Recently in Condensed matter Category

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

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

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

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

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 (H_x) affects the cantilever's resonant oscillation. As shown in the bottom panel, they found that scanning H_z, the field normal to the ring, without an additional static H_x induced magnetization steps that were of the right size and position to be full-quantum vortices. When they added a sufficiently strong static H_x, 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

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

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

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

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

Further reading

• The discovery paper: K. S. Novoselov et al., Science 306, 666 (2004).
• Review of graphene's properties: "Graphene: Exploring Carbon Flatland," A. K. Geim and A. H. MacDonald, Physics Today, August 2007, p. 35.
• News story about graphene's quantum Hall effect: Physics Today, January 2006, p. 21.

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

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

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 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

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

One hallmark of Albert Einstein’s genius is his 1905 theory that the kinetic energy of pollen grains, dust, and other similarly sized objects in thermal equilibrium depends solely on temperature—the classic definition of Brownian motion. Einstein then concluded that the instantaneous velocity of such particles would be impossible to physically measure, and for more than a century, it seemed that he was right. But now, Mark Raizen and his colleagues at the University of Texas at Austin have used optical tweezers in a vacuum chamber to trap a 3-μm-diameter silica bead, observe its ballistic (inertial) motion at short time scales, and determine its instantaneous velocity. The bead is held at the focal point of two noninterfering laser beams, similar to the setup in the image. When the bead makes a random move, it deflects the beams, which allows its position to be traced and the instantaneous velocity to be measured. From those measurements, the researchers calculated root mean square velocities; even when taken at varying air pressures, the results agreed with each other and with the theoretically predicted value, proving that in the ballistic regime, the bead’s mean velocity is solely dependent on temperature and not on pressure or the inertial effects of the surrounding air molecules. Raizen says they will next attempt to cool the particle’s motion to the quantum ground state and confirm that the kinetic energy will be nonzero even at 0 K. (T. Li et al., Science, in press, doi:10.1126/science.1189403.)—Jermey N. A. Matthews

Much of the light emitted from stars and other astrophysical objects is absorbed by dust and reemitted at far-IR or submillimeter wavelengths—radiation that is notoriously difficult to detect. Last year researchers from the Jet Propulsion Laboratory proposed a new type of detector for that regime, with an eye toward future, more sensitive space missions. The team has now built a prototype microdevice (see figure), called a quantum capacitance detector (QCD), which would be one pixel in an eventual array. The detection chain goes like this: Photons are received at an antenna and fed into a superconducting absorber where they break Cooper pairs and generate quasiparticles. A superconducting island, called a single Cooper-pair box (SCB), is connected to the absorber in such a way that, at most, one quasiparticle at a time can tunnel onto it; that changes the island’s capacitance, which is so small that the charging energy of a single electron has a large effect. With a resonant circuit, the physicists monitor the frequency of capacitance changes from which they can determine the density of quasiparticles in the absorber and thus the photon flux at the antenna. The device’s performance is already comparable to that of other superconducting detectors. The advantage of the QCD, say the researchers, is the ease with which it can be read out from an array of detectors. For example, each pixel detector could be fabricated with a different resonance and simultaneous readout could be done with a frequency comb. (J. Bueno et al., Appl. Phys. Lett. 96, 103503, 2010.) —Stephen G. Benka

Laser radiation can induce cooling not only in dilute gases of atoms but also in certain transparent solids. The left panel of the figure shows the basic scheme: A laser photon excites a transition from an upper level of one state to a lower level of another, and a higher-energy photon is emitted, with phonons making up the difference. Now Mansoor Sheik-Bahae (University of New Mexico), Mauro Tonelli (University of Pisa), and colleagues have cooled a solid to 155 K, a new temperature record, using a laser-based system with no moving parts. The previous record, 208 K, was set in 2005 in the ytterbium-doped glass ZBLAN (composed of zirconium, barium, lanthanum, aluminum, and sodium fluorides). The Yb³⁺ ions’ lowest two energy levels are split by the surrounding atoms into seven sublevels, as shown in the right panel. ZBLAN’s appeal was that it had been synthesized to high purity for use in optical fibers. But its potential for cooling is limited: Its amorphous structure broadens the Yb³⁺ energy levels, so the peak absorption is weak. The new record was set using Yb-doped yttrium lithium fluoride (Yb:YLF), whose regular crystal structure makes the Yb³⁺ sublevels much sharper and the resonant absorption much stronger. But synthesis of high-purity Yb:YLF is relatively undeveloped, and existing high-power lasers fall just short of the Yb³⁺ E4–E5 transition’s 1020-nm peak. The researchers speculate that improvements in those areas should allow cooling to 77 K—the boiling point of liquid nitrogen. (D. V. Seletskiy et al., Nat. Photonics 4, 161, 2010.) —Johanna Miller

Boron's next-door neighbor in the periodic table, beryllium, forms a simple metal lattice at 0 K. Boron's other next-door neighbor, carbon, forms another simple structure at 0 K, graphite. As for boron itself, its most stable form at 0 K is unknown. Compounding the mystery, the lowest-energy phase that experimenters have found, the β-rhombohedral phase, is stunningly complex and defect riddled: Each hexagonal unit cell has 423 atomic sites; on average only 320 of them are occupied. Now, Tadashi Ogitsu of Lawrence Livermore National Laboratory and his collaborators have explained why the stable β-rhombohedral phase has so many empty sites. If boron were simple, the defects—vacancies and interstitial atoms—would disappear as boron attained its perfect crystalline structure. But according to Ogitsu's calculations, which he carried out on a Livermore supercomputer, the defects actually stabilize the β-rhombohedral phase. It turns out the defect sites in the crystal are arranged in a particular geometric configuration, a double-layer expanded kagome lattice (see figure). Ogitsu and his collaborators realized that the problem of how boron atoms fill empty sites is essentially the same as another problem: how antiferromagnetically coupled spins align themselves on an expanded kagome lattice, whose ground state is degenerate and disordered. Like spin ices, and ordinary water ice, boron's β-rhombohedral phase is geometrically frustrated. Ogitsu notes that the hopping of defects between nearly degenerate configurations can also account for some of boron's peculiar and long-puzzling transport properties. (T. Ogitsu et al., Phys. Rev. B, in press.)—Charles Day

The fractional quantum Hall effect with conductance plateaus at fractional rather than integer multiples of the conductance quantum e²/h values took experimenters aback when it first appeared at two-dimensional semiconductor interfaces nearly 30 years ago. The FQHE was soon explained by invoking strong correlations among the electrons that led to the formation of a collective state with an effective charge of 1/3 (see PHYSICS TODAY, October 1997, page 42). Recently, rather than being surprised by the FQHE, researchers had become frustrated by their inability to see it in a new 2D system—graphene. Consisting of a single layer of carbon atoms in a hexagonal lattice, graphene is expected to have intriguing electronic properties, produced by electrons that behave as massless relativistic particles (see PHYSICS TODAY, August, 2007, page 35). Researchers have been eager to see manifestations of those exotic particles, starting with the FQHE as proof of their expected strong electron interactions. Alas, the intensive search for the FQHE had come up empty. Now, two experiments have finally succeeded, as reported by Eva Andrei and her colleagues at Rutgers University and by Philip Kim and his coworkers from Columbia University. Two steps seem to have been key. One was to free graphene from disruptive perturbations of the substrate by suspending it in vacuum. The second was to measure the Hall-effect voltages using just two terminals rather than the conventional four. Experimenters hope to find ways to return to four-terminal measurements, which give more complete information. (X. Du et al., Nature 462, 192, 2009; K. I. Bolotin et al., Nature 462, 196, 2009.)—Barbara Goss Levi

The tunable elasticity and porosity of colloidal gels lead to some interesting applications, among them tissue scaffolding and drug delivery. Conventionally, colloidal particles interact and assemble under entropic and electrostatic forces to form predictable structures. But greater control can be achieved from an approach developed by Paul Clegg, Michael Cates, and their collaborators at the University of Edinburgh in the UK. The researchers disperse silica particles in the single-phase region of two partially miscible solvents—water and the organic base 2,6-lutidine. When the solution is heated above a critical temperature, the solvents separate and the particles become trapped at the liquid–liquid interfaces. The bulky particle domains then jam together and arrest the solvent separation, forming a two-phase network the researchers call a bijel. But cool the solution and remix the solvents too soon and the distinct structure disappears, as shown in movie 1 and the two left images in which the colloids appear green, the water black, and the lutidine red. Now the researchers have discovered an approach to stabilize the bijel structure. When the phase-separated solution is allowed to sit for at least 24 hours before it is cooled, the bijel surprisingly keeps its shape, as shown in the two right images and movie 2. From Monte Carlo simulations, the researchers deduce how the resulting network of colloidal monolayers, or monogel, stays intact: the particles become compressed by capillary forces, remain attracted by van der Waals forces, and are kept from collapsing into each other by repulsive electrostatic forces. (E. Sanz et al., Phys. Rev. Lett., in press.) —Jermey N. A. Matthews

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Movie 2

In the absence of an applied voltage, an induced electrical current rapidly decays thanks to the scattering of electrons from defects, phonons, and each other. But in a cold metal ring smaller than the electron’s coherence length, it’s possible to induce a dissipationless current, even if the metal is not superconducting. The trick, theorists predicted in the early 1980s, is to thread the ring with a magnetic field, which breaks time-reversal symmetry. The current is revealed only by its magnetic moment μ. And although researchers confirmed the effect early on, mostly using superconducting quantum interference devices (SQUIDs), complete agreement between theory and experiment, and even among experiments, has remained elusive. Jack Harris and colleagues from Yale University and the Free University of Berlin have now developed an elegantly simple alternative measurement scheme. The team deposited aluminum rings on a cantilever whose vibration frequency can be precisely monitored. In a magnetic field B, each ring’s current produces a torque τ = μ × B, recorded as a shift in the cantilever’s resonance frequency of vibration. From that frequency shift, the researchers deduce the current with a precision two orders of magnitude greater than is possible using SQUIDs. For a magnetic flux threading the ring, the current exhibits an Aharonov–Bohm effect, measurable as oscillations, shown here, whose period corresponds to the addition of one flux quantum h/e through the ring. In experiments taken over a broad range of fields, temperatures, and ring sizes, Harris and coworkers find perfect agreement with a noninteracting electron model. (A. C. Bleszynski-Jayich et al., Science 326, 272, 2009.)—R. Mark Wilson

Colossal magnetoresistance is aptly named. By subjecting a piece of appropriately doped manganite to a strong magnetic field and a moderately low temperature, one can raise its electrical conductivity by 10 000%. Despite its prodigious magnitude, CMR has not led to any commercial devices since its discovery 15 years ago. Still, it continues to fascinate physicists. At its most basic level, CMR arises as a paramagnetic insulating phase yields to a ferromagnetic conducting phase. But evidence suggests that a third, charge-ordered antiferromagnetic phase could play a role too. To elucidate the issue, Jing Tao of Brookhaven National Laboratory and her collaborators developed a new experimental technique. Called scanning electron nanoscale diffraction (SEND), their technique combines electron diffraction's ability to reveal the presence of ordered structures with scanning microscopy's ability to reveal those structures' real-space distribution. The patches in the figure correspond to charge-ordered regions. As the temperature approaches the 253-K CMR transition, the volume occupied by the charge-ordered phase increases. Simulations by Elbio Dagotto of Oak Ridge National Laboratory and his collaborators suggest an explanation: The charge-ordered phase vies with the ferromagnetic phase to become the predominant phase below the transition temperature. Although it loses the battle, the charge-ordered phase nevertheless delays and thereby intensifies the onset of CMR. (J. Tao et al., Phys. Rev. Lett. 103, 097202, 2009.)—Charles Day

Two-dimensional spectroscopy of materials, in its simplest form, measures the change in a sample’s interaction with radiation at one frequency caused by excitation at another. First used in nuclear magnetic resonance and later applied at IR and optical frequencies, the technique can separate overlapping peaks in a complicated spectrum or probe the flow of energy through systems such as the light-harvesting molecules involved in photosynthesis. (See Physics Today, July 2005, page 23.) In 2D Fourier-transform optical (FTOPT) spectroscopy, the sample is excited not with monochromatic light but with a pair of phase-related femtosecond pulses separated by a variable delay t₁; the time-domain data are transformed into a spectrum as a function of frequency ω₁. The second field is likewise replaced by a pair of phase-related pulses, one of which is a so-called local oscillator that allows the radiated field to be recorded as a function of time t₃ or frequency ω₃. Now, Keith Nelson and colleagues at MIT have produced 3D FTOPT spectra, in which the signal is a function of ω₁, ω₃, and also ω₂, corresponding to the time t₂ between the two pulse pairs. Applying their technique to semiconductor quantum wells—a system in which electrons are excited from nondegenerate valence-band states into the same conduction-band states—they found that important pathways whose signals were indistinguishable in a 2D spectrum could often be separated in a 3D spectrum. (D. B. Turner et al., J. Chem. Phys., in press.) —Johanna Miller

At absolute zero, a two-dimensional array of identical particles will crystallize into a hexagonal close-packed lattice. At high temperature, the lattice melts. What happens in between has interested physicists for decades. In the 1970s, theorists predicted that 2D melting would proceed via a so-called hexatic state in which the crystal breaks up into patches of local orientational order. The hexatic state's existence has been inferred from changes in resistance and other sample-averaged quantities. Now, the whole melting process—from a crystal through a hexatic state to a liquid—has been directly imaged. Isabel Guillamón of the Autonomous University of Madrid, Spain, and her collaborators melted a lattice of superconducting vortices that forms in a thin film of tungsten under a magnetic field. For their imager, they used a scanning tunneling microscope, which can distinguish the non-superconducting vortex cores from the vortices themselves. As the movie shows, the vortices start off with hexagonal order. As the temperature increases, pentagonal (gold) and heptagonal (green) defects appear that cause dislocations (solid magenta lines). On further heating, the vortices become mobile. Just above 2 K, they move too quickly for the STM to track; they appear as white stripes, whose ordering resembles a liquid crystal's smectic phase. By 3 K, which is 1 K below the film's T_c, the lattice melts completely and the vortices move freely in a gray, undifferentiated blur. (I. Guillamón et al., Nat. Phys., in press.)—Charles Day

The hallmark of a conventional crystal such as table salt is translational symmetry. Quasicrystals do not have that symmetry and so can exhibit a wider structural variety than their more constrained brethren. But quasicrystals, like crystals, do have long-range correlations and display sharp, structure-revealing diffraction patterns. To date, more than 100 quasicrystals have been synthesized in the lab. Now Luca Bindi of the Natural History Museum of Florence has teamed up with Paul Steinhardt and colleagues from Princeton and Harvard universities to present evidence for a natural version of one of those quasicrystals: icosahedral Al₆₃Cu₂₄Fe₁₃. The material, a 100-μm grain, is from a mineral assemblage (left figure) excavated from the Koryak Mountains in Russia and now housed in the Florence museum; the very complexity of the sample argues for its natural formation. In consultation with his US-based colleagues, Bindi identified the sample as possibly hosting a quasicrystal. The US team then probed a small piece of it with transmission electron microscopy. Diffraction patterns such as shown in the right figure identified quasicrystal regions; the 10-fold symmetry cannot be generated by crystals. Subsequent analysis of x rays scattered off pure quasicrystal grains determined the material’s chemical formula. Geologists and physicists have much to learn about the conditions under which quasicrystals form. The study of natural materials can help address that question and may turn up new, never-before contemplated structures. (L. Bindi et al., Science 324, 1306, 2009.) —Steven K. Blau

Below 68 °C vanadium dioxide is an insulator. Above that temperature it’s a metal. The nature of the transition has long remained elusive, though, because bulk VO₂ has a domain structure that complicates its behavior. David Cobden and colleagues from the University of Washington have found an elegant way to avoid that difficulty and map the effective phase diagram. The team grew rectangular nanobeams that were thinner than the characteristic domain size of a few microns. They then suspended each beam between two electrical contacts. The metallic and insulating phases differ in lattice constant, but the constrained geometry creates a uniform stress field in a VO₂ beam such that the two phases coexist in a range of temperatures between 68 °C and 105 °C. Thanks to the dramatic change in optical properties that accompanies the transition, Cobden’s team could visually track the nucleation and growth of the metallic phase as a function of temperature. The figure here shows five snapshots of a 20-μm-long beam (red indicates the insulating phase). By measuring the electrical resistance in the coexistence regime, the researchers found that the resistivity of the insulating phase is independent of temperature. That remarkable result, they argue, implies that the phase transition occurs at a fixed carrier density in the material and is consistent with a picture in which electron–electron interactions drive the transition. (J. Wei, Z. Wang, W. Chen, D. H. Cobden, Nat. Nanotech., doi:10.1038/nnano.2009.141, in press.) —R. Mark Wilson

Decades ago, theorists predicted that under some circumstances, solids could flow like superfluids. In 2004 Moses Chan and Eun-Seong Kim found evidence of such so-called supersolidity: When a torsion oscillator filled with solid helium was cooled below 200 mK, its resonant frequency increased. Some of the He’s mass appeared to have decoupled from the rest. But subsequent experiments revealed a more complicated picture with many aspects unexplained. For example, the oscillator’s dissipation (related to the damping strength) depended on temperature in a way that the theory hadn’t predicted. Now, Cornell University’s Séamus Davis and colleagues have developed a torsion oscillator, shown in the figure, whose position sensor is a superconducting quantum interference device rather than the usual capacitor. The SQUID allows them to measure the dissipation more accurately and to explore a broader range of frequencies and amplitudes than was previously possible. Among their results is the discovery that when the temperature is abruptly lowered, the oscillator’s resonant frequency and dissipation share the same response time constant, which increases steeply with decreasing temperature—much like the characteristic flow time of cooling molten window glass. Some theorists have postulated that He’s behavior results from an ordinary glass transition, not from supersolidity. The relative magnitudes of the changes in frequency and dissipation rule out that possibility. But the ultraslow low-temperature behavior suggests that a glasslike phase may be involved. (B. Hunt et al., Science 324, 632, 2009.) —Johanna Miller

Late Stone Age metal smiths added a little tin to copper to usher in the eponymous Bronze Age; over the ensuing five millennia, many new combinations and applications of the two metals have appeared. Today, for example, a thin tin coating on a copper substrate often serves to interconnect electronic components of various kinds, such as are found in medical devices and satellite equipment. Unfortunately, micron-sized tin whiskers (see figure) sometimes arise spontaneously and can short out the equipment, with great technological and economic repercussions. After decades of widespread effort, the actual mechanism underlying such whisker growth has only now been elucidated. Led by Eric Mittemeijer, a group from the Max Planck Institute for Metals Research in Stuttgart, Germany, working with the Robert Bosch company, examined growing whiskers and their crystallographic environment. Using Laue diffraction measurements made at the Advanced Photon Source at Argonne National Laboratory in Illinois, the researchers noted that at the Cu–Sn interface, Cu₆Sn₅ develops along the tin grain boundaries and is most pronounced directly beneath a whisker's root. That observation, coupled with residual strain measurements, led the team to propose the following mechanism: Deep penetration of Cu₆Sn₅ into the 3-μm-thick tin layer induces in-plane compressive strains near the Cu–Sn interface and in-plane tensile strains nearer the surface. Out-of-plane and in-plane strain gradients—not the strains themselves—then provide the driving force that leads to whisker growth by transporting Sn atoms to the whisker nucleation site as a strain-relief mechanism. (M. Sobiech et al., Appl. Phys. Lett., in press.) —Stephen G. Benka

Before they form snowflakes and other hexagonal crystals, water molecules nucleate in smaller configurations. Determining the structure of those precursors—even in the outwardly simple case of water on a clean metal surface—is an area of ongoing interest and controversy. For example, at submonolayer coverage on a copper (110) surface, water molecules form chains that can grow to many tens of nanometers in length but are just 1 nm wide. The chains’ structure has been a mystery, since no arrangement of water molecules into hexagonal units entirely explains the experimental data. Now, Andrew Hodgson and colleagues of the University of Liverpool in the UK have collaborated with Angelos Michaelides’ group at University College London to find the structure. Michaelides and postdoc Javier Carrasco ran calculations on some 50 possible chain structures; they found that the most energetically stable one also gave the best fit to the Liverpool group’s high-resolution scanning tunneling microscopy images (as shown in the top panel) and vibrational spectra. That structure (bottom panel) is an arrangement of pentagons, not hexagons. The water molecules shown in red and yellow are perpendicular to the Cu surface—the hydrogen atoms pointing up are responsible for the bright spots in the STM images, and the ones pointing down (not visible in the figure) interact with the Cu atoms. The researchers suggest that nonhexagon arrangements might be involved at other water–metal interfaces where the structure of water is unknown. (J. Carrasco et al., Nat. Mater., doi:10.1038/nmat2403.) — Johanna Miller

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

Quantum communication networks and other quantum information processing will require coherent and efficient transfer of information between light and matter, and the realm of light-matter interfaces is an active area of research. Much of the activity has focused on the mapping of quantum information onto atomic systems (see, for instance, Physics Today, March 2001, page 17). Nicolas Gisin and colleagues at the University of Geneva in Switzerland have now demonstrated the coherent storage and retrieval of information using a solid-state system. The team's quantum memory was an ensemble of roughly 107 neodymium ions trapped in a crystal of yttrium vanadium oxide (YVO₄). In such an environment, the resonant frequencies of the rare-earth atoms are inhomogeneously shifted, which broadens the absorption spectrum. That's normally undesirable, but the researchers turned it to their advantage. By optically pumping some of the Nd atoms out of the ground state, they sculpted the spectrum into a series of regularly spaced absorption peaks--an "atomic frequency comb." An incident weak light pulse, with on the order of one photon or less on average, will be uniformly absorbed by the comb and generate a coherent superposition of collective optical excitations, each at a slightly different frequency. The superposition will initially dephase but will get reestablished after a time determined by the comb spacing; once rephased, the atoms will collectively reemit a light pulse that conserves the coherence and phase of the original pulse. Gisin and company achieved storage times of up to a microsecond. Furthermore, they showed that the ensemble can simultaneously store multiple light fields, and they have proposed a means of on-demand retrieval. With such capability, the authors view solid-state systems as a promising contender for quantum storage. (H. de Riedmatten et al., Nature 456, 773, 2008; M. Afzelius et al., http://arxiv.org/abs/0805.4164.) — Richard J. Fitzgerald

At the very moment a droplet of water breaks away from a dripping faucet, a singularity is formed. The dynamics leading up to the singularity are governed by the competition between the water’s inertia and its surface tension. (Water’s viscosity is low enough that it does not play a role.) In the reverse setup—an air bubble breaking away from an underwater nozzle—the pinch-off process is driven instead by the difference in pressure between the air and the water. As a result, the bubble and droplet systems differ both in the shapes formed and in the dependence on time. Now, Justin Burton and Peter Taborek of the University of California, Irvine, have observed both bubble-like and droplet-like behavior in a single continuously variable system: xenon bubbles in water over a range of pressures (and hence xenon densities). At low pressures, xenon bubbles behave like air bubbles, as shown in the top row of the figure. At 68 atmospheres, the highest practical pressure for the system, the xenon bubbles are 70% as dense as water and look like upside-down water droplets, as shown in the bottom row. To quantify the behavior of the Xe–water system, the researchers measured the width of the pinch-off region’s neck as a function of time before pinch off. For water droplets, the neck width is proportional to time to the 2/3 power; for air bubbles, it is proportional to time to the 0.57 power. By that standard, the researchers observed a sharp boundary between the bubble-like and droplet-like regimes at a xenon density that is 25% of the density of water. (J. C. Burton, P. Taborek, Phys. Rev. Lett. 101, 214502, 2008.) — Johanna Miller

In an emptying bathtub, water forms a whirlpool around the drain. But circular flow can’t persist to the very center of the vortex; there must be a water-free funnel. In 1985 Wojciech Zurek, following on work of Tom Kibble, suggested that “topological defects” analogous to the whirlpool could be generated spontaneously in a system undergoing a second-order phase transition. For a fast enough process in a large enough system, small regions independently change state, being unable to communicate with other, relatively far off regions. That independence allows parameters such as the quantum-mechanical phase angle to arrange themselves in vortex structures. Researchers have seen spontaneous vortex formation in, for example, superfluid helium-3, nonlinear optical systems, and superconductors (see the article by Kibble, PHYSICS TODAY, September 2007, page 47). Now a new system can be added to the list: the Bose–Einstein condensate. Deliberately inducing a vortex in a BEC is nothing new, but recent joint experimental work at the University of Arizona and numerical work at the University of Queensland in Australia represents the first study of spontaneous vortex formation in that particularly clean system. In the experiment, Chad Weiler and colleagues tweaked standard procedures to maximize the chance of their observing spontaneously formed vortices. After a trapped atomic gas transitioned to a BEC over the course of a few seconds, the group removed the trapping potential and imaged the escaping condensate. The vortices are revealed by dark, zero-density spots in the figure; the rightmost image shows two vortices, the others a single vortex. Continuing experiment and simulation together, Weiler and colleagues hope, will shed light on the universality of spontaneous topological defect formation in phase transitions. (C. N. Weiler et al., Nature 455, 948, 2008.) — Steven K. Blau

Related links:

Bose Einstein Condensation Lab at the University of Arizona College of Optical Sciences

Centre for Quantum-Atom Optics at the University of Queensland

In an emptying bathtub, water forms a whirlpool around the drain. But circular flow can’t persist to the very center of the vortex; there must be a water-free funnel. In 1985 Wojciech Zurek, following on work of Tom Kibble, suggested that “topological defects” analogous to the whirlpool could be generated spontaneously in a system undergoing a second-order phase transition. For a fast enough process in a large enough system, small regions independently change state, being unable to communicate with other, relatively far off regions. That independence allows parameters such as the quantum-mechanical phase angle to arrange themselves in vortex structures. Researchers have seen spontaneous vortex formation in, for example, superfluid helium-3, nonlinear optical systems, and superconductors (see the article by Kibble, PHYSICS TODAY, September 2007, page 47). Now a new system can be added to the list: the Bose–Einstein condensate. Deliberately inducing a vortex in a BEC is nothing new, but recent joint experimental work at the University of Arizona and numerical work at the University of Queensland in Australia represents the first study of spontaneous vortex formation in that particularly clean system. In the experiment, Chad Weiler and colleagues tweaked standard procedures to maximize the chance of their observing spontaneously formed vortices. After a trapped atomic gas transitioned to a BEC over the course of a few seconds, the group removed the trapping potential and imaged the escaping condensate. The vortices are revealed by dark, zero-density spots in the figure; the rightmost image shows two vortices, the others a single vortex. Continuing experiment and simulation together, Weiler and colleagues hope, will shed light on the universality of spontaneous topological defect formation in phase transitions. (C. N. Weiler et al., Nature 455, 948, 2008.) — Steven K. Blau

Related links:

Bose Einstein Condensation Lab at the University of Arizona College of Optical Sciences

Centre for Quantum-Atom Optics at the University of Queensland

The melting transition has long fascinated physicists, both for its ubiquity in nature and industry and for the sophisticated physics of the phase transition in general. Two-dimensional systems can mimic surfaces, which melt differently from bulk matter. One such system is a 2D dusty plasma: Background gas in a vacuum chamber is ionized when RF power is applied to an electrode. With sufficient care, one can levitate a single layer of charged “dust” microspheres above the electrode; electrostatic repulsion spreads the particles apart, usually in a stable 2D crystalline pattern. At Ohio Northern University, Terrence Sheridan came up with a new way to heat only the layer of dust. He modulated the RF power at a resonance frequency so as to jiggle the dust up and down; some of that motional energy coupled to an in-plane acoustic instability, increasing the dusty plasma's effective temperature. The panels show the dust distributions for different modulation amplitude levels. At 1.0%, the entire system oscillates vertically as a crystalline rigid body. As the hexagonal crystal is “heated,” the coupling becomes evident in the central region at 1.6%. The crystal begins to melt at 2.2% and enters a hexatic liquid-crystal phase; it fully melts at 2.8%. For more on dusty plasmas, see PHYSICS TODAY, July 2004, page 32. (T. E. Sheridan, Phys. Plasmas, in press.) —Stephen G. Benka

When you pull hard enough on two objects that are stuck together by an adhesive, they become unstuck. How that happens depends on the properties of the adhesive. A viscoelastic liquid deforms into thin fibrils as air penetrates the bulk of the adhesive, whereas an elastic solid can debond cleanly from the surface of one of the objects as air enters at the interface. Now, Julia Nase, Anke Lindner, and Costantino Creton of the École Supérieure de Physique et Chimie Industrielles in Paris have studied the debonding of adhesives with a range of viscous and elastic properties spanning those two extremes. To tune the properties, the researchers used polydimethylsiloxane with varying degrees of cross-linking among the polymers: The more cross-links, the more elastic the material. The bulk and interface mechanisms were distinguished by the patterns formed as air fingers penetrated between the surfaces, as shown in the figure. In the viscoelastic case, the characteristic size of the fingers decreased as the pulling speed increased; in the elastic case, the size was independent of the speed. Surprisingly, the researchers observed a sharp transition between the viscoelastic and elastic regimes, with no intermediate mechanism between bulk and interfacial debonding. (J. Nase, A. Lindner, C. Creton, Phys. Rev. Lett., in press.) — Johanna L. Miller

Theorists have long predicted that scattering materials, such as milk and white paint, contain transparent channels that light can pass right through. Ordinarily, very little of the light in an incident beam enters those so-called open channels, and the materials appear opaque. But it should be possible to create a shaped wave that couples more light into the open channels. In fact, Ivo Vellekoop and Allard Mosk of the University of Twente in the Netherlands have done just that. Last year, they demonstrated a method for shaping a light wave, by tuning the relative phases of segments of their beam, so that much of the light transmitted through their scattering sample would be focused to a point. (See PHYSICS TODAY, October 2007, page 26.) Now, using the same algorithm with an improved apparatus, they’ve shown that they can increase the total amount of light passing through the material by as much as 44%. However, their device’s performance is still limited by tiny drifts of the sample with respect to the beam. Extrapolating from their results, they confirmed the theory’s prediction that if they could implement their algorithm perfectly, about two thirds of the incident light would be transmitted, regardless of the thickness of the scattering material. (I. M. Vellekoop, A. P. Mosk, Phys. Rev. Lett., in press.) — Johanna L. Miller

Graphene makes a nearly ideal substrate for transmission electron microscopy (TEM). Composed of a honeycomb carbon lattice just one atom thick, it is the thinnest, most transparent support possible for adsorbed atoms and molecules. But it’s also strong and conductive, able to trap the atoms long enough for good images to be captured and yet suffer minimal charging effects from the electron beam. Researchers from the University of California, Berkeley, and Lawrence Berkeley National Laboratory have taken advantage of those properties to reveal individual hydrogen and carbon adatoms that appear as if suspended in free space. In order to enhance the signal-to-noise ratio, the team summed multiple scans.As pictured here, carbon (black) and hydrogen (gray) adatoms show up as dark spots on a bright background, and atomic vacancies created from electron irradiation appear as white spots. The visualization of the low-contrast light atoms allowed the team to follow the dynamics of individual adsorbed atoms and organic molecules in real time for several minutes. The demonstration presents a straightforward way of using TEM to study atomic-scale chemical diffusion and reactions that occur under electron irradiation. (J. C. Meyer, C. O. Girit, M. F. Crommie, A. Zettl, Nature 454, 319, 2008.) — R. Mark Wilson

Sodium is volatile. It easily burns and boils and diffuses. Meteorites are hardy, and the type known as chondrites are also primitive, dating back to the very early solar system. Chondrites contain a high density of so-called chondrules—roughly millimeter-sized spheres like the one shown here in polarized light—that were flash-melted at temperatures around 2000 K and subsequently cooled and incorporated into a meteorite's parent object, typically an asteroid. The heating mechanism is unknown but could involve shocks or lightning. Mostly made of silicate minerals such as olivine and pyroxene and of the metals iron and nickel, chondrules are expected to be deficient in volatile elements like sodium. But researchers at the Carnegie Institution of Washington, the US Geological Survey, and the American Museum of Natural History say it isn't so. Using electron microprobe spectroscopy, they studied 26 chondrules from the Semarkona meteorite that fell in India in 1940 and found significant sodium throughout. The only way that could happen, they say, is if the chondrules formed as closed systems at densities in the solar nebula (the disk of gas and dust from which the planets formed) that were far higher than previously thought. That way, the cooling droplets would be crowded together in an area of saturated sodium vapor. The required ambient densities range from 10 to hundreds of grams per cubic meter, far exceeding the standard assumption of 0.1 g/m³ or less. At the much higher densities, astronomically tiny regions just a few thousand kilometers across can collapse under their own gravity. Thus chondrule formation seems to be intimately linked to planetesimal formation, the first step in making planets like Earth. (C. M. O'D. Alexander et al., Science 320, 1617, 2008 [MEDLINE].) — Stephen G. Benka

Related links:
Department of Terrestrial Magnetism