no small part responsible for the shape and feel of the natural world we live in. Certainly, granular materials, of which sand is but one example, are ubiquitous in our daily lives. They play an important role in industries, such as mining, agriculture and construction. They also are important in geological processes, such as landslides and erosion and, on a larger scale, plate tectonics, which determine much of Earth's morphology. Practically everything we eat started out in a granular form and the clutter on our desks is often so close to the angle of repose that a chance perturbation can create an avalanche onto the floor.
Nuclear Contamination from Weapons Complexes in the Former Soviet Union and the United States
Authors: Bradley Don J. , Frank Clyde W. , Mikerin Yevgeny
Published: January 2008

Over a 50-year period, the Soviet Union and the United States developed the largest nuclear weapons complexes in the world. In doing so, they also created the world's largest inventories of radioactive waste. Although some of the waste has been stored in safely managed systems such as tanks or converted into stable and storable forms such as glass, significant amounts of it have been released into the environment. This article focuses primarily on these weapons-related discharges because of their size and less-well-known nature.
From Newton's Moon to Einstein's Moon
Authors: Nordtvedt Kenneth
Published: January 2008

Three centuries ago Isaac Newton and his contemporaries sought to understand the details of lunar motion in terms of Newton's theory of gravity. Today, our most precise confirmations of key aspects of Albert Einstein's relativistic theory of gravity are achieved by measuring features of the Moon's orbit to 1-centimeter precision using a technique known as lunar laser ranging (LLR). These present-day measurements address some of the same questions posed by Newton. Do Earth and the Moon (figure 1) fall toward the Sun at the same rate? What, in detail, produces the precession of the lunar orbit's major axis?
Charge-Density-Wave Conductors
Authors: Thorne Robert E.
Published: January 2008

When metals are cooled, they often undergo a phase transition to a state exhibiting a new type of order. Metals such as iron and nickel become ferromagnetic below temperatures of several hundred degrees Celsius; electron spins order to produce a net magnetization in zero field. Other metals, such as lead and aluminum, become superconductors at cryogenic temperatures; electrons form Cooper pairs of opposite spin and momentum, leading to electrical conduction with zero resistance and to expulsion of magnetic fields.
Build the International Thermonuclear Experimental Reactor?
Authors: Sessler Andrew M. , Stix Thomas H. , Rosenbluth Marshall N.
Published: January 2008

Controlled fusion represents an opportunity to replace fossil fuels and nuclear fission as energy sources. Intensive research aimed at the peaceful utilization of thermonuclear energy, now in its 46th year, has benefited enormously from vigorous international scientific exchange and collaboration. Much has already been accomplished. In the approach to controlled fusion that uses magnetic confinement—the subject of this opinion piece—plasma temperatures obtained in experimental devices have risen from a few electron volts to over 40 keV; energy confinement times have stretched from tens of microseconds to over a second; total nuclear energy output has gone from zero to a few megajoules, amounting to about 30% of the externally supplied plasma-heating energy.
Quantum Computing: Dream or Nightmare?
Authors: Haroche Serge , Raimond Jean-Michel
Published: January 2008

The principles of quantum computing were laid out about 15 years ago by computer scientists applying the superposition principle of quantum mechanics to computer operation. Quantum computing has recently become a hot topic in physics, with the recognition that a two-level system can be presented as a quantum bit, or “qubit,” and that an interaction between such systems could lead to the building of quantum gates obeying nonclassical logic. (See PHYSICS TODAY, October 1995, page 24 and March 1996, page 21.)
How Things Break
Authors: Marder Michael , Fineberg Jay
Published: January 2008

Galileo Galilei was almost seventy years old, his life nearly shattered by a trial for heresy before the Inquisition, when he retired in 1633 to his villa near Florence to construct the Dialogues Concerning ‘Two New Sciences. His first science was the study of the forces that hold objects together and the conditions that cause them to fall apart—the dialogue taking place in a shipyard, triggered by observations of craftsmen building the Venetian fleet. His second science concerned local motions—laws governing the movement of projectiles.
Carbon Nanotubes
Authors: Ebbesen Thomas W.
Published: January 2008

Carbon is an extraordinary element, considering the diversity of materials it forms. Ranging from sparkling gems to sooty filth, these materials have been studied and used for centuries, and carbon science was long thought to be a mature field. So when a whole new class of carbon materials—the fullerenes, such as C60—appeared in the last decade, many scientists were surprised. The consequences have reached well beyond the fullerenes themselves to include major changes in our concepts and understanding of long-known carbon materials. It is in this context that the story of carbon nanotubes starts.
Duality, Spacetime and Quantum Mechanics
Authors: Witten Edward
Published: January 2008

The purpose of this article is to describe some themes in theoretical physics that developed independently for many years, in some cases for decades, and then converged rather suddenly beginning around 1994–95. The convergence produced an upheaval sometimes called “the second superstring revolution.” It is as significant in its own way as “the first superstring revolution,” the period around 1984–85 when the potential of string theory to give a unified description of natural law was first widely appreciated.
Stretch Genes
Authors: Austin Robert H. , Brody James P. , Cox Edward C. , Duke Thomas , Volkmuth Wayne
Published: January 2008

Polymers—macromolecules composed of many small molecules—are vital to life. For example, proteins provide the cell with structure and carry out its mechanical and enzymatic work. Fatty acids store energy in their bonds, and lipids form the membranes that seal off the cell's aqueous interior from its aqueous exterior. Heredity lies in the very long polymeric strands of deoxyribonucleic acid—DNA.
Time Reversed Acoustics
Authors: Fink Mathias
Published: January 2008

The irreversibility of time is a topic generally associated with fundamental physics. Imaging techniques, on the other hand, are usually related to engineering sciences. Remarkably, however, these two domains can be fruitfully connected to yield many applications in medicine, undersea communications, hydrodynamics and material analysis, as well as elegant experiments in pure physics.
Probing the Faintest Galaxies
Authors: Ferguson Henry C. , Williams Robert E. , Cowie Lennox L.
Published: January 2008

The concept of a galaxy as a vast organized system of stars originates with Galileo's Observation in 1610 that the Milky why cound be resolved into “innumerable” faint stars. Confirmation that many of the faint patches of light seen through small telescopes were actually galaxies like our own came in 1923, when Edwin Hubble, working with the just-completed 2.4-meter telescope at Mount Wilson in southern California, measured the brightnesses of individual stars in the Andromeda nebula. At about the same time, Vesto Slipher, working at the Lowell Observatory in Arizona, was making the first systematic measurements of the Doppler shifts of galaxies. Of several dozen galaxies measured, all but two were found to be receding. In 1929, Hubble showed that the recession velocities of Slipher's galaxies were directly proportional to their distances, thus introducing the concept of an expanding universe
Can Phonons Squeeze their Way Into the Company of Photons?
Authors: Levi Barbara Goss
Published: January 2008

Twelve years ago, Richart Slusher and his colleagues at AT&T Bell Labs produced light whose noise was below the vacuum quantum fluctuations, at least in part of the signal. Since then, researchers have been trying to squeeze the uncertainties out of other systems as well. So far they have succeeded in quieting a classical mechanical oscillator and both classical and nonclassical states of a vibrating, trapped ion. Now comes a report of squeezed phonons: By striking a crystal with a femtosecond laser pulse, a group at the University of Michigan believes it has excited an acoustic mode whose variance falls below the standard quantum limit. So far, the noise has been reduced by only 0.01% (the earliest experiments on optical squeezing yielded 20%), but just the concept of squeezed phonons has intrigued many observers.
Radioactive Waste: The Size of the Problem
Authors: Ahearne John F.
Published: January 2008

Exposure to radioactivity is not a new phenomenon: Jewelry workers painting luminous dials on watches in the 1920s were exposed to radium, a naturally occurring radioactive element, until its dangers were identified; in recent decades, homeowners have worried about radon, a naturally occurring radioactive gas that can adversely affect both their health and the resale value of their homes. Today, we must worry as well about the enormous quantities of anthropogenic radioactive materials that have accumulated as waste in this country and worldwide since the 1940s, largely because of the nuclear weapons programs and nuclear power plants. Some of the anthropogenic material is still valuable—for example, as fuel for nuclear reactors. Vast amounts of it, however, consist of waste. Radioactive waste, by definition, is unneeded material that contains unstable elements that decay by emitting alpha, beta or gamma emissions. This article describes the sources of this waste, the types of waste that are of concern and the amounts estimated to be present in the US.
Earth's Upper Mantle: How Low Can It Flow?
Authors: Levi Barbara Goss
Published: January 2008

For years, a debate has raged among Earth scientists about the convection within our planet's thick mantle. Most researchers agree that the 3000 km thick mantle is divided into an upper region and a lower region by an interface at a depth of about 660 km where the mantle materials undergo an isochemical phase change. But they differ as to whether this seismic discontinuity is a barrier to convection.
Quantum State Reconstruction of Squeezed Light
Authors: Collins Graham P.
Published: January 2008

These five images are probability distributions for quantum states of light, as measured by a group at the University of Konstanz in Germany. The top distribution is a coherent state and the other four are a variety of squeezed states.
How Nature Harvests Sunlight
Authors: Hu Xiche , Schulten Klaus
Published: January 2008

It is through photosynthesis that Earth's biosphere derives its energy from sunlight. Photosynthetic organisms—plants, algae and photosynthetic bacteria—have developed efficient systems to harvest the light of the Sun and to use its energy to drive their metabolic reactions, such as the reduction of carbon dioxide to sugar. The ubiquitous green color of plants is testimony to the key molecular participant in the light harvesting of plants, chlorophyll. More hidden in this respect, but no less widespread, is a second participating molecule, carotenoid. In green leaves, the color of the carotenoids is masked by the much more abundant chlorophylls, whereas in ripe tomatoes or the petals of yellow flowers, the carotenoids predominate. Chlorophyll molecules exist in slightly different chemical structures in various photosynthetic organisms, as chlorophyll a or b in plants or algae, and as bacteriochlorophyll a or b in photosynthetic bacteria. Molecules such as chlorophyll and carotenoid that absorb light and impart color to living matter and other materials are called pigments.
Top-ology
Authors: Quigg Chris
Published: January 2008

The top quark os a most remarkable Particle even for a quark. A single “top,” as we will casually call it here, weighs 175 GeV, about as much as an atom of gold. But unlike the gold atom, which can be disassembled into 79 protons, 79 electrons and 118 neutrons, top seems to be indivisible. With an experimental resolution approaching 10−16 cm, we have not discerned any structure. Top's predicted lifetime of about 0.4 yoctoseconds (0.4×10−24 s) makes it by far the most ephemeral of the six varieties of quarks. The compensation for this exceedingly brief life is a measure of freedom: Top decays before it suffers the confining influence of the strong interactions. In spite of its fleeting existence, the top quark nonetheless helps shape the character of the everyday world.
Experiments Find Hysteresis and Precursors in the Stick-Slip Friction of a Granular System
Authors: Lubkin Gloria B.
Published: January 2008

What happens to a granular layer as you increase an applied shear force? In a recent experiment at Haverford College, Satoru Nasuno, Arshad Kudrolli and Jerry Gollub were able to image the particle dynamics directly. Their paper, published in the 4 August issue of Physical Review Letters, describes a stick-slip behavior at low velocity gradients across the layer, with a transition to continuous motion at higher velocity gradients. Furthermore, within individual slip events, they found that the frictional force is a multivalued function of the velocity.
Superconductor-Insulator Transitions in the Two-Dimensional Limit
Authors: Goldman Allen M. , Marković Nina
Published: January 2008

The investigation of superconductivity in the presence of disorder began 60 years ago with the work of Alexander Shal'nikov at the Institute for Physical Problems in Moscow. The subject has played an ongoing role in condensed matter physics over the years. Interest has recently been heightened by the possibility that the disorder-driven or magnetic-field-driven quenching of superconductivity in systems at the limit of zero temperature and two dimensions might be quantum phase transitions. That would link the physics of the superconductor-insulator transition in thin films to other systems believed to exhibit quantum phase transitions—for example, helium-4 in porous media, high temperature superconductors, Josephson-junction arrays, two-dimensional electron gases and various spin systems.
Adrift on the Ice Pack, Researchers Explore Changes in the Arctic Environment
Authors: Levi Barbara Goss
Published: January 2008

It was a relatively warm winter in the western Arctic this year; the lows were only -42°C. That was still chilly enough to sting the fingers and toes of the more than 150 scientists participating in the SHEBA project—a multiprong, yearlong international study of polar environmental conditions whose data-gathering phase ended this October. (SHEBA stands for Surface Heat Budget of the Arctic Ocean.) The base of operations for the project was a Canadian icebreaker named Dea Groseilliers that was intentionally frozen into a region of multiyear ice and set to drift with the ice pack. With the ship serving as a hotel (featuring, we're told, gourmet meals), power source, communications base and repair facility, researchers fanned out over the ice to deploy radiometers, launch balloons, poke instruments into the ice and even descend in diver's sear into the frigid ocean waters.
Exposing Life's Limits with Dimensionless Numbers
Authors: Vogel Steven
Published: January 2008

The impressive performance of evolution as a design mechanism needs no belaboring. Physics, though, constitutes a larger reality that evolution can no more transcend than a cow can jump upward at escape velocity. Enzymes cannot act as Maxwellian demons, nor can birds turn off gravity. Physics limits life's designs no less rigidly than it constrains our own technology.
When Liquids Stay Dry
Authors: Dell'Aversana Pasquale , Neitzel G. Paul
Published: January 2008

If you are in the habit of watching your drip coffee maker prepare your morning beverage, you'll occasionally witness something interesting: As the coffee drips into the pot, you'll often see one of the drops land on the surface and remain intact for a second or so before coalescing with the bulk liquid. This temporary noncoalescence of two bodies of the same liquid is neither an isolated phenomenon nor one that has been observed only in the era of drip coffee makers. As early as 1879, Lord Rayleigh examined how water jets bounce over each other, and in 1881 Osborne Reynolds observed what he called floating drops. (See the box on page 40.) A century later, “a scientific curiosity” was how Jearl Walker referred to the phenomenon.
Ultrahigh-Energy Sound Waves Promise New Technologies
Authors: Ladbury Ray
Published: January 2008

Perhaps because we are constantly bombarded by sound, it is easy to forget that sound waves actually represent quite small pressure variations. The sound of a jet engine a few meters away measures only about 20 Pa (about 0.0002 atmospheres). As one increases the energy going into a sound wave, nonlinear processes in the gas in which the wave propagates direct more and more energy into harmonics of the drive frequency. The harmonics distort the sound wave and ultimately form shock waves. It is these shocks that limit the amplitudes attainable. Soundwaves’ low energy levels and compression ratios (defined as the ratio of the waveform's peak and minimum pressures) have limited their usefulness in high-power applications such as compressors and pumps. Accordingly, many researchers have wondered whether the acoustic saturation imposed by shock formation can be circumvented in some special circumstances.
Inventing US Science Policy
Authors: Blanpied William A.
Published: January 2008

Throughout 1995, speeches, articles and symposiums celebrated the 50th anniversary of Science—The Endless Frontier, known more familiarly as the Bush Report, in honor of Vannevar Bush, its principal author. While all the commentators recognized the report as the centerpiece of post-World War II's science policy, some went on to express regret about its legacy for our present time: Bush's strategy was mapped in the heat of war and served as a battle-cry during the cold war, but it is likely to be inadequate as a vigorous defense of scientific research in the 21st century.
Stellar Motion Very Near the Milky Way's Central Black Hole
Authors: Schwarzschild Bertram
Published: January 2008

The extraordinary pair of images at right are near-infrared exposures, taken two years apart, of stars in the central 3×3-arcsecond field of our Galaxy. Andreas Eckart and colleagues at the Max Planck Institute for Extraterrestrial trial Physics in Garching, near Munich, have been monitoring the proper and Doppler motions of these stars since 1992 with spectacularly high-resolution infrared speckle photography and spectroscopy, using European Southern Observatory telescopes, in Chile, optimized for infrared observation. At visible wavelengths, the central precincts of the Galaxy are completely hidden by dust.
The Comprehensive Test Ban Treaty
Authors: Sullivan Jeremiah D.
Published: January 2008

The Comprehensive Test Ban Treaty awaits the advice and consent of the Senate for ratification by the US. The CTBT offers the US substantial security and political benefits by arresting the further development of nuclear weapons by the nuclear powers and by uniting the US and the vast majority of other states in an effective global regime against nuclear proliferation.
Quantum Theory without Observers—Part Two
Authors: Goldstein Sheldon
Published: January 2008

Albert Einstein believed in the possibility of a quantum theory without observers—a version of quan tum theory for which the notions of measurement, observation and observer are not invoked in its very formulation, but rather emerge from an analysis of more fundamental concepts. Niels Bohr believed that such a theory was “in principle” impossible. In part one of this article, I described one approach to such a theory, that of decoherent histories (DH). Although much progress has been made, it could be argued that this approach has not yet yielded a theory that is sufficiently well defined to provide decisive support for Einstein's view. The theories I discuss in this final part of the article are completely well defined and hence provide a conclusive refutation of Bohr's impossibility claims.
Are Stripes a Universal Feature of High-Tc Superconductors?
Authors: Levi Barbara Goss
Published: January 2008

As if the superconducting copper oxides weren't mysterious enough, experiments within the past decade have revealed a magnetic structure—at least in members of the lanthanum strontium copper oxide family—whose period is different from that of the underlying lattice. One explanation that has attracted increasing attention is the possibility that stripe phases may form spontaneously when experimenters dope these high critical-temperature (Tc) materials. According to this postulate, the added charges line up in rows in the copper oxide plane, sandwiching between them regions of copper atoms whose spins are aligned antiferromagnetically, with nearest neighbors having opposite spins. The repeat pattern of these spin regions is generally different from that of the crystal lattice, thus explaining the incommensurate peaks seen in diffraction studies.
The Hipparcos Astrometry Mission
Authors: Perryman Michael
Published: January 2008

Much of what we know about the universe has come from astrometry, the precise measurement of star positions. Astrometry has been an important scientific tool for more than two millenia. The name of the European Space Agency's High Precision Parallax Collecting Satellite—Hipparcos for short—honors the great prechristian astrometer Hipparchus of Nicea. Tycho Brahe, the last great naked-eye astrometer, died just nine years before Galileo first turned his telescope to the heavens.
Strong Evidence for Flavor Oscillation in Atmospheric Neutrinos
Authors: Schwarzschild Bertram
Published: January 2008

As we go to press, the applause has barely subsided for a 5 June talk by Takaaki Kajita at the Neutrino ‘98 conference in Takayama in the Japanese Alps, not far from the 50-kiloton Super Kamiokande underground water-Cerenkov neutrino detector. Speaking for the Japanese-American Super Kamiokande collaboration, Kajita reported by far the most convincing evidence to date of oscillation between different neutrino flavors. The evidence comes from the high-statistics observation of neutrinos created by cosmic-ray showers in the atmosphere. It appears that muon neutrinos on their way to the detector are exhibiting oscillatory metamorphosis into something other than electron neutrinos. Neutrino oscillation cannot happen if all flavors are massless. Details in the next issue.
A Surface View of Etching
Authors: Boland John J. , Weaver John H.
Published: January 2008

Chemical etching has been practiced since at least the late Middle Ages. In its early form, it involved coating an object, such as a metal plate, with wax, carefully patterning the hardened wax by cutting down through it with a sharpened tool to expose but not penetrate the object's surface and then exposing the object to an etching solution, typically an acid. With time, the etchant molecules in the solution would react with atoms of the exposed surface to form reaction products that would dissolve, thereby removing material from the surface.
Self-Trapping of Optical Beams: Spatial Solitons
Authors: Segev Mordechai , Stegeman George
Published: January 2008

Although people have always been fascinated by visual manifestations of nonlinear wave phenomena, such as tsunamis and tidal waves, the first scientifically documented report of a self-trapped wave did not come until 1834, when a Scottish scientist, John S. Russell, observed a “rounded smooth and well defined heap of water” propagating in a narrow and shallow canal “without change of form or diminution of speed.” The water was calm on both sides of this unusual wave, and Russell noted that it had the form of a “solitary elevation.”
New Magnetic Superconductors: A Toy Box for Solid-State Physicists
Authors: Canfield Paul C. , Gammel Peter L. , Bishop David J.
Published: January 2008

Over the past decade, discussion of the interaction between superconductivity and magnetism has been overshadowed by the omnipresence of the oxide-based, high-temperature superconductors. But interest in the interaction between these two generally competing effects has a history that predates high-Tc materials by several decades. Starting with seminal work by Bernd Matthias and his coworkers, it was found that magnetic impurities strongly suppress superconductivity in pure elements and binary compounds. This rapid suppression of the superconducting transition temperature Tc was due to the local magnetic moment of the impurity preventing the formation of the spin-up/spin-down conduction-electron pairs that are responsible for superconductivity. The early measurements were made on dilute alloys, and for many solid-state physicists of the time, the quest was to find compounds in which superconductivity coexists with an ordered lattice of local magnetic moments.
Search at SLAC Finds No Millicharged Particles
Authors: Schwarzschild Bertram
Published: January 2008

The quantization of electric charge is very well tested: The proton and electron charges ±e are equal and opposite to better than a part in 1021, and the neutrality of the neutron, photon and neutrino have been experimentally affirmed with comparable precision. And all attempts to find naked fractionally charged quarks have failed thus far, as the theory of quark confinement tells us they must.
Everyone's Magnetism
Authors: Geim Andrey
Published: January 2008

If you were to tell a child playing with a horseshoe magnet and pieces of iron that his uncle has a much bigger magnet that can lift everything and everybody, the child would probably believe you and might even ask for a ride on the magnet. If a physicist were present at such a conversation, he or she—armed with knowledge and experience—would probably smile condescendingly. The physicist would know well that only a very few materials—such as iron or nickel—are strongly magnetic, while the rest of the world's materials are not; to be precise, the rest of the world is a billion (109) times less magnetic. This number seems obviously too large to allow common substances (water, for example) to be lifted even by the most powerful magnets; a billionfold increase in magnetic fields can be found only on neutron stars. In this case, however, knowledge and experience would mislead the physicist: In fact, all materials can be lifted by using magnetic fields that are rather standard these days. In principle, even a child can be levitated by a magnet, as we shall see below.
Physics Nobel Prize Goes to Tsui, Stormer and Laughlin for the Fractional Quantum  Hall Effect
Authors: Schwarzschild Bertram
Published: January 2008

This year's Nobel Prize in Physics is shared by Robert Laughlin (Stanford), Horst Stormer (Columbia University and Bell Laboratories) and Daniel Tsui (Princeton), for their roles in the discovery and explanation of the fractional quantum Hall effect. In 1982, when Stormer and Tsui were experimenters at Bell Labs, they and their colleague Arthur Gossard discovered this totally unexpected quantum effect in the transport properties of two-dimensional electron gases at low temperature in strong magnetic fields.’ (See PHYSICS TODAY, July 1983, page 19.)
Experiments and Simulations Track Crack Propagation in Silicon Wafers
Authors: Day Charles
Published: January 2008

Modeling a piece of silicon as it cracks and breaks is doubly difficult. Not only are huge numbers of atoms involved, but the whole process comes down to the rapid snapping of complicated covalent bonds. Compounding the difficulty, a fully prescriptive quantum mechanical treatment of crack propagation in bulk material, though possible in principle, will remain for some time beyond the number-crunching power of even the largest supercomputers.
Nobelium-254 Forms High-Spin Rotational States
Authors: Lubkin Gloria B.
Published: January 2008

Why should elements beyond, say, atomic number Z = 100 exist at all? One might think that the Coulomb repulsion of so many protons ought to make such nuclei fission. But elements with a Z as high as 112 have been created, and they prefer to decay by alpha emission instead of fission. So it's widely believed that the relative stability of these high-Z nuclei is a striking example of nuclear shell structure, which lowers the energy of the ground state, thereby creating a fission barrier. Without the shell-correction energy, the heaviest nuclei wouldn't be stable.
New Views of Neutron Stars
Authors: Bildsten Lars , Strohmayer Tod
Published: January 2008

Born amid the violence of supernova explosions, neutron stars are the extremely dense endpoints of stars that initially had a mass of more than about 18–10 M⊙ (M⊙ is the mass of the Sun, about 2×1033 g). During its final collapse, such a star expels most of its material, leaving behind a remnant—a neutron star—packing a typical mass of 1.4 M⊙ into a radius of only 10 km. With a density comparable to that of an atomic nucleus, a neutron star provides an extreme environment for fast and violent phenomena: Matter orbiting a neutron star can have a period as short as a millisecond; when it crashes into the star (that is, when it is “accreted”), this matter can be moving at one-third the speed of light. Because their behavior can vary over readily observable time scales, neutron stars can be rich sources of information about nuclear physics, general relativity and astrophysics.