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Houston's structures thwart cleansing breezes

Posted: June 16th, 2011, 9:29am MDT by Physics Today

On 30 August 2000, as the sun beat down on Texas’s largest city, ozone concentrations soared to unhealthy levels. Usually in summer, as the city heats up, sea breezes blowing in from nearby Galveston Bay and the Gulf of Mexico refresh the air. But the prevailing winds over Houston, although mild, tend to counteract the sea breeze. Thus, if the breeze collides with the prevailing winds, stagnation sets in over the city and pollutants can build up. Now a numerical study led by Fei Chen of the National Center for Atmospheric Research suggests that the materials of the urban environment are partly to blame for ozone pollution. Chen and colleagues validated their computer model by comparing their simulation of the August 2000 pollution event against extensive data collected in the Texas Air Quality Study 2000. Then, to understand how various environmental features affect the development of the sea breeze, they simulated conditions that were wetter or dryer than normal, and in one simulation they replaced the urban landscape with cropland. The substitution of crops for concrete had the greatest impact on boosting the sea breeze and reducing periods of stagnation. (It also increased the efficacy of the nighttime land breeze that blows pollutants out to sea.) Compared with green space, the researchers found, the urban environment is hotter. That effect actually tends to enhance the sea breeze, but the enhancement is more than offset by the frictional damping from Houston’s buildings. (Fei Chen et al., J. Geophys. Res. [Atmospheres], in press, doi:10.1029/2010JD015533; image from J. Geophys. Res. [Atmospheres], in press, doi:10.1029/2010JD015533; [www.utexas.edu] .)—Steven K. Blau

Stirring superfluids

Posted: June 13th, 2011, 9:52am MDT by Physics Today

If you chill fermions enough, they can pair up to form bosons and settle into a single collective ground state, a Bose–Einstein condensate. In the case of helium-3 atoms, the resulting BEC is a superfluid that flows without dissipation—provided the flow is not so energetic that it breaks the pairs apart or destroys the ground state's coherence. Until now, theorists could characterize placid flows in fermionic superfluids, but not the vigorous turbulence that results from shaking or stirring. Aurel Bulgac of the University of Washington in Seattle and his colleagues have adapted density functional theory—a computational approach originally devised to calculate molecular energy levels—and applied its time-dependent extension to model turbulent fermionic superfluids. Although the underlying quantum mechanical equations are straightforward, solving them required the use of one of the world's most powerful supercomputers, Jaguar at Oak Ridge National Laboratory in Tennessee. In their simulations, Bulgac and his colleagues agitated a fermionic superfluid by shooting spherical projectiles through it or by stirring it with a laser beam. Turbulent superfluids are known to harbor tubes of quantized vorticity. As the figure below shows, the simulation could track how two vortex tubes (marked a and b) joined to form a ring, which then opens in a manner reminiscent of the unzipping of a DNA molecule during transcription. Bulgac's model could help astronomers understand another agitated superfluid: the interior of a rapidly spinning neutron star. (A. Bulgac et al., Science 332, 1288, 2011.)—Charles Day

Fluorescing diamonds inside living cells

Posted: June 9th, 2011, 9:50am MDT by Physics Today

The point defect in diamond known as the NV center—a nitrogen atom substituted for a carbon atom and adjacent lattice vacancy—has become a promising ingredient in recent efforts to develop atomic-scale sensors. When optically excited, the defect exhibits stable fluorescence, even in a crystal as small as 5 nm. And its ground state is magnetically sensitive—the spin 0 level is separated from degenerate spin ±1 levels by a microwave transition of 2.9 GHz. That sensitivity allows one to detect weak magnetic fields by observing the quantum spin state, which can be manipulated by microwave pulses and then read out optically by monitoring the fluorescence intensity. (The intensity depends on which of the three spin states is populated.) Researchers led by the University of Melbourne’s Lloyd Hollenberg have now performed such magnetic resonance experiments on individual nanodiamonds placed inside human cells. The quantum spin levels of the defects acted both as local magnetometers and as fingerprints to spectrally distinguish each nanodiamond in the complicated cell environment. Using a confocal microscope, the researchers were able to identify and track at nanometer precision individual NV centers in the cells from the optical emission (red). They were also able to measure the coherence time of the spin states; that work sets the stage for sensing the cells’ local magnetic fluctuations in response to, for example, the transport of charge through cell membrane ion channels, which are important drug targets. (L. P. McGuinness et al., Nat. Nanotechnol., in press, doi:10.1038/nnano.2011.64.)—R. Mark Wilson

Domain walls on the fast track

Posted: June 6th, 2011, 9:42am MDT by Physics Today

Magnetic moments don’t necessarily point in the same direction everywhere in a ferromagnet. More often, domains of different orientations coexist, separated by thin domain walls. Moving those walls with spin-polarized current is potentially a convenient way to write bits to magnetic random-access memory or to shuttle sequences of bits to and fro in three-dimensional memory devices. But such applications require that domain walls be moved quickly and with minimal current. Unfortunately, the materials best suited to yield such highly mobile domain walls are also the most susceptible to Walker breakdown, a turbulence-triggering instability that slows domain-wall speeds to a crawl. Now, researchers led by Gilles Gaudin and Ioan Mihai Miron of Spintec laboratory in Grenoble, France, have figured out a way around that problem. They crafted 500-nm-wide nanowires consisting of cobalt, the active ferromagnetic layer, sandwiched between platinum and aluminum oxide, as shown here. The resulting inversion asymmetry produces an out-of-plane electric field that gives rise to a fortuitous spin–orbit coupling: As electrons pass along a nanowire, their spins tend to tilt to one side, producing a magnetic torque that stabilizes the domain wall even at large current densities. Unconstrained by Walker breakdown, the domain walls reached speeds of up to 400 m/s, more than fast enough for memory applications. The researchers say they’ll now work toward achieving comparable speeds with less current. (I. M. Miron et al., Nat. Mater. 10, 419, 2011.)—Ashley G. Smart

High-spin early stars

Posted: June 2nd, 2011, 10:38am MDT by Physics Today

ESO

Stellar nucleosynthesis by exothermic fusion ends with iron, the most tightly bound of all nuclei. Stars produce heavier species by two neutron-capture mechanisms—the rapid “r-process” and the slow “s-process”—and expel them in supernovae or stellar winds. The r-process avails itself of the enormous neutron flux during the supernova to convey nuclei up the atomic-number scale by way of short-lived stages faster than those stages can decay back down. The s-process, making do with weaker neutron fluxes inside quiescent stars, requires long-lived intermediate stages. It was long thought that the s-process occurs only in stars of modest mass. But now Cristina Chiappini (Leibniz Institute for Astrophysics, Potsdam, Germany) and coworkers have reported anomalously high abundances of two s-process products—yttrium and strontium—in spectra of a cluster of Milky Way stars so old that their Y and Sr must have been created in an early generation of very massive stars that exploded less than a billion years after the Big Bang. Yet model calculations of the s-process in early massive stars yield far too little Y and Sr to account for the overabundances—unless those stars were spinning very fast. Chiappini and company suggest that first-generation stars were not only very massive, as is generally thought, but also very rapidly spinning, with surface speeds as high as 800 km/s. They calculate that the resultant centrifugal mixing of interior layers boosted the s-process’s efficiency by the requisite four orders of magnitude. The spectra were taken with the European Southern Observatory’s Very Large Telescope, shown here. (C. Chiappini et al., Nature 472, 454, 2011.)—Bertram Schwarzschild

Bose-Einstein condensation in the textbook and in the lab

Posted: May 26th, 2011, 8:30am MDT by Physics Today

Albert Einstein’s description of Bose–Einstein condensation is based on a statistical argument. In a gas of identical bosons, the statistical weighting of each state is such that the total occupation of the excited states is capped at an upper bound Nc. If the number of identical particles exceeds Nc, all additional particles must occupy the ground state. That textbook picture doesn’t include interparticle interactions, but it does assume thermal equilibrium, which cannot exist without some form of interaction, so it’s no surprise that the picture doesn’t describe real systems exactly. Now, Zoran Hadzibabic (Cambridge University, UK) and colleagues have taken a closer look at how the role of interactions in a Bose–Einstein condensate relates to the textbook picture. The researchers found, as shown in the figure, that for a gas of potassium-39 atoms in an optical trap, the thermal component (excited-state atoms) consistently exceeded the textbook upper bound. Qualitatively, they attribute the difference to repulsion between the thermal and condensed components, which turns the harmonic trap into a Mexican hat potential and thereby increases the number of thermally accessible excited states. Quantitatively, they found that when they tuned the interparticle interaction strength, by applying a magnetic field near a Feshbach resonance, an extrapolation to the zero-interaction limit recovered the textbook picture. (N. Tammuz et al., Phys. Rev. Lett., in press.)—Johanna Miller

Optical pump-probe diagnosis for melanoma?

Posted: May 23rd, 2011, 10:21am MDT by Physics Today

Whether rosy or rich, the color of human skin comes from a pigment called melanin, which occurs in two forms: Pheomelanin acts as a photosensitizer for UV radiation, while eumelanin has a protective role against light and seems to be overabundant in melanoma, a dangerous form of skin cancer. Melanoma is typically diagnosed by removing a lesion and examining it microscopically, layer by layer. Several noninvasive imaging techniques have been explored to detect the cancer, but they lack the molecular specificity needed to distinguish the two melanins. Enter researchers from Duke University who address that shortcoming with a nonlinear optical pump–probe technique. They excite the molecules with an ultrafast laser pulse and, after a varying time delay, measure the absorption of a second pulse—the probe. The contrast between the two melanins arises from their different time-delay absorption profiles. The white-light image on the left shows a melanoma lesion on human skin grafted onto a live mouse. The false-color image on the right, taken at a depth of 45 µm with the new technique, shows eumelanin in red, pheomelanin in green, and multiphoton fluorescence in blue. Previously the group used this method to image the tiny blood vessels in a mouse's ear. Such architectural views of skin coupled with the new functional mapping could provide the basis for a useful noninvasive diagnostic and screening method, particularly when removing tissue is problematic—as with melanoma in the eye. (T. E. Matthews et al., Biomed. Opt. Exp. 2, 1576, 2011.)—Stephen G. Benka

Gravity Probe B concludes its 50-year quest

Posted: May 19th, 2011, 8:54am MDT by Physics Today

Results of an experiment conceived around 1960 to test general relativity and launched in 2004 were announced at a NASA press conference earlier this month: Albert Einstein’s theory passed. The experiment featured four (for redundancy) gyroscopes—spinning, niobium-covered spheres—orbiting 642 km above Earth (see the figure). The goal was to measure the precession induced in the gyroscopes by two general relativistic effects. The easier-to-measure geodetic effect influences any spinning object orbiting a mass. The second effect, frame dragging, arises when the spacetime-distorting mass, here Earth, is itself spinning. Gravity Probe B was not the first to measure the two effects, but it was designed to measure them independent of each other and to extraordinary precision. The gyroscopes are the most perfectly spherical objects ever fabricated. They needed to be, lest the general relativistic precessions be swamped by those arising from Newtonian torques. To measure the spin of those featureless spheres, the experimenters cooled them below niobium’s superconducting transition; the superconducting metal then produces a magnetic field, parallel to its spin axis, that can be measured with a superconducting quantum interference device. In the end, the experiment was a qualified success. It measured the geodetic effect to 0.3% precision, but stray charges on the gyroscopes and their housings limited the precision of the frame-dragging measurement to 20%. In both cases other efforts have achieved comparable results. (C. W. F. Everitt et al., Phys. Rev. Lett., in press.)—Steven K. Blau

New sensor for detecting trace gases

Posted: May 16th, 2011, 10:48am MDT by Physics Today

Practitioners in many fields, including environmental science, homeland security, and health care, are keenly interested in identifying exquisitely tiny amounts of certain gases in, for example, the atmosphere or one's breath. Laser-based techniques are useful because they can selectively probe spectral lines with great sensitivity. In the mid-IR, where many molecules have a multitude of lines, the quantum cascade laser is fast becoming the instrument of choice. (For more on QCLs, see the article in Physics Today, May 2002, page 34.) Typically, an optical cavity is loaded with the gas at millibar pressure and the laser light is finely tuned to resonate with both a single cavity mode and a spectral line of interest. In a new twist, Gottipaty Rao and Andreas Karpf (Adelphi University, New York) use gases at atmospheric pressure, combining so-called off-axis cavity enhanced spectroscopy with multiple-line integrated absorption spectroscopy, which they developed. The off-axis alignment of the laser excites a near-continuum of cavity modes in addition to reflecting many times for a long path length through the gas. After tuning the QCL to a dense spectral region, the spectroscopists enhance the sensitivity by integrating over the resulting absorption spectrum, which includes more than 100 transitions. Their proof-of-principle detection of nitrogen dioxide has a sensitivity of 28 parts per trillion, which compares favorably with other laser-based systems. Unlike other systems, this one works well at high pressure, is less sensitive to instrumental vibrations, does not require fast electronics, and is effective for molecules with dense, poorly resolved spectra. (G. N. Rao, A. Karpf, Appl. Opt. 50, 1915, 2011.)—Stephen G. Benka

Clouds of atoms act like bouncing balls

Posted: May 12th, 2011, 10:48am MDT by Physics Today

To study the transport of spin in an ultracold gas, Martin Zwierlein and colleagues at MIT separated a cloud of spin-up atoms from a cloud of spin-down atoms and sent them traveling toward one another. Surprisingly, the two puffs, each a million times thinner than air, ricocheted off one another like billiard balls. The figure shows the spin-up (red) and spin-down (blue) clouds at 1-ms intervals during the first bounce. Such extreme behavior occurs when the atoms have the strongest possible interactions. The researchers cooled a trapped gas of fermionic atoms to nanokelvin temperatures, where the gas was degenerate, with atoms filling most of the lowest energy levels. Then they tuned an applied magnetic field to the so-called Feshbach resonance, where the atoms have their strongest possible interactions. The strong scattering between atoms with opposite spins, combined with the absence of interactions between those of the same spin, explains how the repulsion can be large enough for the clouds to rebound. After about six increasingly damped bounces, the clouds penetrated one another and the spins became homogeneously mixed. It took nearly a second for the spins to equalize. The diffusion time determines the spin diffusivity, which reached a minimum quantum value of ℏ /m, where m is the atomic mass. Zwierlein and his colleagues hope experiments like this can lend insight into the hydrodynamic properties of other strongly interacting systems, such as quark–gluon plasmas or neutron stars. (A. Sommer et al., Nature 472, 201, 2011.)—Barbara Goss Levi

Light does the twist to detect chirality

Posted: May 9th, 2011, 9:16am MDT by Physics Today

Chiral enantiomers—molecules that, like our left and right hands, are mirror images of each other but aren’t superimposable—behave interchangeably for the most part but distinguish themselves when interacting with chiral electromagnetic fields. For example, one way to discern a protein’s chirality is to measure its chiral dissymmetry, its differential absorption of left- and right-handed circularly polarized light (CPL). But measured dissymmetries are often quite small, partly because CPL twists at a rate that’s only faintly perceptible at molecular length scales; from a molecule’s perspective, left- and right-handed CPL don’t look very different. A year ago, Adam Cohen and his graduate student Yiqiao Tang (both at Harvard University) proposed that fields with a high ratio of optical chirality—the pair’s measure of twistiness—to electric field intensity could yield larger dissymmetries. Such fields can be produced with just a laser and a mirror: As illustrated here, the superposition of CPL with its partial reflection results in an electric field that describes an elliptical helix in space and rotates circularly about its axis. Cohen and Tang predicted that dissymmetry should be enhanced at the minima, where the electric intensity diminishes but optical chirality doesn’t. Now they’ve experimentally confirmed the theory. Tested on a chiral, naphthalene-based compound, the reflection scheme yielded a dissymmetry 10 times greater than that obtained with plain CPL. Combined with metamaterial and plasmonics technologies, the researchers’ concepts could lead to even more sensitive dissymmetry techniques. (Y. Tang, A. E. Cohen, Science 332, 333, 2011.)—Ashley G. Smart

Observing antimatter helium-4

Posted: May 5th, 2011, 9:38am MDT by Physics Today

When relativistic heavy nuclei collide, they fleetingly interact to produce hot, dense matter—often interpreted as the quark–gluon plasma—containing roughly equal numbers of quarks and antiquarks. As the matter cools, it changes phase to a hadronic gas that includes nucleons and their antiparticles. And those antinucleons, when close enough in position and momentum, can form a stable bound state. The STAR collaboration, a team of hundreds of scientists from 54 institutions worldwide, has now found evidence for antihelium-4 in the debris created in high-energy collisions at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider. Consisting of two antiprotons and two antineutrons, ⁴He is the heaviest antinucleus yet detected. The experiment’s central detector, situated in a solenoidal magnetic field, is used to image the ionization trail left by charged particles and antiparticles as they traverse a gas-filled chamber. From measurements of the energy loss and the time of flight for the antiparticles to reach a secondary detector composed of 23 000 sensors, the collaboration unambiguously identified 18 ⁴He nuclei in a sample of 10¹² tracks from a billion gold-on-gold collisions; the figure shows such tracks, including one (red) from a ⁴He nucleus, for a typical event. The yield is consistent with expectations from thermodynamic and nucleosynthesis models and provides a benchmark for any future observations of ⁴He, or even heavier antimatter nuclei, from cosmic radiation. (H. Agakishiev et al., STAR collaboration, Nature, in press, doi:10.1038/nature10079.)—R. Mark Wilson

CERN collider achieves record collision rate

Posted: May 2nd, 2011, 10:31am MDT by Physics Today

Image courtesy of CERN.

The Large Hadron Collider at CERN has reached a milestone in its quest to find—or lay to rest—the Higgs boson predicted by particle theory’s standard model. The LHC produces 7-TeV head-on collisions between protons in countercirculating beams stored in its 27-km-circumference ring, part of which is shown here. On 22 April, the colliding beams achieved a record luminosity of 4.7 × 10³²/(cm² s), surpassing for the first time the previous record held by the 2-TeV Tevatron collider at Fermilab. The new record luminosity (defined as the collision rate per unit scattering cross section) translates into 50 million proton–proton collisions per second at each of the LHC detectors. If the standard-model Higgs exists, it will be produced in only a tiny fraction of those collisions. But given the new record luminosity, the LHC detectors should accumulate enough data by the end of 2012 to provide a statistically robust sighting of the Higgs somewhere in the mass range (115–155 GeV) from which it has not already been excluded—or to demonstrate that Nature here parts company with the standard model. To allow that decisive accumulation of data, the start of the 18-month shutdown required to bring the LHC up to its full 14-TeV design energy has now been postponed from the end of this year to the end of 2012. (W. J. Murray et al., in Proceedings of the Chamonix 2011 Workshop on LHC Performance, [https:]] .)—Bertram Schwarzschild

Dry ice on Mars

Posted: April 28th, 2011, 8:56am MDT by Physics Today

Earth’s obliquity—the angle between its spin and orbital axes, currently 23.4°—doesn’t change much, due to the stabilizing influence of the Moon. Mars, in contrast, doesn’t have a large moon, so its obliquity is less constrained, wobbling by up to tens of degrees on time scales of 10⁵–10⁶ years. The resulting variation in solar heating at the Martian poles has a profound effect on the polar icecaps: Substantial amounts of material sublimate and refreeze, creating a complex structure of overlapping layered deposits. Most of those layers are primarily water ice. But now, radar data from the Mars Reconnaissance Orbiter reveal at least one large deposit, shown in the figure, of solid carbon dioxide. Because CO2 ice and H2O ice have different indices of refraction, a layer’s composition affects how the radar sees the layers under it. If the deposit in question were H2O ice, it would yield unrealistic distortions in the underlying topography; the refractive index that gives the most realistic topography is an excellent match to CO2 ice. If the deposit were vaporized—as it certainly was at some point in the past, possibly around 600 000 years ago when the obliquity was particularly large—it could nearly double the mean atmospheric pressure, from 6 mbar to 10.5 mbar. Such an increase would have important implications for the frequency and intensity of dust storms and the stability of liquid water on the Martian surface. (R. J. Phillips et al., Science, in press, doi:10.1126/science.1203091.)—Johanna Miller

Resolving the tension between droplets and solids

Posted: April 25th, 2011, 11:47am MDT by Physics Today

In 1805 Thomas Young derived the relationship between the static forces of a liquid droplet at rest on a solid substrate. As shown in the schematic, the droplet’s contact angle, θ, can be determined by balancing the horizontal solid–liquid and solid–vapor forces and the horizontal component of the liquid–vapor force, determined by the surface tension, γ/v (see Physics Today, February 2007, page 84). However, for more than two centuries, theories yielded unrealistic singularities for stress and strain at the three-phase contact line. That’s because Young’s equation does not account for the vertical, out-of-plane force pulling on the solid substrate, which naturally should be balanced by the substrate’s elastic response. Now, researchers at Yale University and at consumer products manufacturer Unilever have experimentally and theoretically resolved the out-of-plane contributions. Using a confocal fluorescence microscope, the researchers, led by Yale’s Eric Dufresne, laced a 20-micron-thick film of silicone gel with fluorescent beads and measured the deformation due to a water droplet. At equilibrium, a one-micron-high ridge, illustrated in the inset, formed in the gel at the contact line. When the researchers factored the gel’s surface tension and thickness into a linear elastic model, they arrived at a nonsingular theoretical solution for stress that closely fit their experimental data. Their model, however, underestimates the deformations in the solid-liquid contact plane, which they believe are caused by pinning or viscous drag. (E. Jerison et al., Phys. Rev. Lett., in press.)—Jermey N. A. Matthews

Mending polymers with light

Posted: April 21st, 2011, 10:46am MDT by Physics Today

We know all too well from daily life that materials made from polymers can be damaged: Tires get punctured, garbage bags rip, plastic eyeglass lenses get scratched. But over the past decade, several ways have been developed to heal polymers. Some methods are autonomous, drawing only on resources within the material. More commonly, though, the repair process is externally activated, typically by heat: When heated above their glass transition or melting temperature, the polymer chains can rearrange, diffuse, and re-entangle. Stuart Rowan (Case Western Reserve University), Christoph Weder (University of Fribourg), and colleagues have now demonstrated a healing mechanism activated by light. The team's method exploits so-called supramolecular polymers, in which noncovalent bonds connect large repeating units. In the new work, the repeating units were elastic, hydrophobic hydrocarbon segments terminated by hydrophilic ligands. Those building blocks, termed macromonomers, were joined together into chainlike structures by metal ions, either Zn²⁺ or La³⁺. The chains' hydrophobic and hydrophilic regions phase separated into a tough, cross-linked lamellar pattern. When UV light excited the metal–ligand bonds, most of the absorbed energy went into heating, which locally dissociated and liquefied the macromonomers. Thus freed, the macromonomers could quickly diffuse and mend defects. The researchers found that for some experimental samples, two 30-second bursts of light could completely heal cuts halfway through a thin film and restore the material to its original toughness. (M. Burnworth et al., Nature 472, 334, 2011.)—Richard J. Fitzgerald

New limit on putative dark-matter particle

Posted: April 19th, 2011, 7:50am MDT by Physics Today

Several lines of evidence suggest that dark matter, the mysterious substance that makes up 83% of the mass of the universe, is made up of subatomic particles known generically as WIMPs (weakly interacting massive particles). On cosmic scales, the presumed influence of WIMPs is easy to spot; the particles' collective gravity controls the distribution and motion of stars in galaxies. Detecting single WIMPs is far harder. After running continuously for 100 days, the XENON100 experiment has now yielded the most stringent limits on WIMP properties. Situated in a cavern in Italy's Apennine mountains, XENON100 consists of a bank of photomultiplier tubes that monitor 100 kg of liquid xenon. If a WIMP—or any other energetic particle—were to collide with one of the xenon atoms, the resulting scintillation would be detected. The overlying rock prevents cosmic muons from reaching the xenon, but the rock also contains radioactive elements, whose gamma-ray emission can beget scintillations. Although XENON100 incorporates ways to discriminate between the two sources of scintillations, some residual, unremovable background remains. In a paper submitted to Physical Review Letters, the XENON100 team report detecting three events that survived all the discrimination criteria. Given that the expected background was 1.8 events, the three events don't constitute a firm detection, but they do lower the limits on the WIMP mass and the WIMP–nucleon cross section. Supersymmetry, an extension to the standard model of particle physics, predicts the existence of WIMPs of order 100 GeV/c ², which is both consistent with the XENON100 results and within reach of the Large Hadron Collider. (E. Aprile et al., [arxiv.org] .)—Charles Day

Spin-orbit coupling in an unexpected context

Posted: April 14th, 2011, 8:58am MDT by Physics Today

In a solid-state system, spin–orbit coupling (SOC) is the interaction between the spin of an electron and the effective magnetic field that the electron feels as it moves relative to the charge field of the crystal lattice. Is there an analogous interaction for the neutral atoms in a Bose–Einstein condensate, where no charge field exists? “Yes,” say researchers at the Joint Quantum Institute of NIST in Gaithersburg, Maryland, and the University of Maryland, College Park. Defining SOC more broadly as the interaction between an atom’s spin and its momentum, the research team used a pair of laser beams to create such an interaction. In essence, the laser beams couple a spin-up state to a spin-down state that differs in momentum. The resulting coupling was equivalent to a specific type of SOC in materials. For sufficiently strong interaction, the researchers saw a separation between the up and down spins, as seen in the figure. The ability to create SOC—and other electromagnetic effects—in gases of ultracold atoms takes experimenters one step closer to a cherished goal: the simulation of condensed-matter systems. For example, interfering laser beams can create a periodically varying potential energy in which the atoms sit, reminiscent of electrons in a crystal lattice. The advantage of the atomic system is that it is defect free and allows researchers precise control over its parameters. (Y.-J. Lin, K. Jiménez-García, I. B. Spielman, Nature 471, 83, 2011.)—Barbara Goss Levi

Simple molecules mimic double slits

Posted: April 11th, 2011, 9:39am MDT by Physics Today

The double slit experiment, in which a coherent beam of particles diffracts through closely spaced slits to produce interference fringes, is perhaps the simplest and most famous demonstration of wave–particle duality. It has been shown to work with photons, electrons, and even the comparatively massive buckminsterfullerenes. According to a theory outlined by Howard Cohen and Ugo Fano nearly 50 years ago, interference fringes should also result from the photoionization of a diatomic molecule, provided electrons on each atom are ejected simultaneously and bestowed with enough kinetic energy that their wavelengths are similar to the interatomic distance, as illustrated here. Unfortunately, the direct approach—fixing molecules’ orientation in space, photoionizing them, and detecting fluctuations in the angular distribution of electron density—is, for the most part, impractical. Instead, an international team of scientists led by Fernando Martín (Autonomous University of Madrid) used an elaborate first-principles theory to predict the effect that interference should have on the relationship between a molecule’s photoionization cross section and its vibrational energy. Working at Lawrence Berkeley National Laboratory’s Advanced Light Source, they then used extreme UV light pulses to ionize hydrogen, nitrogen, and carbon monoxide gas. For H2, the simplest molecule of the bunch, the theory almost perfectly described the vibrationally resolved ionization spectra. For N2 and CO, the theory was less accurate, but still in qualitative agreement with the data. The results could help to resolve ambiguities surrounding previous attempts to demonstrate the phenomenon. (S. E. Canton et al., Proc. Natl. Acad. Sci. USA, in press.)—Ashley G. Smart

Fiber lasers deliver pulses at tens of gigahertz

Posted: April 7th, 2011, 10:04am MDT by Physics Today

Fiber lasers are commonly run with their frequency modes locked in phase by a saturable absorber, an optical element opaque to light below a threshold intensity but increasingly transparent above it. The mode locker enforces pulsed operation at a repetition rate inversely proportional to the cavity length. Fiber lasers typically run at tens of megahertz because of the long fiber lengths—on the scale of meters—needed to accumulate gain. But for portable metrology and data-transmission devices, researchers are striving to push that pulse rate higher. The University of Tokyo’s Amos Martinez and Shinji Yamashita now report the latest milestone in that effort: the development of an erbium-doped fiber laser that delivers 20-GHz pulses from a cavity just 5 mm long, as shown here. Key to the achievement are co-doping the fiber with ytterbium and incorporating carbon nanotubes into the laser cavity. Ytterbium’s absorption cross section is two orders of magnitude greater than that of Er³⁺ and thus it generates high gain over short lengths. Thanks to the nanotubes’ subpicosecond charge-carrier dynamics, low losses, and essentially negligible space requirement, a thin film of them functions as a nearly ideal saturable absorber when sprayed onto one of two Fabry–Pérot mirrors that form the cavity. As a demonstration of the laser’s applicability, Martinez and Yamashita use its ultrashort pulses to generate a broadband spectrum of frequencies that may be used as a precise frequency comb. (A. Martinez, S. Yamashita, Opt. Express 19, 6155, 2011.)—R. Mark Wilson

Tracking adsorbed atoms on a nanoscale mass sensor

Posted: April 4th, 2011, 10:47am MDT by Physics Today

An electromechanical system capable of sensing the mass of a single molecule or a few atoms typically consists of a nanometer-thick beam whose resonant frequency measurably shifts in response to the loaded mass. Naturally, such devices are sensitive to the adsorption and desorption of individual analytes or their diffusion along the beam's surface; both processes, shown in the schematic, cause the resonator's frequency to fluctuate. Now, Michael Roukes and colleagues at Caltech's Kavli Nanoscience Institute have determined the contribution of those processes to frequency noise for a nanoscale resonator vibrating at 190.5 MHz. The experiment, which was conducted in a temperature-controlled vacuum cryostat, relied on a nozzle that delivered a steady stream of xenon atoms to a silicon carbide resonator, shown in the inset. As the resonator was cooled, the average number of adsorbed xenon atoms increased; subsequently, so did the magnitude of the frequency shift. The researchers measured the fluctuation of surface adsorbates and found that diffusion is the favored route for atoms leaving the resonator; diffusion thus contributes the most to frequency noise in excess of the resonator's inherent thermal fluctuations. The combination of experimental data and analytical models also revealed previously unknown power-law dependence in the system's noise spectrum, a finding that could be used to probe the sensitivity limits of wide classes of nanoscale frequency-shift sensors. (Y. T. Yang et al., Nano Lett., in press, doi:10.1021/nl2003158.)—Jermey N. A. Matthews

Electron spins in Earth's mantle

Posted: March 31st, 2011, 9:03am MDT by Physics Today

When a transition-metal compound is subject to high pressure, its electronic spin state can change, which in turn can change the compound's material properties. That spin-state crossover is of geophysical relevance because of the iron-bearing minerals in Earth's lower mantle. But the most abundant mantle mineral—Fe-bearing magnesium silicate perovskite (Pv)—is a challenge to study, since it contains three nonequivalent types of Fe atom: Not only can Fe replace either Mg or Si in the crystal lattice, but Fe replacing Mg can be either ferrous (Fe²⁺) or ferric (Fe³⁺). Experiments on spin states under pressure probe the electron configuration indirectly, via its effect on nuclear energy levels, so computational studies are necessary to connect experimental measurements with the correct interpretations. Last year, an experimental study of ferric Fe in Pv yielded results that were at odds with the computational studies to date. Now, Renata Wentzcovitch and colleagues at the University of Minnesota have verified the experimental results computationally and predicted their geophysical consequences. The researchers found that ferric Fe that replaces Si undergoes a spin-state crossover at a pressure somewhere between 40 and 70 GPa, equivalent to a depth between 1100 and 2000 km and consistent with the 50–60 GPa crossover pressure measured experimentally. Since that transition causes the unit cell to shrink in volume by about 1%, it has a significant effect on the mineral’s bulk modulus and thus on the speeds of seismic waves and on mantle convection. (H. Hsu et al., Phys. Rev. Lett. 106, 118501, 2011.)—Johanna Miller

Precision measurement of cosmic-ray spectra

Posted: March 28th, 2011, 10:15am MDT by Physics Today

Broadly viewed, the energy spectra of the dozen or so nuclear species that dominate the flux of cosmic rays (CRs) bombarding the top of the atmosphere all look much the same, exhibiting a featureless E^−2.7 power-law falloff with increasing energy E up to about 10⁶ GeV. That seeming universality has bolstered a standard paradigm, which presumes that CRs originating in the Milky Way come almost entirely from supernova remnants whose expanding shock fronts can accelerate ions to as high as 10⁶ GeV. But now PAMELA, the first satellite-borne magnetic spectrometer dedicated to CR observation, has taken a closer look at the spectra of the two principal CR species: hydrogen and helium. Having analyzed two years of data, the PAMELA collaboration reports results that conflict with the standard paradigm. The power-law falloff of the H spectrum is distinctly steeper than that of the He spectrum, and each spectrum exhibits an abrupt upward kink at a few hundred GeV. Both results had been hinted at by earlier, less precise balloon data, so theoretical speculation has had something of a head start. The data suggest that in addition to supernova remnants, other classes of astrophysical accelerators within the galaxy contribute significantly to the CR flux. The list of candidates includes nova explosions on white dwarf stars, superbubbles of diffuse hot gas hundreds of light-years across, and even larger-scale plasma winds associated with the galaxy’s spiral arms. (O. Adriani et al., Science, in press, doi:10.1126/science.1199172.)—Bertram Schwarzschild

Microresonators as temperature sensors

Posted: March 24th, 2011, 8:53am MDT by Physics Today

In yet another application for micro- and nanostructures, recent experiments have demonstrated the potential for microresonators to serve as ultrasensitive temperature sensors. Last year, Ashok Pandey, Oded Gottlieb, and Eyal Buks of the Technion–Israel Institute of Technology showed that the resonant frequency of a suspended, microfabricated gold–palladium beam, hundreds of microns long but just a micron wide and a fraction of a micron high and supported at each end, was a strong function of temperature. The dominant contribution came from the temperature dependence of the tension in the beam, which is due to the difference between the thermal expansion coefficients of the beam and of the silicon substrate below it. The researchers could measure temperature changes by monitoring the relative frequency shift; as a temperature sensor, the beam's sensitivity was about a third that of commonly used, macroscopic platinum sensors. More recently, a team led by Anja Boisen of the Technical University of Denmark has reported aluminum microresonators, such as the ones seen here, whose resonant frequencies are even more sensitive to temperature. The improvement, of more than an order of magnitude, arises primarily from the larger difference in thermal expansion coefficients and from the Al beam's smaller initial tension compared with that of the Au–Pd beam. With their high quality factors, microresonator-based sensors would potentially have exceptional temperature resolution as well. (A. K. Pandey et al., Appl. Phys. Lett. 96, 203105, 2010; T. Larsen et al., Appl. Phys. Lett. 98, 121901, 2011.)—Richard J. Fitzgerald

An ancient meteorite recorded its changing environment

Posted: March 21st, 2011, 9:03am MDT by Physics Today

In the past decade or so, planetary scientists have significantly changed their view of the protoplanetary disk from which Earth condensed. Once described as having well-defined concentric zones, the disk is increasingly seen as a dynamic place whose materials could travel huge distances away from or toward the Sun. Now former University of California postdoc Justin Simon (currently a research scientist at NASA’s Johnson Space Center) and colleagues have used oxygen-isotope analysis to document what appears to be the wild ride of a meteorite fragment that formed early in the solar system’s history. The relative abundance of oxygen-16 and oxygen-17 indicates the radius at which the primitive material incorporated oxygen. That’s because solar radiation able to liberate ¹⁷O from carbon monoxide (C¹⁷O) is less attenuated as it travels through the disk than is the radiation that encounters the more prevalent C¹⁶O. Regions of the disk far from the Sun thus have an extra bit of ¹⁷O that can be integrated into meteorites. With unprecedented precision, the Simon team determined relative oxygen isotope abundances along four transects through a pea-sized piece of the Allende meteorite. In the figure, colors represent different minerals. Reading from inside to outside, the group found regions that were ¹⁶O rich, poor, then rich again. Conceivably the fragment remained stationary in a part of the protoplanetary disk with a dramatically changing environment. But a more natural interpretation is that the isotope signature represents a stamped, roundtrip ticket of a journey that began within the orbit of Mercury and extended beyond that of Mars. (J. I. Simon et al., Science 331, 1175, 2011.)—Steven K. Blau

Unexpected wear may yield new plasma rockets

Posted: March 17th, 2011, 7:29am MDT by Physics Today

A Hall-effect thruster (HET) is a type of electric propulsion system that produces thrust by the formation of an electron current around a circular channel that interacts with a radial applied magnetic field to create a strong axial electric field. That electric field then accelerates propellant ions to very high speeds. (For an introduction to HETs, see the Quick Study by Mark Cappelli, Physics Today, April 2009, page 76.) HETs have been used on many near-Earth missions, but most deep-space travel requires extended thruster operation, typically for years, which raises a major challenge: Some of the ions smash into the ceramic channel and erode it over time, leaving critical thruster components exposed to high-energy ions. Such erosion is known to limit a thruster's lifetime. However, during recent testing of the commercial HET design shown here, the erosion surprisingly stopped after about 5600 hours of operation, and remained suppressed until the test ended after more than 10 000 hours. Now a team of scientists at NASA's Jet Propulsion Laboratory has developed a new simulation code for HETs that exposed the physics behind those test results. As the channel eroded, the magnetic field topology changed and induced an effective shield against ion bombardment. The scientists suggest that carefully designing the magnetic field in future HETs can reduce channel erosion by several orders of magnitude. (I. G. Mikellides et al., Phys. Plasmas 18, 033501, 2011.)—Stephen G. Benka

Convection homogenizes magma intrusions

Posted: March 11th, 2011, 12:40pm MST by Physics Today

Plutons are mountain-sized formations of igneous rock that poke through Earth's surface. Their origin as solidified eruptions of magma is straightforward to explain. What's puzzling is why plutons are so homogeneous on large scales, despite their immense size and despite being inhomogeneous in their mineral composition on small scales. Alain Burgisser of the Institute of Earth Sciences in Orléans, France, and George Bergantz of the University of Washington in Seattle have proposed an answer. In their model, a mass of viscous, semisolid magma—"mush" is the technical term—lies beneath the surface, hemmed in by walls of more solid rock. Pluton formation begins when the slow churning of the mantle below happens to bring a body of hot magma into contact with the bottom surface of the cooler mush. Over the ensuing decades, the heat rises slowly via conduction, making the mush less viscous and—crucially—less dense. In a process that Burgisser and Bergantz call unzipping, the gradually thickening layer of hot, light mush abruptly undergoes a Rayleigh–Taylor instability, which sends convective plumes of hot mush upward through the nascent pluton. Within a few months, the first plumes reach the top of the nascent pluton, cool, then sink. Only a few successive cycles of overturning suffice to homogenize the mush on plutonic scales. Burgisser and Bergantz's model can plausibly account for the speed with which three real plutons formed, including the one left by the 1991 eruption of Mount Pinatubo in the Philippines. (A. Burgisser, G. W. Bergantz, Nature 471, 212, 2011.)—Charles Day

Fluorescent bursts reveal how biomolecules fold

Posted: March 10th, 2011, 9:03am MST by Physics Today

To gather clues about the structure of a protein, DNA, or other chain-like biomolecule, researchers can tag it with two fluorescent dyes. In what's known as fluorescence resonance energy transfer (FRET), the excitation of one dye, the donor, can cause a nearby partner, the acceptor, to also fluoresce. The resulting two-color burst of photons hints at the biomolecule's shape: The larger the contribution of the acceptor to the total fluorescence, the closer the dyes. Now, two groups have paired single-molecule-resolution FRET with microfluidics to shed light on complex biophysical phenomena. Ashok Deniz (Scripps Research Institute, La Jolla, California), Alex Groisman (University of California, San Diego), and colleagues used a novel microfluidic design to quickly initiate folding of a tagged protein and then track the time evolution of the donor–acceptor distances. Their experiment revealed the three-step process, illustrated here, by which the protein α-synuclein folds from its intrinsic disordered state into an extended helix. Another group of researchers led by Shimon Weiss (UCLA) and Stephen Quake (Stanford University) devised a microscale network of flow channels, valves, and pumps to facilitate high-throughput FRET experiments. The team used the device to investigate effects of chemical environment on bacterial RNA transcription, which could be detected as changes in fluorescence when tagged DNA strands hybridized with matching RNA strands. (Y. Gambin et al., Nat. Methods 8, 239, 2011; S. Kim et al., Nat. Methods 8, 242, 2011.)—Ashley G. Smart

Femtosecond bioimaging

Posted: March 7th, 2011, 9:55am MST by Physics Today

X-ray crystallography is remarkably successful at yielding atomic-resolution structures of proteins and other biological molecules. But that success relies on growing macroscopic crystals. Unfortunately, some molecules crystallize with difficulty or not at all. A decade ago, researchers predicted that the femtosecond pulses from an x-ray laser would be short enough and intense enough to produce a useful diffraction pattern from an uncrystallized biomolecular cluster before vaporizing it. An international collaboration of more than 80 scientists has now used SLAC’s Linac Coherent Light Source free-electron laser to perform two proof-of-concept demonstrations of the feat. In one study, the researchers squirted a suspension of nanocrystals (the photosynthetic protein photosystem I) across the 1.8-keV x-ray beam, recorded the two-dimensional diffraction pattern each time a crystal intersected the beam, and then combined 15 000 single-crystal snapshots to form the 3D projection shown here. From the data, the team reconstructed the protein’s structure at the near-atomic resolution of 8.5 Å, just over the x-ray wavelength. In the second study, the team injected an aerosol stream of 0.45-µm-diameter noncrystalline mimivirus particles across the beam. Thanks to the beam intensity—nearly 10¹³ photons per pulse—the collaboration was able to transform the diffraction pattern of a single virus particle into a real-space projection of its interior, though at the more modest resolution of 32 nm. The studies represent a step toward making molecular movies at atom-by-atom detail using harder and shorter pulses. (H. N. Chapman et al., Nature 470, 73, 2011; M. M. Seibert et al., Nature 470, 78, 2011.)—R. Mark Wilson

Recovery mechanism for Arctic ice

Posted: March 3rd, 2011, 9:10am MST by Physics Today

Global climate change is progressively reducing the Arctic Ocean’s summer ice cover. That retreat harbors an obvious positive-feedback mechanism: Because ice is more reflective than open water, the shrinking cover means more absorption of solar radiation, leading, in turn, to more loss of ice. That raises the prospect of a possible tipping point at which the thus-far relatively gradual retreat of summer ice “runs away,” leaving the Arctic Ocean perennially free of summer ice long before the date—sometime late in this century—generally deduced from climate models. But might not those models be made to reveal such threshold behavior by subjecting them to strong perturbations? Steffen Tietsche and coworkers at the Max Planck Institute for Meteorology in Hamburg, Germany, tried that with a widely used climate model. What would happen, they asked, if by a random fluctuation in some year, the Arctic Ocean became completely free of ice on 1 July? The figure, plotting ice cover in September, when it’s typically least, shows the result (in blue) when that initial perturbation is imposed in a particular year. In every case, the September cover reverts to its gradually falling unperturbed level (black curve) within about two years. Tietsche and company attribute such prompt recovery primarily to a negative-feedback mechanism that damps the albedo reinforcement: During the long, dark winter, the lack of insulating ice produces an anomalously warm arctic atmosphere, whose top radiates heat away faster and whose sides receive less wind-driven heat from temperate latitudes. So, they conclude, a tipping point at which the loss of summer sea ice becomes sudden and irreversible is unlikely. (S. Tietsche et al., Geophys. Res. Lett. 38, L02707, 2011.)—Bertram Schwarzschild

Building an atomic geomagnetometer from the ground up

Posted: February 28th, 2011, 7:16am MST by Physics Today

Measurements of the geomagnetic field at the smallest scales are used to locate sunken ships and mineral-rich geological formations. Large-scale measurements probe properties of Earth’s core. At length scales of tens to hundreds of kilometers, geomagnetic maps yield clues about the chemical dynamics in Earth’s outer mantle and the effects of ionic currents on ocean circulation. To avoid ground-based electromagnetic interference, geomagnetometers are typically placed aboard orbiting satellites, which are deployed sporadically and at a relatively high cost. Now, an international team of scientists led by James Higbie (Bucknell University), Domenico Bonaccini Calia (European Southern Observatory), and Dmitry Budker (University of California, Berkeley) has proposed a lower-cost ground-to-space system that exploits the interaction of beams from ground-based lasers with sodium atoms in the mesosphere, about 90 km above Earth's surface. The team’s system would harness the existing and expanding infrastructure of high-powered lasers that generate artificial stars for optical telescopes by exciting mesospheric sodium. As shown in the image, an optically pumped laser would spin polarize the sodium atoms, and then a ground-based telescope would measure the fluorescence intensity, which is dependent on the atoms’ precession frequency in the magnetic field. The researchers calculate that such a system would achieve subnanotesla sensitivity and could lead to the formation of a global network for continuous mapping and monitoring of mesospheric magnetic fields. (J. M. Higbie et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1013641108.)—Jermey N. A. Matthews

Entanglement cuts through the noise

Posted: February 24th, 2011, 8:22am MST by Physics Today

Quantum entanglement, by itself, cannot be used to communicate, but it can enhance the capacity, accuracy, or security of communication. In one example, researchers led by William Matthews (University of Waterloo, Canada) and Andreas Winter (University of Bristol, UK) showed last year that when two communicating parties—Alice and Bob—share pairs of entangled qubits, they can improve their chances of successfully transmitting a message over a complex noisy classical communication channel. Now, Robert Prevedel, Kevin Resch, Matthews, and other Waterloo colleagues have developed a similar protocol for a simpler channel, and they've implemented that protocol experimentally. The researchers used an electronic circuit to create a noisy classical channel that takes a pair of bits from Alice and tells Bob either the first bit, the second bit, or the parity of their sum—each with equal probability—and which of the three he received. Without the benefit of shared entanglement, Alice’s best strategy for transmitting a single bit to Bob succeeds 5/6, or 0.833, of the time. But if Alice and Bob each possess one of a pair of entangled qubits—polarization-entangled photons, in the experiment—then they can make better use of the channel and achieve a theoretical success rate of (2 + 2^−1/2)/3, or 0.902. The experimental success rate was 0.891 ± 0.002, much higher than the classical limit and not much lower than the entanglement-enhanced maximum. (R. Prevedel et al., Phys. Rev. Lett., in press.)—Johanna Miller

Wavefunction’s unconventional statistics manifested

Posted: February 17th, 2011, 7:00am MST by Physics Today

In three dimensions, exchanging identical particles has a simple effect on a wavefunction: no change for bosons, multiplication by −1 for fermions. In two dimensions, things are more complicated. Consider the two ways to switch identical particles “A” and “B” shown in the figure. Because the clockwise and counterclockwise switches can’t be continuously deformed into each other, 2D exchange doesn’t just swap coordinates; it also involves a topological component. When many particles are involved, the topological issues are correspondingly more complex, and exchange operations might not commute. In that case the particles are said to have non-abelian (that is, noncommuting) anyon statistics. Non-abelian anyons are more than a mathematical curiosity: Condensed-matter physicists have plausibly argued that the quasiparticles that participate in the so-called ν = ⁵⁄2 fractional quantum Hall state are objects of that type (see the article by Sankar Das Sarma, Michael Freedman, and Chetan Nayak in Physics Today, July 2006, page 32) . Now, Nayak (Microsoft Station Q and the University of California, Santa Barbara) and colleagues have, in the first calculation of its kind, explicitly demonstrated the compatibility of a specific popular candidate ν = ⁵⁄2 wavefunction with non-abelian anyon statistics. The key step, says MIT’s Frank Wilczek, was to map the wavefunction to a rather different physical system amenable to attack with a well-established battery of mathematical tools. Does the wavefunction studied by the Nayak team actually describe the ν = ⁵⁄2 state? That ball is in the experimentalists’ court. (P. Bonderson et al., Phys. Rev. B 83, 075303, 2011.)—Steven K. Blau

Spin doctors operate on a quantum dot

Posted: February 14th, 2011, 1:55pm MST by Physics Today

A quantum dot is a nanostructure that confines a single conduction-band electron in three spatial dimensions. Researchers have long sought to use the spin on that electron as a quantum bit—or qubit—to store binary information for a quantum computer. First, however, they must show that they have precise and rapid control over the quantum-dot qubit. Unfortunately, flipping the spins with oscillating magnetic fields requires high-frequency fields and very low temperatures, posing a challenge to experimenters. An alternative is to use purely electric fields and exploit the spin–orbit coupling of electrons: The orbital motion relative to the background of the semiconductor’s nuclear charges causes the electron to see a magnetic field, which couples to its spin. In traditional gallium arsenide quantum dots, the spin manipulation times obtained with spin–orbit coupling are too slow. Recently, Leo Kouwenhoven and his colleagues at Delft University of Technology and at Eindhoven University of Technology, both in the Netherlands, have turned to indium arsenide, which is known to have much larger spin–orbit coupling. Furthermore, the team formed the qubits in an InAs nanowire, which offers interesting possibilities for combining with other semiconductors. For example, one might make an optoelectronic device that converts the spin state to a photon for long-distance transportation. A more exotic prediction is that InAs nanowires might be useful for topological quantum computing. (S. Nadj-Perge, S. M. Frolov, E. P. A. M. Bakkers, L. P. Kouwenhoven, Nature 468, 1084, 2010.)—Barbara Goss Levi

Chemical pattern formation in three dimensions

Posted: February 7th, 2011, 1:36pm MST by Physics Today

Irving Epstein and his coworkers at Brandeis University in Waltham, Massachusetts, have shown that a chemical mechanism for producing patterns in two dimensions also works in three. Proposed in 1952 by Alan Turing, the mechanism relies on the competition between a slow-diffusing chemical that activates a reaction and a fast-diffusing chemical that inhibits the reaction. Nudging the reaction–diffusion system into a metastable state yields stable stripes, spots, and other periodic patterns. Turing's analysis and its subsequent experimental confirmation was for two-dimensional systems. Although computer simulations suggest the mechanism also operates in 3D, proving it does so in the lab is challenging: The extra spatial dimension makes it difficult to see patterns inside the medium. To meet that challenge, the Brandeis team used optical tomography to view a medium made up of aqueous droplets embedded in oil. Turing's model doesn't ordinarily apply to such an inhomogeneous medium. However, by coating the droplets with a surfactant, the team ensured that the slow-diffusing activator and fast-diffusing inhibitor leaked in and out at rates that sustained pattern formation on scales larger than the droplets themselves. To monitor the system, the team rotated the reaction vessel (a quartz cylinder) in front of a camera that took a sequence of 2D images. Tomographic reconstruction of the system under different initial conditions revealed a gallery of structures, including the labyrinthine worms shown here. Epstein anticipates that the 3D version of Turing's model may explain the formation of some biological patterns, such as the process by which Hydra regrows its tentacle-tipped head after decapitation. (T. Bánsági, V. K. Vanag, I. R. Epstein, Science, in press.)—Charles Day

Saving food webs by subtraction

Posted: February 7th, 2011, 9:20am MST by Physics Today

An ecosystem is a dynamic, complex tangle of predators and prey in which the depletion of one species can trigger a cascade that leads to the extinction of several others. Some cascades are structural—they propagate when an extinction leaves some predators with no prey. Others are dynamic—they occur when an initial extinction leaves the system in an unstable state that, in mathematical parlance, is drawn by an attractor toward multispecies extinction, even though more favorable steady states might exist. Now, simulations by Adilson Motter and Sagar Sahasrabudhe (both at Northwestern University) suggest that many dynamic cascades can be mitigated with the strategic removal or suppression of a second species. Some rescues are intuitive, such as removing a second species that’s a predator or prey of the first. But the ideal rescue species need not share such a direct link with the cascade-instigating species; it can even be a species that would have gone extinct in the cascade anyway. Sometimes just partial suppression of a species can spare the entire food web. The team demonstrated how its approach might work in the Coachella Valley ecosystem, depicted as a network in the figure. Arrows point from each prey to its predator. The larger circles indicate species that, if removed, would most likely instigate a cascade; the yellow circles, those most likely to mitigate one. The results suggest that smart, sometimes counterintuitive population-control measures could be instrumental in conserving ecosystems. (S. Sahasrabudhe, A. E. Motter, Nat. Commun. 2, 170, 2011.)—Ashley G. Smart

Backward lasing in air

Posted: February 3rd, 2011, 3:13pm MST by Physics Today

A central challenge in detecting hazardous gases and vapors that indicate the presence of hidden explosives is obtaining a strong enough signal from a distant, safe location. Optical techniques commonly rely on the backscattering of incident laser light. At long distances, though, the backscattered signal can be prohibitively weak due to the omnidirectional nature of fluorescence. Arthur Dogariu and his Princeton University colleagues have now developed an approach that could produce much stronger signals. They demonstrate that a thin 1-mm-long patch of oxygen molecules can be optically pumped from afar to emit laser light in the forward and backward directions—the backward-going beam sampling the air as it returns. As outlined in the figure, a remote UV laser (blue) does double duty: It drives both the two-photon dissociation of molecular oxygen and the two-photon excitation of one of the atomic fragments, which then emits in the IR (red) back toward the pump laser. Stimulated IR emission from atomic O is itself not new; it was first observed in combustion experiments in the late 1980s. But the Princeton team realized, from their own combustion experiments a few years ago, that lasing in room-temperature, atmospheric-pressure air might be possible without the need for a local, molecule-dissociating flame. Thanks to the backward-going laser’s high gain, its signal intensity is roughly a million times greater than the intensity of concomitant fluorescence collected in the same solid angle. (A. Dogariu et al., Science 331, 442, 2011.)—R. Mark Wilson

Gamma-ray flares from the Crab

Posted: January 31st, 2011, 10:56am MST by Physics Today

The Crab nebula, shown here in a Hubble Space Telescope image, is the remnant of a supernova explosion so nearby that Chinese astrologers took note of it in AD 1054. Powered by the rapidly spinning neutron star at its heart, the nebula shines with extraordinary luminosity at all photon energies from radio to TeV gammas. The apparent stability of the Crab’s brightness over that entire range has made it a convenient reference for calibrating astronomical instruments; the brightness of other sources is often quoted in “millicrabs.” But now two teams that operate orbiting gamma-ray telescopes—the Italian Space Agency’s AGILE and NASA’s Fermi—have reported seeing powerful short-duration flaring of the Crab nebula at gamma energies ranging from 100 MeV to 1 GeV. Lasting a few days or weeks, and apparently occurring once or twice a year, the outbursts manifest synchrotron radiation by electrons and positrons in the nebula that have somehow been accelerated to extremely high energies on an astonishingly rapid time scale. The flares rudely challenge the prevailing theory of how charged particles are accelerated, very gradually, in shock fronts within supernova remnants. The theory goes back to Enrico Fermi’s 1949 attempt to explain the origins of cosmic rays. Now the Crab has been put under close surveillance by orbiting x-ray and optical telescopes as well as AGILE and Fermi, in hopes of pinpointing longer-wavelength clues within the nebula during the next gamma outburst. (M. Tavani et al., AGILE collaboration, Science, in press, doi:10.1126/science.1200083; A. A. Abdo et al., Fermi collaboration, Science, in press, doi:10.1126/science.1199705.)—Bertram Schwarzschild

Half-quantum vortices

Posted: January 27th, 2011, 10:54am MST by Physics Today

In most known superconductors, electrons pair up to form spin singlets: combinations of spin up and spin down with zero angular momentum. But in a few materials, including strontium ruthenate (Sr2RuO4, or SRO), the electron pairs form spin triplets (see the article by Yoshiteru Maeno, Maurice Rice, and Manfred Sigrist in Physics Today, January 2001, page 42). In SRO, the triplets are thought to take a form that can be represented as two weakly interacting superfluid condensates: one of spin-up pairs, the other of spin-down pairs. That arrangement can support half-quantum vortices (HQVs), in which one condensate has one more quantum of vorticity than the other. Because of HQVs' potential applications to quantum computing, they’ve been extensively studied by theorists and sought by experimentalists. Now, researchers led by Raffi Budakian (University of Illinois at Urbana-Champaign) have found the first experimental evidence of HQVs in SRO. They mounted a micron-sized SRO ring on a cantilever, as shown in the figure’s top panel, and monitored the ring's magnetic moment by observing how a weak oscillating magnetic field along the cantilever (Hx) affects the cantilever's resonant oscillation. As shown in the bottom panel, they found that scanning Hz, the field normal to the ring, without an additional static Hx induced magnetization steps that were of the right size and position to be full-quantum vortices. When they added a sufficiently strong static Hx, the steps split into half-height increments, consistent with the presence of HQVs. To strengthen the case, the researchers repeated the experiment with a different superconductor and with a larger SRO sample—neither of which should support HQVs—and saw no half-height steps. (J. Jang et al., Science 331, 186, 2011.)—Johanna Miller

An ultrasonically focused optical probe

Posted: January 24th, 2011, 11:05am MST by Physics Today

When imaging, monitoring, or stimulating samples in a scattering medium, even the most powerful optical microscopes and probes are hindered by the diffusion limit, the length scale beyond which the incident light uncontrollably scatters. To overcome that limit, some techniques focus the wavefront as it propagates through the sample; others iteratively shape it to amplify the target signal. Lihong Wang and colleagues at Washington University in St. Louis have developed a new approach that combines time reversal with ultrasound, whose waves scatter weakly in biological tissue, to focus light to a controllable position. (For an introduction to multiwave imaging, see the article by Mathias Fink and Mickael Tanter in Physics Today, February 2010, page 28.) In the team's setup, laser light, frequency-shifted by two acousto-optic modulators in series, entered the sample medium, where the diffused light was further modulated by an ultrasonic wave tuned to the frequency shift. The interaction between the light and the ultrasound produced a virtual point source within the sample. From a holographic record of the modulated diffused light, the researchers generated a time-reversed trajectory that produced optical focusing at the virtual source location. The new technique, known as time-reversed ultrasonically encoded (TRUE) optical focusing, generated a noticeably higher contrast for objects inside a slab of synthetic biological tissue than was attained by conventional ultrasound-modulated optical tomography, which cannot focus light. (X. Xu, H. Liu, L. V. Wang, Nat. Photonics, in press, doi: 10.1038/nphoton.2010.306M.)—Jermey N. A. Matthews

A standard candle slowly burns down

Posted: January 20th, 2011, 7:05am MST by Physics Today

Cepheid stars are remarkable beacons. Each pulses with a regular frequency related to its intrinsic brightness. An astronomer can therefore determine the distance to a Cepheid by comparing its observed and intrinsic luminosities. Cepheids, in turn, calibrate other classes of luminosity-signaling standard candles that can be viewed from farther away. Iterating the process yields a cosmic distance ladder that enables distance estimation across the cosmos. Astronomers had long thought the behavior of Cepheids was unchanging, but Pauline Barmby of the University of Western Ontario and colleagues now report that of 29 Cepheids that the team studied, about one-fourth are losing mass. As they do so, they shroud themselves with dust that could introduce errors in distance estimations. The figure, from an earlier, related study, shows an IR image obtained with the Spitzer Space Telescope of one such star, Delta Cephei. The red region is a shock created as the star plows through interstellar gas and dust. The detailed form of the shock reveals that it was shaped by a stellar wind emanating from Delta Cephei that’s a million times stronger than that from our Sun. So, is a lower rung on the cosmic distance ladder rotten, making the ladder itself unreliable? To the contrary, argued team member Massimo Marengo at last week’s American Astronomical Society meeting in Seattle. Even with its uncertainties, the Cepheid rung is strong and observations based on the Cepheids remain trustworthy. Furthermore, now that astronomers are gaining a deeper understanding of those stars, they will be able to better determine their brightness and construct an even sturdier ladder. (M. Marengo et al., Astrophys. J. 725, 2392, 2010; P. Barmby et al., Astron. J. 141, 42, 2011.)—Steven K. Blau

Tabletop measurements of Hawking radiation

Posted: January 13th, 2011, 8:15am MST by Physics Today

Stephen Hawking proposed in 1974 that black holes evaporate. In essence, vacuum fluctuations near a black hole's horizon produce particle–antiparticle pairs. One of each pair falls into the hole while the other escapes. Since the escaping particle has energy, the black hole must lose energy. A blackbody temperature inversely proportional to the black hole's mass can be assigned to the process, yet the temperature is so low—on the order of 100 nK for a solar mass—that the radiation is difficult to observe directly. But William Unruh (University of British Columbia) demonstrated an analogy between the behaviors of waves near the black hole and sound waves in moving fluids. Now, physicists Silke Weinfurtner, Matthew Penrice, and Unruh and engineers Edmund Tedford and Gregory Lawrence at UBC have used another analogous system, surface waves, to study the Hawking process. They put a streamlined object shaped like an airplane wing into a channel of flowing water to create a region of high-velocity flow. Long-wavelength surface waves created downstream could propagate upstream toward that region but were blocked by the obstacle and converted into short-wavelength waves. The figure shows the converted waves (bottom) and the interference between them and the incoming wave (top). The obstacle behaves like a so-called white hole, which, as a time-reversed black hole, lets no radiation in but does let radiation escape. The conversion is the analogue of stimulated emission, and the team's measurements of the amplitudes of the converted waves matched the expected thermal distribution. Moreover, despite the system's nonlinearities, turbulence, and viscosity, and along with prior numerical work by various groups, the new results demonstrate the generic nature of Hawking radiation. (S. Weinfurtner et al., Phys. Rev. Lett. 106, 021302, 2011.)—Richard J. Fitzgerald

Record power in the terahertz gap

Posted: January 10th, 2011, 8:47am MST by Physics Today

The universe teems with terahertz radiation, though here on Earth you’d never know—partly because most of it is absorbed by atmospheric molecules, partly because it’s difficult to detect. Heterodyne receivers can do the trick, but they must be driven by a narrowband source at the frequency of interest. Optical devices such as lasers make suitable sources at frequencies above 4 THz or so, and electronic devices work well at frequencies below about 0.8 THz. But nestled in between, just longer than the IR but shorter than microwaves, is an elusive region known as the terahertz gap. Now Jerome Moloney (University of Arizona, Tucson) and an international team of colleagues have designed a room-temperature source that delivers narrowband, milliwatt beams at terahertz-gap frequencies. They exploited a nonlinear optics effect whereby two frequencies of light are mixed in a nonlinear crystal to emit a beam at the difference frequency, so that appropriately spaced frequencies of IR laser light beget a terahertz beam. Though not alone in having adopted such an approach, the team achieved record power by positioning all the necessary photomixing elements—the nonlinear crystal; a Brewster window, which polarizes light; and an etalon, which establishes two IR lasing frequencies—inside the laser cavity, where the beam is most intense (see illustration). The device, dubbed a terahertz external-cavity surface-emitting laser, or TECSEL, could help to reveal previously obscured cosmic structures and events. (M. Scheller et al., Opt. Express 18, 27112, 2010.)—Ashley G. Smart

A Bose-Einstein condensate of photons

Posted: January 6th, 2011, 10:08am MST by Physics Today

As massless and chargeless particles of integer spin, photons are the simplest of bosons. Moreover, Satyendra Nath Bose had photons in mind in 1924 when he proposed a new way of counting indistinguishable particles, work that led to Albert Einstein’s prediction of the state of matter known as a Bose–Einstein condensate (BEC): When a collection of bosons becomes cold enough, a macroscopic fraction of them congregate into a single quantum state. But until now, a BEC of photons has never been observed. Blackbody photons, when cooled in a cavity, simply disappear into the walls. And, unlike atoms, photons don’t usually interact with each other, which prevents them from reaching thermal equilibrium apart from a blackbody. Martin Weitz and colleagues from the University of Bonn have now overcome those obstacles to a photon BEC. By confining laser light within an optical cavity filled with dye, they create the conditions for light to equilibrate as a gas of conserved particles. The cavity mirrors’ reflectivity ensures that photons live long enough to scatter among dye molecules, which exchange energy with the photons by repeatedly absorbing and reemitting them. To make a BEC, Weitz and company increase the number of scattered photons in the cavity by dialing up the laser power. Above a critical density, the photon energy spectra exhibit an emerging Bose peak at the cavity’s lowest-energy mode, alongside a broad thermal distribution of uncondensed photons. Shown here, beside the spectra, are spatial profiles of the light emission, below the onset of condensation (top) and above it (bottom). (J. Klaers et al., Nature 468, 545, 2010.)—R. Mark Wilson

Elucidating cell-free protein synthesis

Posted: January 3rd, 2011, 8:58am MST by Physics Today

The chemistry of life is complicated. Gene expression, in which information is transcribed from DNA to messenger RNA and then translated to produce a protein, involves more than 100 different molecules. Gaining a quantitative understanding of the process through observation of living cells is a daunting challenge. Now, Vincent Noireaux (University of Minnesota), Roy Bar-Ziv (Weizmann Institute of Science in Israel), and colleagues have used a cell-free system to carry out a complete gene-expression reaction, and they’ve developed a simple model of the reaction dynamics. Cell-free protein synthesis itself is not new; it’s been used for 15–20 years to produce proteins for research and medicine. Typical cell-free systems, which are available commercially, are therefore optimized to produce a lot of protein quickly rather than to reproduce reactions as they occur in vivo. The systems combine molecules from different organisms, and they don’t allow control over biologically important reactions such as mRNA inactivation and protein degradation. Noireaux and his student Jonghyeon Shin developed their own cell-free system, using molecules only from Escherichia coli bacteria and including enzymes for inactivation and degradation. Bar-Ziv and his student Eyal Karzbrun carried out experiments to determine how each component’s concentration affects the amount of protein produced. They found that they could describe the system’s main features by treating each process—transcription, translation, inactivation, and degradation—as if it were catalyzed by a single enzyme. The researchers’ next step is to extend the system to study more complicated processes, such as in circuits of several genes that affect one another’s expression. (E. Karzbrun et al., Phys. Rev. Lett., in press.)—Johanna Miller

No microscopic black holes yet

Posted: December 30th, 2010, 8:59am MST by Physics Today

From among 10¹³ proton–proton collisions at 7 GeV in its first year of operation, the Large Hadron Collider (LHC) at CERN has as yet yielded no evidence of black hole production. The detectable creation of microscopic black holes at the LHC follows from speculative but attractive theories that seek to explain the puzzling weakness of gravity by positing curled-up extra spatial dimensions accessible only to gravitons. In such theories, the intrinsic strength of gravity would be comparable to those of the electromagnetic and weak interactions at energies near 1 TeV, where electroweak unification occurs. But now the collaboration that runs the LHC’s Compact Muon Solenoid (CMS) detector, having found no evidence of black holes, has published the first experimental lower limits on their masses. A black hole produced in a 7-TeV collision would decay by Hawking radiation within 10^-27 s into perhaps half a dozen extraordinarily energetic particles—mostly quarks and gluons manifesting themselves as jets of hadrons. Such a spectacular decay would be conspicuous not only by the number of emerging ultrahigh-energy jets, but also by their unusually isotropic distribution. Even in the absence of true black hole events, however, 10¹³ collisions will create many imposters. So determining limits on black hole production requires painstaking estimation of the resulting backgrounds. The figure shows the minimum black hole mass deduced from the CMS null result as a function of two parameters of the extra-dimension theories: the number n of extra spatial dimensions, and the characteristic mass scale MD of the putative unification of the gravitational and electroweak interactions. (CMS collaboration, [arxiv.org] .)—Bertram Schwarzschild

Superplastic mantle minerals

Posted: December 24th, 2010, 7:58am MST by Physics Today

Superplasticity is the ability of some crystalline materials to stretch up to several times their own length when heated. Although the minerals in Earth's mantle don't endure such large strains, circumstantial evidence suggests that superplasticity helps them respond to the subduction of continental plates and other tectonic processes. Now, a team led by Takehiko Hiraga of Tokyo University and Hidehiro Yoshida of Japan's National Institute for Materials Science has found direct evidence that mantle minerals are indeed superplastic. Like other superplastic materials—real or presumed—those in the mantle are polycrystalline aggregates. For their study, Hiraga, Yoshida, and their team sintered nanoscale powders to make two analogues of mantle minerals, both of which consisted mostly of forsterite (Mg2SiO4). In the absence of strain, a superplastic material is made up of nanoscale grains of the majority component interspersed with smaller grains of the minority component. When heated under strain, the majority and minority grains both grow by merging with their neighbors. That response ensures that grains continue to abut each other, forestalling failure of the bulk material. As the accompanying figure shows, samples that consisted of 90% forsterite and 10% periclase (MgO) could withstand strains of more than 500%. Moreover, two electronic diagnostics, electron back-scattered diffraction and transmission electron microscopy, revealed that grains in the mantle analogues grew like grains in materials whose superplasticity is established. Having measured the temperatures and strain rates under which mantle analogues become superplastic, the team estimated that superplasticity could help Earth's mantle accommodate a 200-km slab that takes 60 million years to penetrate 3000 km. (T. Hiraga, T. Miyazaki, M. Tasaka, H. Yoshida, Nature 468, 1091, 2010.)—Charles Day

What killed top-kill?

Posted: December 23rd, 2010, 10:41am MST by Physics Today

Global news coverage of the blown-out BP-operated Macondo well documented several efforts to stem the roughly 8 million liters per day of crude oil that gushed into the Gulf of Mexico from April to July 2010. One notable failure was the “top-kill” method to plug the well by pumping a dense slurry of drilling mud into it—reportedly, much of the mud was swept out by the spewing oil. A recent study by scientists at Lawrence Livermore National Laboratory and Washington University in St. Louis points to fluid shearing as the chief culprit in top-kill’s failure. Turbulent eddies, caused by the velocity differences between the counterstreaming mud and the oil, likely sheared the mud, breaking it up into packets of fluid whose settling velocities were an order of magnitude smaller than the upward velocity. In laboratory experiments, the researchers confirmed their theory and demonstrated a possible solution, adding a viscoelastic polymer to an aqueous cornstarch mixture to represent the drilling mud. As the images show, the control fluid (left, in green) suffered turbulent breakup, but the polymer-laced fluid (center) descended as a coherent slug or, at lower flow rates (right), as stringy, connected globules. The researchers calculate that a polymer-doped slug of drilling mud at the Macondo well would have descended with a terminal velocity of roughly 7 m/s, nearly double the estimated 3.7 m/s ascent of escaping oil. (P. Beiersdorfer et al., Phys. Rev. Lett., in press.)—Jermey N. A. Matthews

Counting a mole of silicon atoms

Posted: December 20th, 2010, 9:39am MST by Physics Today

Avogadro’s number, NA, links the microscopic and macroscopic worlds by specifying how many individuals make up a mole. Now an international team of metrologists has obtained NA with an unprecedented precision of 30 parts per billion. The result: NA = 6.02214078(18) × 10²³. The idea behind the new experiment is simple. A sample from a crystalline silicon-28 boule, shown in the figure, was subjected to x-ray interferometry, which yielded the volume of the 8-atom Si unit cell. Other bits of the boule were painstakingly fabricated into spheres whose volumes and masses were carefully measured. The spherical volume divided by the unit-cell volume gives the number of Si atoms; the mass gives the number of moles. Voilà, atoms per mole. The devil, of course, is in the details. The team needed to measure and account for such flaws as pointlike defects in the boule and surface oxidation on the spheres. Furthermore, uncertainty in the isotopic composition of the silicon translates into an uncertainty in the mass of a mole. Indeed, questions of isotopic composition plagued earlier, similar experiments. In their most recent determination, the researchers worked with a crystal that was highly enriched in ²⁸Si and applied an innovative suite of mass spectrometry techniques to measure the minute remainders of ²⁹Si and ³⁰Si. The new NA does more than tweak the size of a mole; in combination with other precision experiments, it will be used by metrologists to refine the values of several other fundamental constants. (B. Andreas et al., Phys. Rev. Lett., in press.)—Steven K. Blau

Energetic gamma rays on Earth

Posted: December 16th, 2010, 8:00am MST by Physics Today

Terrestrial gamma-ray flashes (TGFs) are the source of the highest-energy nonanthropogenic photons produced on Earth. Associated with thunderstorms—and, in fact, with individual lightning discharges—they are presumed to be the bremsstrahlung produced when relativistic electrons, accelerated by the storms’ strong electric fields, collide with air molecules some 10–20 km above sea level. According to the prevailing theoretical model of that process, and to some previous observations, the TGF energy spectrum should follow a power law at low energies but decay exponentially at energies above about 7 MeV, as shown by the red line in the figure. Now, researchers working with data from the Italian Space Agency’s AGILE satellite find a rather different and theoretically challenging spectrum. Based on data from 130 TGFs collected over a 20-month period, their observed spectrum (black dots) is well fitted at the high-energy end by a second power law (blue line) that extends to at least 100 MeV with no sign of an exponential cutoff. According to the theory, the highest-energy electrons must have had a long history of flying through the electric field, colliding with air molecules, and releasing electrons with relatively low energies. But so many low-energy electrons would produce many more low-energy photons than the AGILE team observed. (M. Tavani et al., Phys. Rev. Lett., in press.)—Johanna Miller

Iron in the night sky

Posted: December 13th, 2010, 12:13pm MST by Physics Today

Even in the absence of the Moon and stars, the night sky would not be black. Chemical reactions in the upper atmosphere emit faint light called airglow, whose spectrum has been studied for more than a century. Much of the emission comes from atomic and molecular oxygen, hydroxyl radicals, sodium, and, at higher latitudes, nitrogen dioxide. But after subtracting those species' contributions from the airglow spectrum at lower latitudes, observers have noted a broad band of unaccounted-for emission in the orange part of the visible spectrum, between roughly 550 and 650 nm. Using measurements from the Optical Spectrograph and Infrared Imager System (OSIRIS) on Sweden's Odin spacecraft, Dick Gattinger and Ted Llewellyn (University of Saskatchewan) and colleagues have now identified the source of that band: iron monoxide. Several factors supported the team's conclusions. Nitrogen dioxide was ruled out since sunlight destroys it at lower latitudes and, moreover, it has a different spectral signature. Laboratory measurements of FeO, on the other hand, yield a similar emission spectrum. Iron's atmospheric abundance is comparable to that of sodium, a known airglow contributor. And an orange emission band observed a decade ago originating from iron-containing meteors was also matched to the laboratory FeO spectrum. Subsequent ground-based measurements show that the FeO emission exhibits dramatic temporal fluctuations. The team postulates that FeO is generated through reactions between atmospheric iron and ozone. (W. F. J. Evans et al., Geophys. Res. Lett. 37, L22105, 2010, doi:10.1029/2010GL045310.)—Richard J. Fitzgerald

Exploring the extremes of turbulence

Posted: December 9th, 2010, 8:42am MST by Physics Today

Force a fluid gently and its response is an orderly, laminar flow. Disturb it vigorously and that well-organized procession gives way to turbulent whirls and eddies such as those shown in the image. Now, new insights into turbulent flow arrive by way of two independent experiments—one by Detlef Lohse and colleagues at the University of Twente in the Netherlands, the other by Daniel Lathrop and Matthew Paoletti at the University of Maryland, College Park. Both teams studied Taylor–Couette flows, in which fluid is sheared in the gap between concentric, rotating cylinders, and both achieved Reynolds numbers on the order of 10⁶—records for such a device. Lohse and company were intrigued by the parallels between Taylor–Couette and Rayleigh–Bénard cells, the latter comprising a fluid confined between two horizontal plates and heated from below. They found that angular momentum transport in turbulent Taylor–Couette flows obeyed the same scaling laws that had been predicted, but never conclusively observed, for heat transfer in Rayleigh–Bénard flows. Lathrop and Paoletti explored a regime of Taylor–Couette flow in which the innermost fluid rotates faster than the outermost, but with less angular momentum—conditions that approximate the Keplerian trajectories ubiquitous in astrophysics. Their data suggest that such flows, long presumed laminar, might actually become turbulent at large Reynolds numbers. If so, that would help to explain the behavior of accretion disks and other astrophysical flows. (D. P. M. van Gils et al., Phys. Rev. Lett., in press; M. S. Paoletti, D. P. Lathrop, Phys. Rev. Lett., in press; image courtesy of Dennis van Gils.)—Ashley G. Smart

Trapping antihydrogen

Posted: December 6th, 2010, 10:51am MST by Physics Today

Are there any unexpected differences between matter and antimatter? The international ALPHA collaboration has taken an important step toward answering that question by constructing an apparatus at CERN that can confine freshly made atoms of antihydrogen, the bound state of an antiproton and a positron, for nearly 0.2 seconds—long enough for the antimatter to be examined spectroscopically. A hot plasma of roughly 10⁴ antiprotons—produced by slamming 26-GeV protons into a metal target—is cooled and introduced into one end of the apparatus, while about 10⁶ low-energy positrons from the decay of radioactive sodium are introduced into the other. Electric fields gently nudge the charged species together in the heart of the device, pictured here, where they mix at cryogenic temperatures and form antihydrogen. If their kinetic energies are low enough—in temperature units, less than 0.5 K—the antihydrogen atoms are held in the grip of a superconducting octupole magnet and solenoidal “mirror” coils that together interact with the atoms’ magnetic moments. When the magnetic fields are abruptly turned off, the atoms are released and their spatial distribution captured by a three-layer silicon detector, which locates the atoms’ annihilations and distinguishes them from events triggered by lone antiprotons and stray cosmic rays. In 335 trial runs, the researchers confirmed that 38 antihydrogen atoms had survived in the trap for at least 172 ms. Although the trapping rate per atom produced is low—about 10⁻⁵—the achievement sets the stage for precision spectroscopy and antihydrogen tests of fundamental symmetries and gravitation. (G. B. Andresen et al., Nature 468, 673, 2010.)—R. Mark Wilson

Probing water under tension

Posted: December 2nd, 2010, 8:45am MST by Physics Today

It’s easy to think of water as a typical liquid, but many aspects of its behavior are highly atypical. For example, its freezing and boiling points are high compared to those of similarly sized molecules. To better understand water’s anomalous nature, and to test models that connect its microscopic and macroscopic properties, researchers look at the liquid’s behavior under tension, or negative pressure. When liquid water, with no air or vapor present, is forced to occupy a larger and larger volume, it enters a metastable state: The water could lower its potential energy by forming a vapor bubble that allows the liquid to shrink to a smaller volume, but an energy barrier bars the bubble’s formation. However, negative-pressure experiments present a mystery of their own: Different methods of stretching water achieve vastly different maximum tensions before vapor bubbles form. To try to resolve the discrepancy, researchers at the École Normale Supérieure in Paris measured the equation of state (the relationship between temperature, pressure, and density) of stretched water. Working at constant temperature, they launched intense ultrasound waves from a hemispherical piezoelectric transducer (the red element in the photograph), thereby creating a region of strongly oscillating positive and negative pressure at the sphere’s center. There, they performed two independent optical measurements (using the horizontal green beam and the vertical optical fiber) from which they deduced the pressure and density. But the results pose even more questions: The ultrasound method can’t generate tensions beyond −30 MPa before vapor bubbles form, but the measured equation of state agrees with models predicting much greater tensions should be possible. (K. Davitt et al., J. Chem. Phys. 133, 174507, 2010.)—Johanna Miller

Heaviest neutron star

Posted: November 29th, 2010, 9:22am MST by Physics Today

With diameters of less than 30 km, neutron stars harbor the densest matter in the cosmos. Much of it is surely just neutrons packed together at nuclear densities. But there has been lively speculation that more exotic phases—nuclear matter with hyperons, Bose–Einstein condensates of mesons, or plasmas of free quarks—reside at neutron-star cores. Much of that speculation has now been laid to rest by just one precision mass determination at the National Radio Astronomy Observatory in Green Bank, West Virginia. The NRAO measurements of the millisecond pulse train from a rapidly spinning neutron star orbited by a white-dwarf companion revealed that the neutron star’s mass was a record 1.97 ± 0.04 times that of the Sun. This unexpected record mass exceeds the neutron-star mass limits prescribed by almost all of the proposed equations of state that yield the exotic phases. The precision measurement exploits an unusually strong manifestation of a general relativistic effect first pointed out by Irwin Shapiro in 1964: Light is not only bent but also delayed as it passes near a massive object. In this case, as shown in the figure, radio pulses along the line of sight from the neutron star are manifestly delayed once every nine days when they pass very close to the orbiting companion. The happy accident that the binary system’s orbits are seen almost perfectly edge-on makes possible the unusually clear Shapiro-delay signal that yields the precise measurement of both stellar masses. The white dwarf is about ¼ as heavy as the neutron star. (P. B. Demorest et al., Nature 467, 1081, 2010.)—Bertram Schwarzschild

Cell growth, gene expression, and their interdependence

Posted: November 22nd, 2010, 8:45am MST by Physics Today

If given ample food, bacteria proliferate exponentially. If given little or low-quality food, bacteria proliferate sluggishly. Behind that adjustable growth lies a complex web of interconnected molecular pathways, the elucidation of which is a major goal in biology. But, as Terence Hwa of the University of California, San Diego, and his collaborators demonstrate, you can accurately model the outcome of those complex reactions without knowing the details. Hwa and his colleagues based their model on experiments that revealed the relationships between proliferation rate, nutrient quality, and the ratio of RNA to protein inside the bacteria. That ratio, which is straightforward to measure, is significant because all proteins are made by RNA-rich ribosomes: The more ribosomes in a cell, the faster the cell can grow and multiply. The mathematical relations that emerged from the UCSD team's analysis are simple and linear, and they reflect how cells allocate resources. In a nutrient-rich environment, the cell can afford to synthesize ribosomes at the expense of metabolic proteins, but when nutrients are scarce, metabolic proteins are synthesized at the expense of ribosomes. The balance between the two classes of biomolecule determines the growth rate. To test their model over a broad range of parameter space, Hwa and his colleagues observed two kinds of mutant: one whose ribosomes translate more slowly and one that overproduces a useless protein. The model passed. Hwa likens it to Ohm's law, which enables electrical engineers to design circuits without knowing the band theory of electrical conduction. Given that mutant bacteria are used to make drugs and other useful chemicals, the UCSD model could prove similarly useful. (M. Scott et al., Science 330, 1099, 2010.)—Charles Day

The highs and lows of graphene's strength

Posted: November 18th, 2010, 10:42am MST by Physics Today

Since Andre Geim and Konstantin Novoselov first touched off the graphene “gold rush” in 2004—their pioneering work earned them this year’s Nobel Prize in Physics—researchers have been pursuing ways to scale up its production. Among graphene’s remarkable properties is its roughly 100-GPa tensile strength, which is 40 times greater than the value for steel. That, however, is for defect-free graphene sheets; when formed by chemical vapor deposition, a proven industrial technique, graphene sheets contain crystallites separated by grain boundaries (see the news story in Physics Today, August 2010, page 15). Now, a computational study by Rassin Grantab and Vivek Shenoy at Brown University and Rodney Ruoff at the University of Texas at Austin reveals that graphene sheets with highly misaligned boundaries are actually stronger than slightly misaligned ones. As the image shows, misaligned grain boundaries consist of repeating pairs of 5- and 7-member rings separated by hexagonal rings. In simulations of the stress–strain curves as a function of the misalignment, the researchers found that, surprisingly, tensile strength increases with increasing misalignment angle. According to their model, stress failure begins at critical bonds within the 7-member rings; and critical bond length, which decreases with increasing misalignment angle, is proportional to initial material strain. In one simulation, a graphene sheet with a boundary angle of 28.7° and strained by 15% resisted stress up to 95 GPa; conceivably, it might be more efficient for researchers to engineer controlled defects into a graphene sheet rather than trying to make a perfect one. (R. Grantab, V. B. Shenoy, R. S. Ruoff, Science 330, 946, 2010.)—Jermey N. A. Matthews

Positronium in confinement

Posted: November 15th, 2010, 8:15am MST by Physics Today

Porous materials have numerous scientific and technological uses, including catalysis, filtering, and, it turns out, atomic physics. By confining atoms within pores, one can reduce the effects of various mechanisms that broaden spectral lines. Moreover, porous materials can be very efficient for making positronium (Ps). As incident high-energy positrons scatter off the material's nuclei and electrons, they slow down and can bind with one of the electrons. The resulting Ps atoms subsequently diffuse into the pores, where they get trapped, further thermalize, and can have long lifetimes, upward of 50 ns. David Cassidy, Allen Mills, and colleagues at the University of California, Riverside and San Diego State University have now looked at the spectroscopic consequences of Ps confinement, in particular the effect on the 1S–2P Lyman-α transition. Working with a silica film with pores on the order of 5 nm, they compared the spectral lines of Ps atoms trapped in the film with those of Ps atoms that had escaped the film. The team found that the line is significantly shifted and that confinement indeed narrows the width. Calculations incorporating confinement's effect on not only a Ps atom's orbitals but also its center of mass explained the energy shift: Whereas the 1S state tends to keep away from the walls, the 2P state tends to fill cavities smaller than about 5 nm, and the wall interactions raise the state's energy. (D. B. Cassidy et al., Phys. Rev. Lett., in press.)—Richard J. Fitzgerald

The evolving structure of global trade

Posted: November 11th, 2010, 9:41am MST by Physics Today

To a significant extent, the cells, tissues, organs, and so forth that make up a living organism act independently as they perform their related tasks. According to spin-glass models of evolution, modular structures analogous to those in biological systems generically arise for systems in changing environments. Moreover, such structures help ensure that a system is better able to cope with changes to come. Now Michael Deem of Rice University and his student Jiankui He have applied those conclusions to global trade. To do so, they came up with a parameter—the cophenetic correlation coefficient—that quantifies the modularity of the global trade network. As the figure shows, the system becomes more modular (the CCC increases) in response to environmental change—in this case, global recession. (The red bars indicate more severe recessions; the green bars, less severe. Data for the 2008 recession were not available.) But the overall trend during the past 40 years has been a mostly steady decline in the CCC as insular trade blocs—the modules—dissolved in favor of freer trade. According to evolution theory, and in contrast to much current economic thinking, the decreasing modularity implies that the global trade network is becoming less resistant to recessionary shocks such as a 1% dip in the US gross domestic product. And indeed, looking back at US recessions that have occurred during the past 30 years, Deem and He find that the most recent ones have had the greatest and longest lasting global impact. (J. He, M. W. Deem, Phys. Rev. Lett. 105, 198701, 2010.)—Steven K. Blau

Single-molecule absorption three ways

Posted: November 8th, 2010, 8:31am MST by Physics Today

Since its advent 20 years ago, room-temperature optical detection of single molecules has found application in biology, materials science, and other fields. Typically, the molecules are detected by their fluorescence, but not all molecules fluoresce. Now, three groups have independently detected single molecules by their optical absorption. Each group used a different technique. Vahid Sandoghdar and his colleagues at ETH Zürich in Switzerland measured the absorption directly, by detecting the minute intensity change of a laser beam passing through the sample. Michel Orrit and his colleagues at Leiden University in the Netherlands used a two-laser technique based on the photothermal effect: The molecule to be detected absorbed photons from the first laser and converted their energy to heat, thereby changing the refractive index in the surrounding material. The researchers detected the scattering of a second laser beam off that refractive-index inhomogeneity. And Sunney Xie and his colleagues at Harvard University used a different two-laser technique called ground-state depletion microscopy. One laser, whose amplitude was modulated at 1.75 MHz, repeatedly pumped the molecule out of its ground state, which caused the absorption of a second laser to be modulated at the same frequency. By isolating the high-frequency fluctuations of the second laser, the researchers separated the absorption signal from the laser’s inevitable intensity noise. (P. Kukura, M. Celebrano, A. Renn, V. Sandoghdar, J. Phys. Chem. Lett 1, 3323, 2010; A. Gaiduk, M. Yorulmaz, P. V. Ruijgrok, M. Orrit, Science 330, 353, 2010; S. Chong, W. Min, X. S. Xie, J. Phys. Chem. Lett. 1, 3316, 2010.)—Johanna Miller

Dueling theories for electricity-breathing microbes

Posted: November 4th, 2010, 9:20am MDT by Physics Today

The bacterium Shewanella oneidensis enjoys a unique respiratory versatility: not only can it use oxygen as a terminal electron acceptor, as do humans, but it can also transfer electrons to extracellular minerals such as iron oxides. Researchers hope to use that flow of electrons to, say, power microbial fuel cells or remediate soil contaminants, but they remain conflicted as to how the feat is carried out. Two theories have emerged: that microbes secrete shuttle molecules that diffuse to a metal’s surface, deposit electrons, and then return to start the process anew; and that on contacting a suitable metal, they pass electrons directly across the cell membrane. A team led by Charles Lieber (Harvard University) and Bradley Ringeisen (US Naval Research Laboratory) has produced new evidence in favor of the shuttle hypothesis. The researchers conducted a tiny fuel cell experiment with two types of nanoelectrodes—one exposed via openings too small for microbes to access, shown in the figure, and the other with large, accessible openings. Both yielded roughly equal current. Meanwhile, experiments by Mohamed El-Naggar (University of Southern California), Yuri Gorby (J. Craig Venter Institute), and colleagues support the direct contact mechanism. Applying voltage drops along the bacteria’s pili—long filament-like appendages that might attach to metal surfaces—they observed lengthwise conductivities sufficient to keep pace with bacterial respiration. It’s possible, maintain both teams, that microbes can choose either strategy based on the demands of their environment; the groups are now collaborating to find out. (X. Jiang et al., Proc. Natl. Acad. Sci. USA, 107, 16806, 2010; M. Y. El-Naggar et al., Proc. Natl. Acad. Sci. USA, 107, 18127, 2010.)—Ashley G. Smart

Injecting picoliter-volume reagents into droplets

Posted: November 1st, 2010, 11:23am MDT by Physics Today

Shuttled around a microfluidic circuit, water droplets can serve as test tubes in which controlled chemical reactions occur. That microenvironment has proven useful in applications such as genetic sequencing and the screening of huge libraries of molecular compounds in the search for new drugs. The central challenge in reactor applications is controllably adding reagents to the drops, which is particularly problematic in those cases where drops are coated with an oily surfactant layer to ensure that the small chemical factories remain isolated from each other. A Harvard University team led by David Weitz has now designed a pressurized microfluidic system that injects, with subpicoliter precision, an aqueous reagent into individual drops in the presence of an electric field. The figure illustrates the technique. As tightly packed drops pass through a narrow channel in single file, carrier oil is added to space them apart. Downstream, a reagent-filled injector channel tapers to a small opening and connects to the main channel. The watery reagent and oily carrier fluid are immiscible, and interfacial tension keeps the reagent, except for a small bulge, out of the main channel. As a drop flows under the bulge and voltage is applied to the electrodes, the electric field destabilizes the drop’s oil–water interface and reagent flows into it. By adjusting the drop’s velocity, the injection pressure, and the rate at which the field is switched on and off, the researchers can control how much reagent is added—between 0.1 and 3 pL in their device—and selectively target particular drops. (A. R. Abate et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1006888107.)—R. Mark Wilson

Radio pulses from cosmic rays

Posted: October 28th, 2010, 9:29am MDT by Physics Today

Ultrahigh-energy cosmic rays (UHECRs), with energies exceeding 10¹⁹eV, are rare. A random square kilometer on Earth is hit by a few dozen per century. But their trajectories, relatively undeflected by intergalactic magnetic fields, might reveal their enigmatic sources. So astrophysicists build enormous ground-detector arrays. But even the flagship 3000-km² Pierre Auger Observatory in Argentina, which has been recording UHECRs since 2004, has not yet harvested enough to yield robust evidence of anisotropy that might reveal a class of sources (see Physics Today, May 2010, page 15). Now a serendipitous discovery by the ANITA collaboration opens the prospect of surveilling much larger areas. While the balloon-borne ANITA radio detector was circumnavigating Antarctica at an altitude of 37 km, searching for signs of cosmogenic neutrinos interacting in the ice, it found something quite unexpected. In less than three weeks of monitoring for the nanosecond radio pulses expected from neutrino interactions, ANITA recorded 16 pulses whose polarization and phase characteristics convincingly marked them as coming from atmospheric UHECR showers. Interferometry with the gondola’s antenna array fixed each shower’s direction to within about 2°, revealing that most of the radio pulses had been reflected off the ice (at the red diamonds on the map). But two came directly from atmospheric showers (at the black squares). In either case, one learns the cosmic ray’s incident celestial direction. Year-round radio surveillance from an orbital platform that monitors reflections from the open oceans as well as the polar ice should greatly enhance the rate at which the highest-energy cosmic rays are harvested. (S. Hoover et al., Phys. Rev. Lett. 105, 151101, 2010.)—Bertram Schwarzschild

A new twist on magnetic tweezers

Posted: October 25th, 2010, 10:20am MDT by Physics Today

Like their optical-tweezer cousins, magnetic tweezers have become a standard tool for stretching individual biological molecules to gain insight into their physical properties and behavior. Magnetic tweezers (and optical ones, too) can also apply torques to rotate or twist specimens. Last year, Sean Sun of the Johns Hopkins University and colleagues developed a technique to control and measure the applied torque with magnetic tweezers. In their approach a biomolecule, such as DNA, was connected to the middle of a 2-µm-long nanorod that was magnetically latched at one end to a superparamagnetic bead. In a dipole magnetic field, the bead held the nanorod and the top of the molecule in place while the experimenters introduced a controlled amount of twist by rotating the substrate bound to the molecule's other end. Nynke Dekker and her coworkers at the Delft University of Technology have now presented a new take on magnetic torque tweezers, this time using standard spherical beads. A nonmagnetic bead 1 µm in diameter served as a landmark on the 2.8-µm-diameter superparamagnetic bead to which it was attached (a third, nonmagnetic reference bead corrected for mechanical drift). The larger bead was tethered to a substrate by a single DNA molecule, and the beads were placed in a slightly asymmetric dipole magnetic field. Rotating the magnet twisted the DNA, and by monitoring the beads' orientation the team could extract the DNA's torsional stiffness and the torque. With their tweezer setup, Dekker and colleagues could document torque-induced twisting, buckling, and denaturing of DNA under a wide range of torques and stretching forces and could study protein–DNA interactions. (J. Lipfert et al., Nat. Meth., in press, doi:10.1038/nmeth.1520.)—Richard J. Fitzgerald

Braking winds over a roughening Earth

Posted: October 21st, 2010, 9:44am MDT by Physics Today

Despite global warming, the rate of water evaporation over land surfaces has steadily declined in the past few decades. That unexpected trend, observed by farmers and climate-change scientists alike, has been linked to a decline in surface wind speeds over the same period. The challenge of quantifying the stilling-winds phenomenon on a global scale was recently taken up by Robert Vautard and his colleagues at the Climate Science and Environment Laboratory in France and the European Centre for Medium-Range Weather Forecasts in the UK. By analyzing data from more than 800 weather stations, the researchers found that 73% reported that wind speeds measured 10 meters above the ground had declined by up to 15% from 1979 to 2008. As the image shows, some regions in Europe experienced declines of as much as 5 m/s per decade. After studying climate-model simulations, the researchers attribute much of the slowdown to an increase in topographical surface roughness from a surge in vegetation growth induced by excess atmospheric carbon and also anthropogenic activities such as urbanization. Less of a slowdown, or even an increase, was seen in regions that did not experience significant vegetation growth. The analysis assigned a lesser role to reduced atmospheric circulation caused by global warming. Wind energy enthusiasts should not necessarily be worried, say the researchers, since most wind turbines operate at 50–100 m, where the analysis did not detect any noticeable trend. (R. Vautard et al., Nat. Geo., in press, doi:10.1038/ngeo979.)—Jermey N. A. Matthews

Fluid flow around swimming microorganisms

Posted: October 18th, 2010, 7:30am MDT by Physics Today

As tiny creatures swim, the flows they create in their liquid environment can affect the motion of their neighbors and the viscosity of the ambient suspension. Those flows have been mathematically modeled, but, until recently, they had not been imaged near a swimming microorganism. The feat has now been accomplished by two research teams working independently, one at the University of Cambridge in the UK and one at Haverford College in the US. Both obtained flow fields by taking sequential photographs of microscopic tracer particles displaced by the alga Chlamydomonas reinhardtii, which swims by executing a breaststroke-like motion with its two approximately 10-µm-long flagella. The image, from the Cambridge group, shows the streamlines created by the swimming alga, averaged over a stroke cycle (left). That fluid flow, the researchers note, is well described by a simple model (right) in which C. reinhardtii exerts two point forces to balance the drag force on its body. (The Cambridge team also studied the larger alga Volvox carteri.) The Haverford collaboration focused on the changes in the alga’s flow field during the course of a swimming cycle (see the accompanying video). The group discovered, for example, that as it carries out its forth-and-back motion, C. reinhardtii expends about four times the energy it would if it could swim steadily along its trajectory. (K. Drescher et al., Phys. Rev. Lett. 105, 168101, 2010; J. S. Guasto, K. A. Johnson, J. P. Gollub, Phys. Rev. Lett. 105, 168102, 2010.)—Steven K. Blau

Iron's structure at the temperatures and pressures of Earth's core

Posted: October 14th, 2010, 1:00pm MDT by Physics Today

A team from Japan has measured the crystal structure of iron under conditions that prevail in Earth's solid inner core—that is, at temperatures and pressures higher than 5000 K and 300 GPa. To reach those extreme values, Shigehiko Tateno and Kei Hirose of the Tokyo Institute of Technology and their collaborators placed Fe powder inside the 20-μm-wide cell of a diamond anvil. Tightening the anvil's screw squeezed the sample to pressures up to 377 GPa, while two 100-W ytterbium fiber lasers raised the sample's temperature as high as 5700 K. Placing the cell in a beamline at the SPring-8 synchrotron in Sayo, Japan, yielded the structural information and enabled the team to fill in the uncharted top corner of Fe's phase diagram. Under ambient conditions, Fe adopts a body-centered cubic (bcc) structure (the red region in the bottom left corner of the phase diagram). As the temperature increases, the pressure needed to forestall melting increases too. Previous measurements (solid diamonds) had shown that Fe switches from a bcc to a hexagonal close-packed (hcp) structure (blue region) at modest temperatures and pressures. That the hcp structure survives at inner core conditions has now been established by the SPring-8 measurements (open symbols). If Fe in Earth's inner core really is hcp, then the lengthening of the crystal's c-axis parallel to Earth's rotation would naturally account for a certain anomaly in seismic signals that pass through the core. (S. Tateno, K. Hirose, Y. Ohishi, Y. Tatsumi, Science 330, 350, 2010. )—Charles Day

Time dilation seen at just 10 m/s

Posted: October 11th, 2010, 9:32am MDT by Physics Today

There’s no reason to think that the lengthening of time predicted by the theory of relativity does not hold even for everyday speeds, but the effect is so minuscule that it has taken two of the world’s most accurate optical clocks to measure it. Trapped at the heart of each clock is an aluminum ion: Displacing the ion in one clock just slightly from the trap’s center induces a relative average speed difference between the two clocks’ ions of 10 m/s (22 mile/hr). The resulting fractional frequency difference is on the order of 10⁻¹⁶. That small shift was measured recently by James Chin-Wen Chou and his colleagues at NIST in Boulder, Colorado, using NIST’s newest optical clock, whose accuracy is 8.6 x 10⁻¹⁸, and a slightly less accurate older clock. The group also measured the frequency difference between the clocks’ ions when one clock was raised by 33 cm relative to the other. The measurements not only demonstrate the high performance reached by optical clocks but also show that they may play an important role in geodesy, the measurement of Earth’s gravitational potential. The time keepers might be sensitive to elevation changes as small as 1 cm if they can attain the current goal of 10⁻¹⁸ accuracy. Such measurements would complement those of satellite-borne instruments, which also have 1-cm sensitivity but average over large areas of Earth’s surface. (C. W. Chou et al., Science 329, 1630, 2010.)—Barbara Goss Levi

Sampling a network to catch the flu

Posted: October 7th, 2010, 9:21am MDT by Physics Today

Current methods to monitor flu epidemics rely on contemporaneous data from random people seeking health care. But in social networks, some members are more connected than others, and theorists have long known that the central-most hubs, by virtue of their greater exposure, are more likely to catch and spread a contagion (see the article by Mark Newman in Physics Today, November 2008, page 33). The question is how to find the hubs. A phenomenon known as the friendship paradox—“your friends have more friends than you do”—offers a strategic path. To appreciate the paradox, proposed by sociologist Scott Feld in 1991, imagine a group of randomly chosen people, each asked to name a friend. Extroverts are named more often than loners, and the nominated friends will have, on average, more social ties than the nominators. How many more depends on the variance in the distribution of ties. Nicholas Christakis of Harvard University and James Fowler of the University of California, San Diego, have now tested the paradox as a basis for early detection of an outbreak. After the H1N1 epidemic emerged last year but before vaccines were available, they divided 744 Harvard undergraduates into two groups. As shown in the figure, of the 8% eventually infected, those in the friend group tended to get the flu earlier than those in the random group. The trend was discernable a full 46 days prior to the epidemic’s peak, a result that sheds light on spreading processes in networks and how to anticipate them. (N. A. Christakis, J. H. Fowler, PLoS One 5, e12948, 2010.)—R. Mark Wilson

Andre Geim and Konstantin Novoselov win 2010 physics Nobel for graphene

Posted: October 5th, 2010, 6:03am MDT by Physics Today

Andre Geim and Konstantin Novoselov are the winners of this year's Nobel Prize in Physics. Six years ago the two researchers discovered how to make graphene, a honeycomb sheet of carbon atoms just one atom thick. Both researchers are based at the University of Manchester in the UK, where they did the prize-winning work.

Geim and Novoselov's method is simple and cheap. By applying Scotch tape to graphite, they could pull off thin flakes that consist of one, several, or many layers of graphene. To locate the rare one-layer flakes, they took advantage of an optical effect: If the flakes are deposited on silicon dioxide substrate of just the right thickness, one-layered graphene reveals itself through interference fringes.

Thanks to its two-dimensionality and to the symmetry and strength of its lattice, graphene has a host of fascinating electronic properties. Theorists had anticipated some of them decades ago, but by showing physicists that making the material is feasible and straightforward, Geim and Novoselov touched off, and participated in, an explosion of experimental and theoretical work.

The feverish activity continues. As of today, 1476 papers with "graphene" in the title have appeared in Physical Review Letters, the world's most prestigious physics journal. All but 21 of them came out after Geim and Novoselov's 2004 discovery paper.

Interest in graphene isn't limited to its fundamental properties. The material is also a candidate for replacing silicon as a basis of faster, more powerful electronics. Already, 343 papers about graphene have appeared in Applied Physics Letters. Carbon-based electronics is an active area of research at IBM, Samsung, and other device manufacturers.

Opportunity, serendipity, and luck

In an interview last year, Novoselov recounted how he and Geim made their discovery. The project began as a long-shot attempt to find a metallic semiconductor. On paper, graphene was a promising candidate. The challenge was to make it.

The first step toward that goal occurred when a member of Geim's lab, Oleg Shkliarevskii, reminded Geim and Novoselov that he and his fellow electron microscopists routinely make thin samples by applying Scotch tape to a material then peeling it off.

The next, and crucial step, occurred by chance. As a mount for the peeled-off graphite flakes, Geim and Novoselov chose a 300-nanometer-thick substrate of silicon dioxide. The thickness is fairly standard, but, as the researchers were to find out later, if it had been off by more than just 5%, they would not have seen the revelatory interference.

One of the first experiments that Geim and Novoselov did was to confirm graphene's most important electronic property: the unusual relationship between the energy and momentum of its charge carriers. In a crystal, the combination of the atoms' energy levels and the lattice's three-dimensional structure compels electrons to occupy bands in an energy-momentum diagram. For a given energy, only certain values of momentum are allowed.

Silicon and other semiconductors have more or less the same band structure: a valence band, in which electrons are tied to their atoms, and a more energetic conduction band, in which electrons can move freely as if they were in a metal. A narrow energy gap of varying width separates the two bands. At low temperature, the most energetic electrons are stuck in the valence band. But heat or voltage can give them enough extra energy to jump across the gap, turning the material from an insulator into a conductor.

Graphene's band structure consists not of wavy, gap-separated bands but of two cones—one upright, the other upside down—that meet at their apexes. The cones' straight sides imply that the electrons will behave like massless particles and whizz through the material ballistically, as if they were photons travelling in free space.

Geim and Novoselov confirmed graphene's band structure by measuring its conductivity as they varied a voltage applied perpendicular to the sheet. Other experiments followed, including the demonstration that graphene exhibits a quantum Hall effect at room temperature.

One of graphene's surprising properties is mechanical. Theory says a sheet of material one atom thick is unstable above absolute zero. The slightest amount of thermal energy causes the sheet to buckle. Graphene is no exception, but the carbon–carbon bonds are strong enough to limit the buckling to waves no higher than 10 nm.

It's too early to say whether graphene could end up being useful. Exploiting its unusual electronic properties could prove too difficult to pull off in a cost-effective way. Still, the research that Geim and Novoselov's discovery spawned has been remarkably diverse and fruitful.

When asked about what he'd tell the public about his work, Novoselov replied: "That science should be fun, and you don’t always need to do expensive multi-million dollar experiments to be on the cutting edge of research."

Charles Day

Photonic-crystal slab for biodetection

Posted: October 4th, 2010, 9:23am MDT by Physics Today

The public-health and biosecurity communities need biosensors that are sensitive, operate in real time, and can be easily deployed. Many approaches are being pursued, including one by a group from Lawrence Livermore National Laboratory that uses a photonic crystal (PC) slab connected to two waveguides. The idea is for viruses or other tiny pathogens to randomly infiltrate the pores of a silicon PC, whose optical properties—specifically, the in-plane transmission spectrum's band edge—change accordingly. First, the researchers used simulations to determine a PC geometry suitable for a specific virus, vaccinia, and for their laser's wavelength. They then fabricated an appropriate 17 × 17 array of 280-nm pores and exposed it to a flux of polystyrene beads with two different sizes; those with 260-nm diameter entered the pores (see the figure, with empty and filled pores enlarged) while 320-nm ones did not. The measured band-edge redshifts were then used to calibrate the simulations and predict detection limits. Theoretically, as few as 10 vaccinia viruses could be detected with the PC, comparable to other biodetection methods. Advantages accrue, however, from the small sensor size, the ability to tune the geometry for different particles, and the ease of integration into lab-on-a-chip setups. The authors say that their random-binding scheme is more practical than methods that rely on binding organisms to single PC defects. (S. E. Baker et al., Appl. Phys. Lett. 97, 113701, 2010.)—Stephen G. Benka

Particles hitch a ride inside a beam of light

Posted: September 30th, 2010, 8:01am MDT by Physics Today

Since its introduction in the 1970s, optical trapping has become a mainstay in science research. It has been used to tug at strands of DNA, to levitate aerosols, and to cool atoms to microkelvin temperatures. It has not, however, been very effective at manipulating light-absorbing particles, particularly in air or other gases. That's because conventional traps rely on radiation pressure, the momentum imparted by refracting light, to pull a particle into an optical beam. But photophoretic forces, which result from the imbalanced momentum exchange between an anisotropically heated particle and its surrounding gas, tend to push an absorbing particle away from intense light. Andrei Rode and colleagues at the Australian National University in Canberra have now developed a technique that exploits photophoretic forces to confine and move light-absorbing particles in air. And unlike conventional techniques that manipulate particles over just millimeter distances, theirs could conceivably send particles gliding across an entire room. The key was to use a Laguerre–Gaussian beam, in which a donut-like ring of light surrounds a dark core. A particle that finds its way into the core, such as the 50-µm carbon-coated glass sphere shown here, gets trapped in the transverse (x–y) plane and propelled in the direction of light propagation (z) by photophoretic pushes. Two beams aimed from opposite directions can hold a particle in place, a potentially useful technique to isolate particles such as carbon soot—a suspected global warming contributor—for sensitive measurements. (V. G. Shvedov et al., Phys. Rev. Lett. 105, 118103, 2010.)—Ashley G. Smart

Oxygen in ancient oceans

Posted: September 27th, 2010, 10:26am MDT by Physics Today

Oxygen in the air and dissolved oxygen in the ocean are crucial for sustaining animal life on Earth. But that oxygen wasn’t always there. Researchers can gain some information about the history of ocean oxygenation from the compositions of sedimentary rocks, because dissolved O2 affects the solubility and precipitation of certain elements. Now, an international team of researchers led by Tais W. Dahl (University of Southern Denmark) has studied the problem using a new technique based on molybdenum isotopes. In oxygenated water, Mo forms the soluble molybdate ion, MoO4²⁻, and precipitates in small quantities by a mechanism that favors lighter isotopes. But when hydrogen sulfide is present instead of O2, all Mo isotopes precipitate quickly. By analyzing the Mo in 180 rock samples with ages spanning almost 2 billion years, the researchers concluded that O2 levels increased twice: 550 million years ago and 400 million years ago, as shown in the figure. The first increase coincides with the emergence of the first motile animals, and the second coincides with the evolution of much larger animals including large predatory fish. The method does not provide a quantitative measure of O2 levels over time. Still, based on the physiological requirements of fish, the researchers suggest that prior to 400 million years ago the ocean and atmosphere contained O2 at just 15–50% of their present levels, which means that the earliest animals survived with much less oxygen than we breathe today. (T. W. Dahl et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1011287107.)—Johanna Miller

Synchronized cameras catch messenger RNA on the run

Posted: September 23rd, 2010, 11:35am MDT by Physics Today

Messenger RNA (mRNA) is the shuttle that carries genetic information across a cell’s nuclear membrane and into the cytoplasm, where the information is translated into a protein sequence. However, the movement of mRNA, which is about 25 nm in diameter, through the nuclear pore complex, roughly 120 nm in diameter, has been difficult to resolve visually since both mRNA and the NPC are well below the 200-nm diffraction limit for optical microscopes. Now, Robert Singer at Yeshiva University’s Albert Einstein College of Medicine in New York and David Grünwald at the Delft University of Technology in the Netherlands have developed a new nanometer-resolution imaging technique that they used to track mRNA’s passage. Emission signals from mRNA and the NPC—labeled with spectrally different fluorescent probes and shown in the image as green and red, respectively—were chromatically separated and tracked by two synchronized high-speed CCD cameras. The researchers achieved 26-nm spatial resolution by resolving the misalignment between the mRNA and the NPC in the images collected before and during tracking by both cameras—a technique they’ve dubbed super-registration microscopy. In their experiments, they also observed that mRNA spent about 5-20 ms crossing the NPC, a fraction of the time it spends at the pore’s entrance and exit—as if, say the researchers, the mRNA was being double screened for quality. That information may support research into understanding how defective mRNA is prevented from escaping the nucleus. (D. Grünwald, R. H. Singer, Nature, in press, doi:10.1038/nature09438.)—Jermey N. A. Matthews

Related link:

Robert Singer explains super-registration microscopy

Efficiency bounds for powerful engines

Posted: September 20th, 2010, 8:30am MDT by Physics Today

Thermodynamics teaches that the efficiency of a heat engine operating between a hot reservoir at temperature Th and a cold one at Tc can be no greater than the Carnot value ηC = 1 - Tc/Th. To achieve its theoretical maximum, the engine must run infinitely slowly and generate zero power—surely an unsatisfactory state of affairs in the real world. Now Massimiliano Esposito (Free University of Brussels) and colleagues have derived efficiency bounds for engines operating at maximum power. They assume that the engine operates in a Carnot cycle and interacts with the hot reservoir for a finite time τh, presumed much greater than the duration of the adiabatic steps. They then express entropy as a sum of the standard term of heat over temperature and a term of the form ah/τh (the cold reservoir is treated analogously); placing the interaction time in the denominator ensures that the reversible-process result is obtained in the infinite-interaction-time limit. After deriving the maximum power as a function of the interaction times, Esposito and company can readily calculate the efficiency, which depends in particular on ac/ah. The figure shows the allowed range of efficiencies at maximum power. The upper bound corresponds to ac/ah = 0; the lower bound to an infinite ratio. The points in the figure give observed efficiencies for several heat engines worldwide. Those engines may not satisfy the assumptions of the Esposito model or even run in Carnot cycles; still, their efficiencies lie within or near the idealized bounds. (M. Esposito et al., Phys. Rev. Lett., in press; available at [arxiv.org] .)—Steven K. Blau

Putting quantum gases under the microscope

Posted: September 14th, 2010, 6:57am MDT by Physics Today

The recently demonstrated ability to image single atoms in an ultracold quantum gas is welcome news to those who hope to use the relatively clean and more highly controlled atomic systems to simulate complex electronic interactions, such as superconductivity, that take place in solids. The simulations would be based on optical lattices, an array of potential wells that hold atoms in much the same way as the potential wells of a solid’s atomic structure hold electrons. Last year, Markus Greiner and his colleagues at Harvard University built a fluorescent microscope that was a step ahead because it not only could see individual atoms in an optical lattice but could distinguish very closely spaced atoms, which are necessary to simulate the strong electronic interactions that researchers most want to explore. Now, two groups have used such high-resolution microscopes to study a quantum gas of atoms as it transitioned from a superfluid to a Mott insulator (MI) state in which particle interactions hinder hopping between sites. Images such as the accompanying pictures of the superfluid (left) and two types of MI (center, right) give insight not gleaned from less direct measurements. The experiments were done by Greiner and his group and by Stefan Kuhr, Immanuel Bloch, and colleagues at the Max Planck Institute of Quantum Physics in Garching, Germany. (W. S. Bakr et al., Science 329, 547, 2010; J. F. Sherson et al., Nature 467, 68, 2010.)—Barbara Goss Levi

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Diffraction-free beams in homogeneous media

Posted: September 12th, 2010, 3:29pm MDT by Physics Today

An axicon lens is flat on one side, conical on the other. After passing through the flat surface, a beam of light with a Gaussian profile emerges from the conical surface with a profile that looks approximately like a zeroth-order Bessel function. Bessel beams interest physicists because of a remarkable property: Thanks to the way the beams' energy and phase components are spread in space, the beams barely diffract—even around certain obstacles. Now, Florian Fahrbach, Philipp Simon, and Alexander Rohrbach of the University of Freiburg in Germany have demonstrated that Bessel beams can propagate without significant diffraction through inhomogeneous media. The accompanying figure shows the intensity of fluorescence emitted by a transparent fluorescent gel when three kinds of beam passed through from left to right: a sheet-shaped beam from a cylindrical lens, a Gaussian beam scanned from top to bottom, and a Bessel beam scanned from top to bottom. The different patterns arose from the beams' interactions with hundreds of 2-μm-diameter glass beads that the Freiburg researchers had dispersed throughout the gel. As the light sheet and Gaussian beam encountered the beads, their scattered components interfered constructively, leading to bright streaks and therefore to what would amount to image artifacts. The Bessel beam had far less streaking. The researchers obtained similar results with a sample of fresh human skin, whose densely packed cells, like those of other biological tissues, are micron sized. Bessel beams could therefore form the basis of a new kind of biological microscopy. (F. O. Fahrbach, P. Simon, A. Rohrbach, Nat. Photon., in press.)—Charles Day

The physics of peeling paint

Posted: September 9th, 2010, 9:12am MDT by Physics Today

The complex mechanical properties of colloidal coatings are hard to measure because they vary spatially and temporally. Paint, for instance, starts as a fluid that, as the solvent evaporates, dries into a brittle solid that can crack and peel away from a substrate. To better understand the stresses that drive the fracture process, researchers led by Yale University’s Eric Dufresne have now adapted a technique from cell biology known as traction force microscopy. In the technique’s biological application, researchers observe a cell crawling across a rubber substrate and monitor the deformations within the rubber. Knowing the rubber’s mechanical properties, the researchers convert the displacement field into a stress field and deduce which parts of the cell exert force on the substrate. Dufresne and colleagues replaced the cell with a drying film of a silica-particle suspension, which they applied to a soft layer of silicone rubber that would deform as the film dried and cracked. To map those deformations and convert them to a three-dimensional stress field, the team monitored the motion of tiny, fluorescent tracers mixed into the rubber. In this plot of stress as a function of distance from the crack front, the normal stress (solid dots) shoots up rapidly just ahead of the crack front—with much greater magnitude than does the in-plane stress (open dots) and, reassuringly, with a scaling that roughly agrees with that predicted by classic fracture theory (red). (Y. Xu et al., Proc. Natl. Acad. Sci. USA 107, 14964, 2010.)—R. Mark Wilson

A piece of pristine Earth?

Posted: September 2nd, 2010, 8:50am MDT by Physics Today

Like a batch of cookie dough, Earth is constantly being stirred: Volcanism, tectonic-plate movements, and thermal convection can all transport material between and within the mantle and crust. Even the lower mantle is not immune to mixing: Studies of seismic waves’ speeds (see Physics Today, August 1997, page 17), which measure the temperature profile across Earth’s interior, show slabs of cool, dense oceanic crust sinking nearly to the core–mantle boundary. But now, Boston University’s Matthew Jackson and collaborators have found that rock samples from Baffin Island in northeastern Canada, the products of an enormous volcanic eruption some 60 million years ago, show all the signs of having come from an ancient mantle reservoir, unstirred since not long after Earth’s formation 4.5 billion years earlier. The researchers based their conclusion on isotopic measurements of several chemical elements, each with some isotopes produced by radioactivity and others that are purely primordial. If any mixing had occurred, it would have changed the relative amounts of a radioactive parent element and its daughter—and thus the daughter’s present-day isotopic composition, which is easier to measure than the parent–daughter ratio. If the Baffin samples did indeed come from a primitive reservoir, geophysical modelers will be challenged to explain how any part of the mantle could have remained so isolated for so long; one possible solution involves convective eddies that trap and preserve the pristine mantle material. (M. G. Jackson et al., Nature 466, 853, 2010.)—Johanna Miller

Resurrecting sterile neutrinos

Posted: August 30th, 2010, 8:54am MDT by Physics Today

An accelerator experiment at Los Alamos National Laboratory in the late 1990s reported the observation of neutrino flavor oscillation on a laboratory length scale—tens of meters. All prior oscillation observations had been over much larger distances. The disquieting implication of the LANL claim was that so small an oscillation length required the existence of a “sterile” neutrino flavor impervious to the weak interactions, which would clutter the attractively neat prevailing theory. So experimenters designed the MiniBoone facility at Fermilab to confirm the LANL result or lay it to rest. In 2007, the MiniBoone collaboration reported that its results were incompatible with the LANL claim. But now a new MiniBoone result suggests that the sterile neutrino’s obituary was premature. The new result looks a lot like the original LANL data. Like the LANL experiment, it was based on a beam of muonic antineutrinos. The 2007 refutation, however, was based on a muonic neutrino beam. The standard theory assumes that neutrinos and antineutrinos oscillate identically. But the new result, though not yet statistically robust, appears to show that antineutrinos, unlike neutrinos, do indeed oscillate on a distance scale that implies one or more sterile neutrino states. In fact, theorists are already considering how interference between two sterile states of different mass might explain such a neutrino–antineutrino asymmetry. The photo shows the photomultiplier tubes in the oil-filled MiniBoone detector that discern the flavors of neutrinos by recording the Cherenkov light of the charged leptons they create in collisions with nucleons. (A. Aguilar-Arevalo et al., [arxiv.org] .)—Bertram Schwarzschild.

An expandable molecular sponge

Posted: August 23rd, 2010, 2:14pm MDT by Physics Today

Zinc ions and some other metal ions can bind to three or four organic molecules at once. If those molecules are long and attach to zinc at both ends, it's possible to create a metal–organic framework (MOF), an open sheet of linked molecules with ions at the vertices. And if those sheets bind to each other and stack in register, the result is a material whose columnar pores can store, catalyze, or otherwise usefully process small molecules.

Matthew Rosseinsky and his coworkers at the University of Liverpool in the UK have made a MOF material, but with a new twist. For its linker, the Liverpool team used a dipeptide—that is, two peptide-bonded amino acids (glycine and alanine; see figure). The team made two versions of the material, one incorporating a solvent (a mix of water and methanol) and one not. X-ray diffraction and nuclear magnetic resonance spectroscopy revealed that adding the solvent caused the dipeptide linkers to straighten, widening the pores to accommodate the solvent ions. Glycine, alanine, and the 18 other naturally occurring amino acids are characterized by side chains that are polar, nonpolar, positively charged, or negatively charged. Given that variety, the Liverpool experiment suggests that peptide-based MOF materials might find uses as expandable sponges for a wide range of molecules. (J. Rabone et al., Science 329, 1053, 2010.)—Charles Day

Color-dependent cyclone activity

Posted: August 23rd, 2010, 11:10am MDT by Physics Today

Under the hot summer sun, the ocean’s surface waters become warmer than the atmosphere above them. As the heat is transferred to the atmosphere, it can strengthen low-pressure disturbances and drive the characteristic weather phenomena known in the Atlantic region as hurricanes and in the Pacific as typhoons or tropical cyclones (see the Quick Study on hurricane formation by Kerry Emanuel in Physics Today, August 2006, page 74). A new model from a research collaboration led by Anand Gnanadesikan at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in New Jersey shows how strongly correlated the sea surface temperature (SST) is to the ocean’s color. The image depicts average concentrations (in mg/m³) of chlorophyll—the green pigment in phytoplankton—from 1997–2000 in the Pacific Ocean, where more than half of the reported typhoon-force winds (greater than 32 m/s) occur. Considering an extreme scenario, the researchers set the concentration of chlorophyll to zero and then studied the evolution of cyclones in the North Pacific Ocean. Without chlorophyll to absorb much of the solar radiation, SSTs drop. Air over the colder water sinks, drying the atmosphere and increasing wind shear, which quenches typhoon formation. Although typhoon frequency increased by 20% along the equator, the simulation predicted an overall drop in the region—up to 70% for areas beyond 15 degrees north of the equator—and a decrease in frequency of the most intense typhoons. (A. Gnanadesikan et al., Geophys. Res. Lett., in press.)—Jermey N. A. Matthews

Plowing through a granular medium

Posted: August 19th, 2010, 9:00am MDT by Physics Today

Children frolicking in a sandbox probably don’t think about the drag forces exerted on their limbs as they displace grains of sand. But physicists Nick Gravish and Daniel Goldman (Georgia Tech) and Paul Umbanhowar (Northwestern University) do think about such forces. Now they have conducted a systematic study of how the drag force on a vertical plate partially submerged in sand-sized glass beads depends on the beads’ packing fraction ϕ. Their study reveals a surprising phenomenon: For a dense packing—that is, when ϕ exceeds a critical value ϕc—the drag force oscillates as the plate moves horizontally. The crucial physics, argue the authors, hinges on the phenomenon of dilatancy: densely packed beads can become less dense when sheared. Dragging a plate through a dense packing creates a “shear plane” that runs from the bottom edge of the plate to the surface of the beads and makes an angle θ with the horizontal. Particles near the shear plane tend to move parallel to it, toward the surface; particles beyond the plane hardly move at all (see the figure). Shear forces arising at the plane cause the local packing fraction to decrease, which makes it easier to move the plate. When the packing fraction dips to ϕc, the shear plane remains stationary at the surface even as its bottom edge moves with the plate; thus θ increases, which causes the drag force to also increase. Once the drag force is high enough, a new low-θ, high-ϕ shear plane forms, and the cycle repeats. Sandboxes, it seems, have pleasures to offer physicists and children alike. (N. Gravish, P. B. Umbanhowar, D. I. Goldman, Phys. Rev. Lett., in press.)—Steven K. Blau

Femtosecond snapshots of a powdered solid

Posted: August 16th, 2010, 8:52am MDT by Physics Today

X-ray diffraction has long been an important tool for finding crystal structures by mapping their electron densities. In recent decades, time-resolved x-ray diffraction has probed ever-faster structural changes in single crystals, including atomic motions on the femtosecond time scale. But many materials of interest, such as the transition metal complexes used in organic photovoltaic cells, can’t easily be made into crystals of sufficient size and quality. Now Michael Woerner, Thomas Elsaesser, and colleagues at the Max Born Institute in Berlin have demonstrated femtosecond x-ray powder diffraction, in which the sample is an ensemble of randomly oriented microcrystals of ammonium sulfate, (NH4)2SO4, and the diffraction pattern is composed of concentric rings rather than discrete peaks. The innovation was in engineering the x-ray source—a laser-driven plasma source that produced an ultrafast x-ray pulse from an equally brief optical pulse—to operate stably and at high repetition rate for long enough to reveal small changes in the diffraction ring intensities. From those changes, the researchers calculated the change in the sample’s electron density. As shown in the figure, which depicts the equilibrium electron density and the resulting changes over one slice through the crystal, electrons briefly pool (red blobs) where no nucleus exists in the equilibrium structure—so a nucleus, specifically a proton, must have migrated there. Ultrafast IR spectroscopy confirmed that NH4⁺ ions were reversibly breaking apart; surprisingly, the observed structural change bears no resemblance to either of ammonium sulfate’s known phase transitions. (M. Woerner et al., J. Chem. Phys., in press.)—Johanna Miller

Quantum effects turn pore diffusion on its head

Posted: August 11th, 2010, 3:42pm MDT by Physics Today

Separating isotopes is difficult business: Since an element’s various isotopes share similar size and shape, separation methods such as thermal diffusion and centrifugation tend to be time and energy intensive. Now, however, a team led by Suresh Bhatia (University of Queensland, Australia) has shed new light on what may prove an attractive alternative—nanoporous materials known as molecular sieves. Normally, molecular sieves aren’t particularly effective isotope separators. Take diatomic hydrogen and its isotopic relative deuterium. As one would expect, the lighter H2 molecules diffuse through the porous molecular sieve faster than the heavier D2, but the difference is slight. That picture changes, though, if the temperature is low enough—and the pores small enough—for quantum effects to set in. If the pore size is on the order of the molecules’ de Broglie wavelength, the molecules' zero-point energy becomes the important barrier for pore diffusion and small mass becomes a disadvantage. Not only does D2 then diffuse faster than H2, it can do so by a substantial margin. Using quasi-elastic neutron scattering and carbon molecular sieves with 3-Å-diameter pores, Bhatia and company directly measured those diffusivities, which differed by nearly an order of magnitude at the coldest temperatures. Although quantum sieve effects—first predicted nearly 15 years ago—had been previously seen in equilibrium adsorption experiments, the research team’s findings represent the first microscopic observations of the kinetic phenomenon. (T. X. Nguyen, H. Jobic, S. K. Bhatia, Phys. Rev. Lett., in press.)—Ashley G. Smart

Watching bubbles puncture a cell membrane

Posted: August 9th, 2010, 10:15am MDT by Physics Today

During the collapse of a cavitation bubble, the gas inside it can reach temperatures of more than 15 000 K—as hot as the surface of a star—and the energy can be released in the form of shock waves, heat, light, turbulent vortices, and high-speed jets of fluid. For decades, medical researchers have worked to harness that energy for therapeutic applications, such as the disintegration of cancerous tumors using focused ultrasound (see the article by Gail ter Haar in Physics Today, December 2001, page 29) and the delivery of drugs or genes into living cells (see the article by Detlef Lohse in Physics Today, February 2003, page 36). Although studies have demonstrated that microbubbles can rupture nearby cells, control over the bubble–cell interaction has remained difficult. Duke University researchers led by Pei Zhong have now demonstrated an approach to puncturing a cell’s membrane that entails carefully manipulating the fluid dynamics around it. The high-speed sequence of photographs captures the process: Two laser pulses, offset in space by 40 μm and time by 4 μs, create two bubbles (B1 and B2) that act in concert. The rapid expansion of the second bubble causes the collapse of the first bubble by pressing against it; the interaction deforms the shape of both. The bubbles’ asymmetric collapse gives rise to two localized microjets—one toward the cell between 6 and 7 μs, one away from it 2 μs later. The researchers can control the microjets’ impact by adjusting the bubbles’ position, spacing, and orientation relative to the cell. (G. N. Sankin, F. Yuan, P. Zhong, Phys. Rev. Lett., in press.)—R. Mark Wilson

Mapping cosmic mass without counting galaxies

Posted: August 5th, 2010, 8:54am MDT by Physics Today

Observations of distant supernovae have revealed that the expansion of the universe is accelerating. Cosmologists seek to confirm that result, and perhaps learn about the dark energy causing the acceleration, by mapping the universe’s large-scale structure. Mass-density patterns, formed when the cosmos was filled with dense plasma that supported sound waves, are visible today in the distribution of galaxies; tracking those patterns over cosmic time could provide constraints on the expansion. Galaxy redshift surveys can reveal large-scale structure but are time-consuming to conduct over large enough volumes. A potentially more efficient strategy, called intensity mapping, is to detect atomic hydrogen by its RF spin-flip transition. That 21-cm emission is an isolated spectral line, so matter at different redshifts can be mapped simultaneously without resolving the component galaxies. Now, Tzu-Ching Chang (Taiwan’s Academia Sinica and University of Toronto), Ue-Li Pen (University of Toronto), and Kevin Bandura and Jeffrey Peterson (both at Carnegie Mellon University) have shown that intensity mapping can work at redshifts near 1—looking back to the time when, models indicate, the expansion started to accelerate. From 15 hours of radio observations on two square-degree patches that were part of the DEEP2 galaxy redshift survey, the researchers removed the terrestrial radio signals and astronomical synchrotron radiation to uncover the much weaker hydrogen emission. Their resulting signal was correlated with the DEEP2 galaxy density—evidence that the technique did in fact detect hydrogen. The team is now observing larger regions of sky and hopes to build a dedicated intensity-mapping telescope. (T.-C. Chang et al., Nature 466, 463, 2010.)—Johanna Miller

Muonic Lamb shift

Posted: August 2nd, 2010, 9:06am MDT by Physics Today

Willis Lamb’s 1947 measurement of the tiny splitting between the 2s and 2p states of atomic hydrogen gave a crucial impetus to the development of quantum electrodynamics (QED). That “Lamb shift” from the Dirac hydrogen spectrum is a 4-μeV increase in the 2s energy level due primarily to vacuum fluctuations of the electromagnetic field. Now Randolf Pohl (Max Plank Institute for Quantum Optics, Garching, Germany) and coworkers at the Paul Scherrer Institute (PSI) in Switzerland have finally measured the analogue of the Lamb shift in the muonic H atom—a proton orbited by a μ⁻ instead of an e⁻. Muons live only microseconds, but they are 200 times heavier than electrons, and their atomic orbits are correspondingly tighter. The muonic Lamb shift is about 200 meV, and its precise value is particularly sensitive to the proton’s finite size. The PSI experiment was accomplished with precision laser excitation of μ⁻ p atoms created by an intense μ⁻ beam stopping in a small volume of H2 gas at very low pressure. The team measured the muonic Lamb shift to a part in 10⁵ and compared it with elaborate QED calculations that parameterize the proton’s finite size with an effective charge radius Rp. They find an Rp about 4% smaller than that measured, with less precision, by conventional H spectroscopy and e⁻–p scattering experiments. The discrepancy is 5 standard deviations. Either the proton really is smaller than previously thought, argue Pohl and company, or there’s something wrong with the QED calculations or their input constants. But the proton is a quark composite whose size and shape are quantum-chromodynamic manifestations beyond the purview of QED. Several QCD theorists suggest that at the extraordinary precision achieved by the PSI experiment, it may not be possible to describe proton-size effects adequately with a single length parameter. (R. Pohl et al., Nature 466 , 213, 2010.)—Bertram Schwarzschild

The noisy expression of genes into proteins

Posted: July 29th, 2010, 1:00pm MDT by Physics Today

Genetic information is transcribed from DNA to RNA and translated from RNA to make proteins. Because each step entails a modest number of molecules, gene expression, as the DNA-to-protein conversion is termed, is inevitably noisy: Identical genes in identical cells don't yield identical numbers of proteins. But how noisy? Sunney Xie of Harvard University and his collaborators have used single-molecule fluorescence microscopy and microfluidics to find out. They started by modifying the DNA of Escherichia coli to create 1018 different strains of the single-celled bacterium. In each strain, the code for a yellow fluorescent protein (YFP) was inserted after the gene for a different protein. To see the rate at which one gene is expressed in one cell of one strain, you’d illuminate the cell with a laser and measure the YFP emission through a microscope. To gather gene-expression statistics for a sample of cells from all 1018 strains, the Harvard team sent streams of cells through channels cut in a microfluidic chip and imaged them. The figure shows sample images for three proteins, YjiE, AtpD, and Adk. Ninety-six strains could be processed at once at a total throughput of 160 cells per second. The team found that the least abundant proteins appear at 10⁻¹ molecules per cell; the most abundant, at 10⁴ per cell. Gene expression is indeed noisy, but with a twist. As you’d expect, the least abundant proteins have the largest cell-to-cell fluctuations. But for proteins whose mean abundance is 10 per cell or higher, the expression noise saturates, presumably because the various molecules that mediate gene expression inside a cell are in limited supply. (Y. Taniguchi et al., Science 329, 533, 2010.)—Charles Day

Space Buckyballs

Posted: July 26th, 2010, 8:29am MDT by Physics Today

The field of nanotechnology is in part rooted in the 1985 Nobel Prize–winning laboratory synthesis of buckyballs—the soccer-ball-shaped carbon molecule, C60—by Rice University chemists Richard Smalley and Robert Curl and their collaborator, University of Sussex chemist Harold Kroto. The synthesis was guided by Kroto’s hypothesis that complex carbon chains could naturally form in the interstellar medium of aging carbon-rich, hydrogen-poor giant branch stars. Now, 25 years later, Jan Cami at the University of Western Ontario and his colleagues have reported the clearest evidence yet of such complex carbon structures in space. The research team analyzed IR spectroscopic data—collected by the Spitzer Space Telescope—of the circumstellar region of a planetary nebula known as Tc 1. As the image shows, the spectrum contains several prominent peaks of C60 (red arrows) and peaks of the rugby-ball-shaped C70 (blue arrows); both molecules were uncharged and in the solid phase. Previous spectra of other carbon-rich planetary nebulae indicated strong emission peaks of volatile polycyclic hydrocarbons, which were completely absent in the monitored region of Tc 1. Cami and his colleagues suggest that the planetary nebula may have ejected its hydrogen envelope a few thousand years ago and that a recent thermal pulse prompted the ejection of the pure carbon dust they’re now observing. (J. Cami et al., Science, in press, doi:10.1126/science.1192035.)—Jermey N. A. Matthews

Rolling ribbons get the bends

Posted: July 22nd, 2010, 7:24am MDT by Physics Today

For thousands of years, children have delighted in hoop rolling. Certainly, most of them have not considered that the rings are subject to gravitational and inertial forces; in any case, the hoops are stiff enough that they maintain their circular form despite those forces. But what happens to a rolling hoop that’s not so stiff? John Bush of the MIT mathematics department, along with visiting student Pascal Raux and colleagues, has answered that question in a recent study of more general systems—rolling bands that may be wider than they are high. Bush and company’s work was both experimental and theoretical. In their experimental investigations they took pictures of a vinyl polysiloxane loop placed on the inner surface of a rotating drum. The figure shows how the form of a representative loop changes as the drum speed is increased; blue corresponds to low speeds; red, high. In their theoretical work, the investigators confirmed the intuitive idea that the rolling band deforms as the inertial or gravitational force overwhelms the internal stiffness force. Indeed, if either gravity or inertial effects are strong enough, the top of the band can make contact with the bottom; new forces then come into play and the team’s analysis is no longer valid. Rolling droplets, tumbling blood cells, and carbon nanotubes deformed by van der Waals forces, the authors note, all display similar shapes to the rolling ribbons; the dynamics of those varied systems may be elucidated by the relatively simple ribbon study. (P. S. Raux et al., Phys. Rev. Lett., in press.)—Steven K. Blau

Bali's beating gong

Posted: July 19th, 2010, 10:19am MDT by Physics Today

At the heart of the Balinese percussive orchestra known as a gamelan is the large gong called the gong ageng wadon. It features a large, protruding dome or boss in the middle; when the boss is struck with a padded mallet, the gong produces a pronounced acoustic beating or ombak (meaning "wave"), as can be heard in this sound file. Using acoustical and vibrometric analyses, David Krueger and his colleagues at Brigham Young University have studied the sources of the ombak. Although some beating was found to come from asymmetric vibration modes with closely spaced frequencies, those appear to contribute mostly to the gong's timbre. The more significant contribution arises from the gong ageng wadon's nonlinear structural response. The gong's two dominant vibration modes, both axially symmetric, have nearly a 2:1 frequency ratio. That relationship gives the gong its perceived pitch, but the ratio isn't exact. So when the gong is struck, causing displacements large enough to produce overtones, the fundamental generates harmonics and interacts with the second axisymmetric mode to yield sum and difference frequencies. The resulting sound spectrum features strong peaks of similar amplitudes that are spaced only a few hertz apart and give rise to the distinctive sound of ombak. (D. W. Krueger, K. L. Gee, J. Grimshaw, J. Acoust. Soc. Am. 128, EL8, 2010.)—Richard J. Fitzgerald

Directly imaged exoplanet challenges formation models

Posted: July 15th, 2010, 9:06am MDT by Physics Today

Two years ago, astronomers in Canada directly imaged what seemed to be a gas giant planet in a very distant orbit—more than 300 times the Earth–Sun distance of one astronomical unit (AU)—around a star much like our Sun. (For comparison, Jupiter's orbit is 5.2 AU, Neptune's is 30 AU.) Such a scenario poses difficulties for all the major planet-formation models in current use: core accretion, gravitational instability, and fragmentation of a pre-stellar core. The main difficulty is that either much larger objects, like another star, or much smaller ones are expected at such a great distance. Now, with further observations in hand from the Gemini North telescope and its adaptive optics, University of Toronto astronomers Ray Jayawardhana, Marten van Kerkwijk, and David Lafrenière (now at the University of Montreal) have confirmed the puzzle: The planet, with about eight times the mass of Jupiter, is moving through space gravitationally bound to the parent star, known by its nickname 1RXS 1609. Besides astrometric observations, the direct imaging (shown here) along with spectroscopic and photometric data allowed the researchers to further characterize the planet and confirm that no other large planets are farther out in the system. A mere toddler at only 5 million years old, 1RXS 1609 is about 500 light-years away. Hundreds of other exoplanets have been discovered in recent years, but this one is expected to keep theorists busy for some time. (D. Lafrenière, R. Jayawardhana, M. H. van Kerkwijk, Astrophys. J., in press.)—Stephen G. Benka

Quantum memory takes a leap forward

Posted: July 12th, 2010, 9:16am MDT by Physics Today

Light is a useful carrier of quantum information, but no method yet exists for storing and retrieving it without significantly corrupting its quantum state. Now, Matthew Sellars (Australian National University, Canberra) and colleagues have taken a step closer to a useful quantum memory by mapping a light pulse’s state onto an ensemble of praseodymium ions doped into a transparent crystal. Capitalizing on the Stark effect, the researchers used an electric-field gradient, as shown in the figure, to shift the ions’ transition energies. Absorption of an input light pulse created a coherent superposition of ground and excited states, whose phase in each ion evolved at a rate proportional to the transition energy. Flipping the electric field’s sign reversed the transition-energy distribution and brought the ions back into phase, at which point the pulse was re-emitted. The innovation was in how the researchers countered the spectral line’s natural broadening due to inhomogeneities in the crystal lattice: Applying a frequency-stabilized laser beam to the side of the long, thin crystal, they pumped atoms whose transition energies lay outside a narrow range into an out-of-the-way hyperfine state. Using the sharpened spectral line, which Sellars says may be “the sharpest optical filter ever demonstrated,” the researchers stored pulses of up to 500 photons more faithfully than could be done classically. When pulses were stored for 1.3 µs, the retrieved ones were 69% as intense—the previous record was 15%. (M. P. Hedges et al., Nature 465, 1052, 2010.)—Johanna Miller

Graphene production goes industrial

Posted: July 8th, 2010, 9:12am MDT by Physics Today

A single plane of carbon atoms, graphene can be isolated using an exceedingly simple method: In 2004, the University of Manchester’s Andrey Geim and colleagues used common, clear cellophane tape to peel off weakly bound layers from bulk graphite. That process can produce millimeter-sized graphene flakes and is still common, particularly among researchers exploring graphene’s astonishing electronic properties. Many applications, though, require a large, continuous sheet of graphene. One promising approach is to grow graphene epitaxially atop some other crystal that can be etched away afterward. Recently, two groups—one led by MIT’s Jing Kong, the other by Byung Hee Hong of Sungkyunkwan University (SKKU) in South Korea—used chemical vapor deposition of methane to grow graphene on thin nickel films. The graphene was then either patterned lithographically or transferred onto silicon or plastic. The SKKU team has now adapted that approach to a scalable industrial manufacturing process that uses copper rather than Ni. In roll-to-roll production, as outlined in the figure, graphene-laden Cu was pressed against a polymer support, bathed in an etchant that removed the Cu, and then dry-transferred to another flexible polymer. To increase the film’s conductivity, multiple layers of graphene were stacked together and chemically doped in a bath similar to that used for etching. As proof of concept, the SKKU group produced a 76-cm-diameter flexible electrode, whose conductivity and transparency make it comparable to the commercial state of the art in touch-screen displays, indium tin oxide. (S. Bae et al., Nat. Nanotech., in press, doi:10.1038/nnano.2010.132.)—R. Mark Wilson

Doubly magic tin

Posted: July 1st, 2010, 11:22am MDT by Physics Today

The shell model of nuclear structure predicts that nuclei with certain magic numbers of neutrons or protons—indicative of closed shells much like the closed electron shells of atomic physics—will be much more stable than their neighbors on the chart of nuclides. In particular, the elite subset of “doubly magic” nuclei, with closed shells of both neutrons and protons, should be rigidly spherical and strongly resistant to deforming excitations. But the validity of such predictions for short-lived nuclei with large neutron excesses has been an open question. A particular challenge for experimenters has been doubly magic tin-132, which has 8 more neutrons than the heaviest stable Sn isotope and a half-life of only 40 seconds. Astrophysicists share the nuclear theorists’ keen interest in ¹³²Sn because it sits very close to nuclides that are presumed to be way stations in the creation of the heaviest nuclei in supernova explosions but cannot be studied in laboratories. Now Kate Jones and coworkers at Oak Ridge National Laboratory’s Holifield Radioactive Ion Beam Facility have demonstrated that ¹³²Sn does indeed have the doubly magic properties predicted by the shell model. They did so by running a beam of energetic ¹³²Sn ions into a deuterium-rich target and showing that single neutrons stolen from the deuterons landed precisely in the ¹³³Sn valence orbits one expects to form around a rigidly spherical nuclear core. The picture shows the Holifield facility’s enormous tandem Van de Graaff accelerator, in which the beam was accelerated. (K. L. Jones et al., Nature 465, 454, 2010.)—Bertram Schwarzschild

How neurons reorganize in growing brains

Posted: June 28th, 2010, 11:33am MDT by Physics Today

Matthias Kaschube of Princeton University and his collaborators have applied concepts from pattern formation and continuum dynamics to address a key question in neuroscience: Do neurons retain their roles in a growing brain? Although an adult human’s brain is four times as large as a baby human’s, it has roughly the same number of neurons. The extra volume accommodates the developed brain’s greater number of blood vessels, nonneuronal cells, and—crucially for memory and intelligence—interneuron connections. To understand the development of those connections, Kaschube and others study ocular dominance (the brain’s preference for input from one eye over the other) in the primary visual cortex (the brain’s principal image processor). Several factors make OD a convenient model system. Not only are OD signals readily induced and tracked; the neurons responsible for OD are grouped in recognizable rows of columns. Data gathered from kittens by Kaschube’s collaborators—Karl-Friedrich Schmidt and Siegrid Löwel of Friedrich Schiller University in Jena, Germany—conflict with the simplest growth model in which OD columns and their separations both expand by the same factor as the visual cortex. Rather, the columns increase in number and keep their separations while the rows lose their original straight configuration and become wavier. Kaschube and his graduate student Wolfgang Keil account for the pattern change by invoking the zigzag instability, which arises when stripes in isotropic systems are stretched. In the simple growth model, individual OD neurons would continue to carry signals from the same part of the visual field of view to the same part of the visual cortex. In Kaschube and Keil’s model, the reorganization of OD neurons implies that individual neurons continuously and flexibly change their signal-carrying roles. (W. Keil, K.-F. Schmidt, S. Löwel, M. Kaschube, Proc. Natl. Acad. Sci. USA, in press.)—Charles Day

Carbon emissions sour ocean water

Posted: June 24th, 2010, 8:02am MDT by Physics Today

It’s not just the spilled oil: Since the Industrial Revolution, Earth’s oceans have been soaking up unprecedented levels of atmospheric carbon dioxide as well as runoff from manufacturing processes. In a new review, Woods Hole Oceanographic Institution scientist Scott Doney describes the impact of human activities on the open ocean and coastal waters. Observations and models show that up to 30% of the carbon released by fossil-fuel combustion is absorbed by the ocean; with that rise in subsurface CO2, a weak acid in seawater, the ocean’s pH plummets. Measurements of CO2 and pH collected over the past two decades off Hawaiian shores show that the ocean is becoming more acidic—30 to 100 times faster than what geological records report. The shift in the ocean’s acid-base equilibrium due to excess CO2 also hinders the precipitation of calcium carbonate, which makes up the shells and skeletons of many marine species. Yet another concern is hypoxia—dangerously low levels of subsurface oxygen caused by climate-warming-induced degradation of organic matter—which leads to anaerobic respiration conditions and produces even more CO2. And yes, oil spills, even naturally occurring seeps from oil wells, release pernicious hydrocarbons into the marine environment. Most such industrial chemicals are “woefully undersampled,” writes Doney, whose summary calls on the oceanographic community to better coordinate its monitoring of the ocean’s biogeochemical cycle. (S. C. Doney, Science 328, 1512, 2010.)—Jermey N. A. Matthews

Protecting the world's largest particle accelerator

Posted: June 21st, 2010, 7:30am MDT by Physics Today

When CERN’s Large Hadron Collider is fully operational, it will accelerate countercirculating protons to energies of 7 TeV, the highest particle energy ever achieved by human ingenuity. By macroscopic standards, 7 TeV (10⁻⁶ J) is tiny. But each beam produced at the LHC will ultimately include some 3 × 10¹⁴ protons; should it go awry, it could seriously damage the LHC and the delicate particle detectors that the accelerator hosts. For that reason, as described in a new paper by Robert Appleby and colleagues at CERN, the LHC includes an elaborate safety system that regulates the beams, monitors them, and dumps them out of harm’s way if they go off course. To give an idea of the system’s complexity, the authors note that 17 distinct subsystems must continually give a virtual OK to a central processer, or else the beam will be dumped. In addition to describing the LHC’s safety features, Appleby and colleagues calculate how the beams would respond to various possible accidents—improperly set magnetic fields, for example—and ask if the protection system would respond quickly and accurately enough to avert disaster. Although no system can protect against all failures, the researchers conclude that the accelerator and its associated detectors are as safe as could reasonably be expected. In addition to the simulated studies, scientists are testing the protection system with real 3.5 TeV beams whose intensity is gradually being ramped up. (R. B. Appleby et al., Phys. Rev. ST Accel. Beams. 13, 061002, 2010.)—Steven K. Blau

Unexpected heterogeneity in a complex fluid

Posted: June 17th, 2010, 7:30am MDT by Physics Today

Newtonian fluids, such as water, are described by the Navier-Stokes equations. But many everyday fluids lack a similar complete description, and researchers still seek better observations and models of their flow. Yield stress fluids (YSFs), a subset of non-Newtonian fluids that includes toothpaste and mayonnaise, hold their shape under low stress but flow under high stress. Some YSFs are also thixotropic, meaning their viscosities decrease with time during continued flow. Thixotropy in a YSF can result in heterogeneous flow—confinement of the fluidlike behavior to part of the material, which flows more and more easily, while the rest remains solid—an important phenomenon to understand and control when handling YSFs industrially. Now, Sébastien Manneville, of the École Normale Supérieure de Lyon, and colleagues have unexpectedly observed similar localized flow in a nonthixotropic YSF subjected to a shear stress. The observed behavior was transient, but it lasted a surprisingly long time: more than a day in one of their trials, several hours in others. Many of the researchers’ observations, such as the power-law dependence of the transient duration on the shear rate, remain unexplained. Even so, the data indicate that nonthixotropic YSFs are more complicated than was previously assumed, and they exemplify the importance of distinguishing between transient and steady-state behavior in YSF experiments. (T. Divoux et al., Phys. Rev. Lett. 104, 208301, 2010.)—Johanna Miller