March 2009 Archives

The closer an object is to its heat source, the hotter it gets—except in the case of the Sun’s corona. Somehow, energy makes the few-thousand-kilometer ascent from the Sun’s 10,000-K photosphere to the corona’s outskirts, raising the temperature to 3 million K. Alfvén waves seem like the ideal energy conveyor. They don’t bend backward as they propagate through regions of varying density and temperature. And the source of their restoring force, the solar magnetic field, extends from the photosphere up into the corona. But until now, no one had seen evidence of the waves in the first leg of their upward journey. David Jess of the Queen’s University Belfast in Northern Ireland and his collaborators observed the Sun with the Swedish Solar Telescope (SST) on La Palma, one of the Canary Islands. They looked at the center of the solar disk in a narrow waveband centered on the hydrogen-α absorption line, which originates from the 1000-km-thick region just above the photosphere. One area, measuring 430 000 km² and shown here, featured bright spots that usually indicate magnetic activity. There, Jess found the telltale signature of an Alfvén wave: a torsional twisting back and forth perpendicular to the propagation. The wave’s speed (20 000 km/s), amplitude (2.6 km/s), and period (400 s) are consistent with theoretical predictions. They also explain the past elusiveness of Alfvén waves: Other telescopes lack the SST’s combination of spatial and temporal resolution. If waves like this one do carry their energy into the corona and are numerous, Alfvén waves may indeed heat the corona. (D. B. Jess et al., Science 323, 1582, 2009.) — Charles Day

How an asteroid reflects sunlight as a function of wavelength reveals something about the asteroid's make-up. Based on reflectance spectra, astronomers have developed a classification scheme for asteroids and noted that those of a given type tend to be formed in the same region of the asteroid belt. The composition of meteorites, most of which come from asteroids, can be determined in detail. Until recently, however, no meteorite could be unambiguously associated with a specific asteroid or even a spectral class. But now scientists have obtained reflectance and composition data for the same asteroid—2008 TC₃, which blew up over Sudan's Nubian Desert shortly before dawn on 7 October 2008. Its story was told recently by an international team led by the SETI Institute's Peter Jenniskens. The asteroid, also called Almahata Sitta, had been sighted 20 hours before it disintegrated—early enough for scientists to take reflectance measurements. And some of Almahata Sitta survived the explosion high above Earth and rained down to the surface. Students from the University of Khartoum, led by professor Muawia Shaddad, gathered 47 of the meteorites, one of which was studied in detail. That remnant's high porosity and dark carbon-rich material are peculiar. Those anomalies may, in time, help physicists understand the processes that took place in the solar nebula in the region where Almahata Sitta and its spectral classmates formed. (P. Jenniskens et al., Nature 458, 485, 2009. Photo courtesy of NASA/Peter Jenniskens.) — Steven K. Blau

Lightness, strength, and moldability are among the most desired material properties for aircraft, sporting equipment, and many structural applications. Those sometimes opposing properties converge in bulk metallic glasses—supercooled amorphous metal alloys that can be cast into complex shapes and are resilient under large elastic strains. However, their toughness is suspect: Under repeated stress, BMGs fatigue and develop fatal cracks much more quickly than crystalline metal alloys do. To control crack propagation, Caltech's William Johnson, Lawrence Berkeley National Laboratory's Robert Ritchie, and their collaborators focused on controlling the microstructure of a particularly tough BMG composite made of zirconium, titanium, and other metals. Its fingerlike crystalline dendrites (67% by volume) are surrounded by an amorphous matrix, as seen in this optical micrograph. By heating the precursor alloys between their melting points then rapidly quenching the solution, the researchers were able to control dendrite size and the spacing between the glassy and crystalline phases. The width of the glassy region between the much tougher dendrite fingers was tailored to be short enough to serve as a "microstructural arrest barrier" for just-formed cracks. Compared with existing dendrite-containing BMGs, the new material holds up under three times more stress cycles and is comparable in toughness to high-strength steel or aluminum. (M. E. Launey et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0900740106.) — Jermey N. A. Matthews

In his "Milkdrop Coronet," strobe-photography pioneer Harold Edgerton famously captured the splash produced by a milk droplet falling into a saucer. But our understanding of the underlying physics remains poor. It's known that before a liquid droplet splashes upward from a surface, a thin sheet of liquid spreads out from the impact point. Four years ago experiments by Sidney Nagel and colleagues at the University of Chicago showed, surprisingly, that splashing on a dry surface can be suppressed by reducing the ambient air pressure. The researchers concluded that compressible effects in the air are responsible for the splashing (L. Xu, W. W. Zhang, S. R. Nagel, Phys. Rev. Lett. 94, 184505, 2005). Now Michael Brenner and coworkers at Harvard University have further looked into the air's role in how droplets splash on a dry surface. Taking into account the compressibility and viscosity of the gas and the surface tension of the liquid, they modeled the behavior of the approaching droplet as it reaches the surface. They find that instead of spreading out over the surface, the liquid spreads over a very thin film of air. When the droplet nears the surface, pressure builds beneath it and the bottom of the droplet deforms by flattening and then becoming dimpled. The droplet's bottom perimeter develops a kink that, still over a layer of air, moves out and creates capillary waves. The calculations don't, however, show any indications of splashing; the researchers suggest that other parameters, such as the droplet viscosity and thermal transfer, must become important after the initial spreading phase. (S. Mandre, M. Mani, M. P. Brenner, Phys. Rev. Lett., in press.) — Richard J. Fitzgerald

Spin–orbit coupling is a double-edged sword to physicists who want to exploit the spin degree of freedom in novel electronic devices. On the one hand, the coupling treats up spins differently from down spins, a necessary feature of any spintronic device. But on the other hand, the coupling nullifies the conservation of spin that prevails in free space. Although the net spin polarization averages to zero in a crystal, it fluctuates randomly and locally. Joseph Orenstein of the University of California, Berkeley, and Lawrence Berkeley National Laboratory and his collaborators have demonstrated a way to restore the conservation of spin in a semiconductor quantum well and extend the lifetime of a coherent spin structure. Two tricks are involved. The first is to build a quantum well in which two types of spin–orbit coupling, Rashba and Dresselhaus, are equal. The second trick is to create a particular kind of coherent signal, a spin helix, and send it through the quantum well with a particular wave vector. Mathematically, the spin helix in its specially tuned well shares the same SU(2) symmetry as an isolated spin. Empirically, the spin helix retains its coherence for 1 ns before it diffuses away. That lifetime may seem fleeting, and it's hardly infinite, but it's an order of magnitude longer than that of a coherent spin signal launched without symmetry's sustaining power. To learn more about the spin helix and two other recently observed spin textures, look for the news story on page 12 of the April issue. (J. D. Koralek et al., Nature, in press.) — Charles Day

Today's laptop computers, cell phones, hybrid vehicles, and other technologies rely on rechargeable batteries. As discussed in Physics Today, December 2008, page 43, batteries— in particular, the popular lithium-ion batteries— typically have a high energy density but a low power density: They can't deliver their stored energy particularly quickly. Often the limiting step in Li⁺ batteries is not getting the ions through the electrolyte and electrode structure but getting them into the active electrode material itself. Using nanoscale materials in the electrodes and doping the materials are among present techniques to improve battery rates. Now Byoungwoo Kang and Gerbrand Ceder of MIT have shown that using particles of a common electrode material, lithium iron phosphate (LiFePO₄), covered with a glassy coating of iron-doped lithium phosphate can significantly increase the charging and discharging rates. Moreover, the particles and coating can be formed together in a single step. In test experiments, the researchers obtained discharge rates 100 times as fast as today's commercial Li⁺ batteries. The researchers suggest that the amorphous coating may improve Li⁺ transport across the surface of the electrode particles; uncoated LiFePO₄, in contrast, conducts ions poorly except in a narrow range of directions. Additionally, they say that the coating may modify the surface potential and provide adsorption sites for a range of ion energies. (B. Kang, G. Ceder, Nature 458, 190, 2009.) — Richard J. Fitzgerald

MRI excels at revealing subtle features in soft tissue. Hydrogen nuclei are detected through the electromotive force induced in a nearby coil when their spins flip from an RF pulse. Typically, one coil transmits the pulse and another detects the induced signal. That configuration has been used in clinical settings for decades with imagers built from 1.5 T magnets. In recent years, imagers have been developed with greater field strength to boost sensitivity. But as the field increases, so does the resonance frequency required to excite nuclei. The corresponding wavelength in tissue in a 7-T magnet is about 12 cm, on par with the size of resonator coils that encircle a human head. The result: interference and standing-wave RF patterns. Those inhomogeneities in the RF field are disastrous because they perturb the image contrast between different types of tissue. A group led by Klaas Pruessmann at ETH Zürich has now solved the problem by removing the RF coils entirely and using the conductive lining in a 7-T MRI cavity as a waveguide with an antenna placed at one end. A patient inside the waveguide is exposed to a homogeneous traveling RF wave launched from the same antenna that subsequently detects the spin signals. The in vivo images of a human leg demonstrate that traveling-wave MRI (left) can excite spins more uniformly than can inductive MRI (right). The researchers speculate that, with the tight-fitting induction coils gone, patients may suffer less from claustrophobia and engineers may enjoy more design freedom. (D. O. Brunner, N. De Zanche, J. Frölich, J. Paska, K. P. Pruessmann, Nature 457, 994, 2009.) — R. Mark Wilson

Synchronized oscillatory processes in populations of living cells can arise in two ways. In one type of transition, individual cells oscillate out of synchrony at low number density and gradually synchronize as their density is increased. In another type, cells exhibit no oscillations at low density, but above a threshold density they suddenly begin oscillating in synchrony. Biological systems' complexity makes understanding the transition mechanisms a challenge. But now, researchers led by Kenneth Showalter of West Virginia University have observed transitions of both types in a simpler nonbiological system. They used a version of the oscillating Belousov-Zhabotinsky reaction based on the catalyst ferroin, which they loaded onto porous particles 200 µm in diameter. When the particles were suspended in a reagent solution, the reaction on each particle oscillated at its own frequency, which could be monitored as the ferroin changed in color. Stirring the solution caused chemicals to be exchanged between each particle and the surrounding solution; as a result, the particles' oscillation cycles could influence one another and thereby synchronize, as shown in the time-sequence images in the figure. When the researchers stirred the solution slowly, they observed synchronization of the first, gradual type. When they stirred more quickly, the transition was of the second, sudden type. The researchers explain their results using a kinetic model of the reaction and species exchange, which may aid in the understanding of biological synchronization. (A. F. Taylor et al., Science 323, 614, 2009.) — Johanna L. Miller

With energies exceeding 10²⁰ eV, the highest-energy cosmic-ray protons are as energetic as well-hit tennis balls. How does a proton become so energetic? Recent cosmic-ray data disfavor the notion that these ultra-energetic protons have exotic origins such as the decay of very massive particles as yet unidentified. So one must seek the proton acceleration mechanism in familiar astrophysical environments. The conventional suggestions—acceleration by relativistic shocks, spinning black holes, or flares on hypermagnetized neutron stars—each have problems accounting for the highest observed energies. Shock acceleration, for example, becomes increasingly inefficient at high energy because the inevitable trajectory bending causes severe synchrotron energy loss. Now theorist Pisin Chen (SLAC and National Taiwan University) and coworkers have demonstrated analytically and by computer simulation that so-called magnetowaves —electromagnetic waves with unusually strong magnetic components in magnetized plasmas—can drive plasma waves in their wake much as laser pulses in the laboratory drive plasma wakefields in experimental plasma-based accelerators (see Physics Today, March 2009, page 44). The mechanism avoids synchrotron loss, and it provides strong accelerating gradients even at very high energy. Chen and company show that a proton surfing a stochastic succession of such plasma wakes can, with luck, be accelerated to 10²¹ eV. Magnetowaves are believed to be produced in the relativistic jets emanating from active galactic nuclei. And the "luck" required for the proton to catch just the right sequence of plasma waves in an AGN jet accords with the observation that ultra-energetic cosmic rays are extremely rare. That's why the detector arrays that study them cover thousands of square kilometers. (F.-Y. Chang et al., Phys. Rev. Lett., in press.) — Bertram Schwarzschild