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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 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

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

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

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

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

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

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.

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 ϕ_c—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 ϕ_c, 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

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

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

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

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

In photonic devices and networks, optical waveguides routinely need to be coupled, split, and switched; several methods are in use to fabricate those junctions. A promising technique for making three-dimensional junctions in transparent materials, such as silica glass, is to write them with femtosecond lasers. Under optimal laser-writing conditions, a focused spot photoexcites the glass through nonlinear absorption and locally changes the refractive index. Slowly moving the material stretches the spot into a line of altered index that acts as a waveguide. For creating junctions, however, it has proven difficult to precisely position the material for each branch. Now, researchers at Japan's Kyoto University have sidestepped that issue by introducing parallel laser writing of multiple branches in three dimensions. The key is computer-generated holograms. With a CGH and a single laser, multiple beam spots can be focused at precise locations in the glass. As the material is moved, CGHs are sequentially swapped in and out to change the spots' locations. Using 256 CGHs, the researchers fabricated a 20-mm-long continuous waveguide that split and spread into four branches about 85 µm apart. A schematic is shown on the left, and the output from a single incident 635-nm laser beam is shown on the right. Optimization of the method is under way. (M. Sakakura et al., Opt. Express 18, 12136, 2010.)—Stephen G. Benka

When chemically modified with water in a process called hydration, cement morphs into the durable binder that holds gravel, sand, and other additives together to form concrete—the most used manmade material in the world. The main constituent of hydrated cement is CaO-SiO₂-H₂O (called C-S-H) in the form of nanoscale colloidal aggregates, the size, shape, and packing of which are crucial to the ultimate strength and stability of concrete. The solid C-S-H nanoparticles are generally thought to be analogous to the claylike minerals tobermorite and jennite, mixed with calcium hydroxide. But new neutron-scattering studies by Jeffrey Thomas and Hamlin Jennings of Northwestern University and Andrew Allen of NIST in Gaithersburg, Maryland, show that C-S-H has a higher-than-expected atomic packing density. The mass density of solid C-S-H is roughly 10% higher than that of a mixture of its widely used mineral analogues with the same composition. The result has important implications for the modeling of cement paste. (See, for example, Physics Today, November 2009, page 23, where that model's starting point is dry tobermorite.) The researchers also investigated the composition and density of C-S-H cured at elevated temperatures and with various additives. In particular, they found that curing the cement at 80 °C led to a lower atomic packing density. Such atomic packing variations suggest the possibility to control chemical shrinkage and the associated cracking of concrete. (J. J. Thomas, H. M. Jennings, A. J. Allen, J. Phys. Chem. C, in press. —Stephen G. Benka

In materials, as the axiom goes, structure follows function: A metal’s tightly bonded atomic crystal lattice gives it strength, and a polymer’s mesh of macromolecular chains makes it elastic. Medical implants, electronic components, and other similar devices call for multifunctional materials that are both strong and stretchy. One such material is the shape-memory alloy (SMA), a polycrystalline arrangement of assorted metals that, when stressed, undergoes a structural phase transition from high to low symmetry. The transition is reversible, and above a critical temperature SMAs are superelastic—they fully recover after being stretched well beyond the reversible-deformation strain values of pure metals. Now, materials scientists at Tohoku University in Sendai, Japan, have presented evidence for an iron-based SMA that is 35 times as elastic as pure metals. The new alloy, which also features nickel, cobalt, aluminum, tantalum, and boron, has an elastic strain of 13%, as shown in the figure, almost double the value of the more expensive commercial-standard nickel–titanium alloy. Furthermore, the material’s yield strength, 800 MPa, is 1.5 times that of the nickel–titanium SMA. The researchers say that microstructured precipitates similar in composition to the bulk matrix and interspersed through it are a key to the improved mechanical strength. The greater elastic strain and strength could be exploited for mechanical damping in building materials. Also, the ferrous SMA’s magnetism is phase dependent, which makes it potentially useful for electromechanical sensing applications. (Y. Tanaka et al., Science 327, 1488, 2010.)—Jermey N. A. Matthews

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

Biological and medical researchers have long sought to study or control cellular function by inserting biomolecular probes inside the cell. But those probes, which include peptides and nucleic acids, must first cross the cell’s highly selective membrane. Traditional approaches to breaching that barrier are to chemically modify the probe or membrane and to pack the probe into a virus, which fuses to a cell’s membrane before depositing its load; both methods induce unwanted side effects and are limited to delivering specific molecular cargo. Now a team of US and South Korean scientists, led by Harvard University’s Hongkun Park, has developed a minimally invasive delivery method that exploits the ability of silicon nanowires to physically penetrate the cell’s membrane. The researchers prepared vertically aligned nanowire arrays with a density of roughly 25 million nanowires/cm² and altered their surface chemistries to enable noncovalent binding of a broad spectrum of molecules. With the nanowire platform, they were able to simultaneously assay the intracellular effects of distinct molecular probes. In one experiment, the researchers layered human fibroblasts, shown green in the scanning electron microscope image, across the nanowires, shown in blue. Nearly all of the cells were impaled within one hour and received the bound probes within 24 hours. Impaled cells continued to grow for several weeks, albeit at a slightly slower rate. (A. K. Shalek et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0909350107.) — Jermey N. A. Matthews

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

Movie 1

Movie 2

Concrete is the most prevalent synthetic material on Earth, yet the detailed nature of its primary binding constituent, hydrated cement, is only poorly understood. When cement, a dry powder that consists mostly of calcium oxide and silicate, is mixed with water, the material hardens through the formation of a complex hydrated oxide called calcium-silicate-hydrate. But the microscopic structure of C-S-H is largely unknown—even its stoichiometry, as suggested by convention with hyphens. C-S-H's structure had been thought to be related to that of two naturally occurring calcium silicate minerals, but those minerals can't explain C-S-H's observed properties. Armed with recent measurements of C-S-H's density and its ratio of calcium to silicon atoms, a team of researchers at MIT has proposed a new molecular model for C-S-H based on atom-scale simulations: Layers of calcium ions (gray in the figure) are surrounded by silicon (yellow) and oxygen (red) arranged as short silica chains one, two, and five units long; between those layers are water (oxygen in blue, hydrogen in white) and interlayer calcium ions (green) that ensure overall neutrality. The model's chemical composition, (CaO)_1.65(SiO₂)(H₂O)_1.75, agrees well with results from neutron scattering experiments. In addition to reproducing the known structural properties of the material, the model also suggests that at short length scales C-S-H should be viewed as a glassy phase. With an atom-level model of the C-S-H structure now in hand, the researchers hope to be able to manipulate the macroscopic properties of concrete, such as its strength and temperature resistance. (R. J.-M. Pellenq et al., Proc. Natl. Acad. Sci. USA, doi:10.1073/pnas.0902180106, in press.)—Richard J. Fitzgerald

Light sensors—photodetectors—have myriad uses in scientific, industrial, and consumer settings: Digital cameras, environmental monitors, remote controls, surveillance equipment, and biosensors are just a few applications. Most photodetectors are made from inorganic semiconductors and are sensitive in some limited waveband in the range between IR and UV. A new photodetector (PD), however, that uses a semiconducting polymer shows good responsiveness from UV (300 nm) to near-IR (1450 nm), as shown in the figure. The polymer is a small-bandgap semiconductor that exhibits photoinduced, ultrafast electron transfer to fullerenes—blended with the polymer in the form of PC₆₀BM. Sandwiched between two electrodes, the two materials form a phase-separated blend with interpenetrating donor and acceptor networks. Because the new photodetector covers nearly the entire solar spectrum at Earth's surface, the researchers—led by Alan Heeger of the University of California, Santa Barbara and CBrite Inc—note that it holds promise for photovoltaic cells. The next step is to make addressable arrays of these broadband, high-detectivity photodetectors. (X. Gong et al., Science, in press, doi:10.1126/science.1176706.) —Stephen G. Benka

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

The unusual stiffness or sponginess of dead and decaying biological tissue is readily apparent to the human touch. However, early detection of such mechanical property changes in a tissue's extracellular matrix could signal the onset of disease. To measure the elasticity of tissue in living patients, needle-based indentation methods are more direct and less expensive alternatives to MRI, ultrasound, and electrical impedance. Such a probe has recently been developed by University of California, Santa Barbara, physicist Paul Hansma, an atomic force microscopy expert, and his collaborators. The handheld tissue diagnostic instrument (TDI) consists of a stainless steel probe, 175 µm to 1 mm in diameter depending on the tissue sample, which longitudinally oscillates at 4 Hz in a needle-thin stationary sheath. The force from the magnetically controlled oscillation of the probe produces a corresponding displacement in the tissue. The tissue's elastic modulus, or stiffness, is proportional to the slope of the force-displacement curve, and energy dissipation in the tissue is proportional to the area under that curve. The researchers measured, with millimeter spatial resolution, healthy and diseased tissue samples ranging in elastic moduli from around 1 kPa to 12 GPa. Among them were mouse breast tissue, which hardens when it becomes tumorous, and human tooth dentin (see schematic), which softens and decays when infection sets in. The researchers say the instrument could be used in the future to simultaneously test and biopsy a tumor or, if the probe is coated with antibodies, to measure single-molecule interaction forces. (P. Hansma et al., Rev. Sci. Instrum., in press.) — Jermey N. A. Matthews

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

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

The several approaches being pursued for fuel cells vary in their chemical reactions, materials, and optimal operating conditions, but they share a basic configuration (see Physics Today, November 1994, page 54, and October 2006, page 38). A fuel, often hydrogen, is oxidized at the anode, where it liberates electrons. The electrons travel through and power an external circuit and eventually reach the cathode, where oxygen is reduced. Meanwhile, to complete the redox reaction, ions travel through an electrolyte that separates the electrodes. In alkaline fuel cells, first developed for the Apollo space missions in the 1960s, oxygen combines with water and electrons at the cathode to form hydroxyl ions (OH^-) that travel through an aqueous alkaline electrolyte to produce water by combining with protons from the anode. Commercialization of those fuel cells, however, has been limited by the high cost of the platinum used for the cathode. New work by Liming Dai of the University of Dayton and colleagues at the Air Force Research Laboratory and the University of Akron has shown that vertically aligned carbon nanotubes doped with nitrogen provide an efficient, lower-cost alternative for the cathode. Nitrogen-doped nanotubes have better long-term stability and, unlike Pt, are not harmed by the presence of carbon monoxide or any fuel molecules that cross the electrolyte from the anode to the cathode. The researchers attribute the catalytic performance to the relatively high positive charge density on the carbon atoms adjacent to the nitrogen atoms. (K. Gong et al., Science 323, 760, 2009.) — Richard J. Fitzgerald

In typical loudspeakers, a coil surrounds the apex of a flexible cone; when a varying current flows through the coil, the cone moves toward and away from a fixed permanent magnet and produces pressure waves we hear as sound. But researchers from Tsinghua University and Beijing Normal University have demonstrated a radically simpler loudspeaker design based on nanotubes: They showed that a thin film of nanotubes can reproduce sounds over a wide frequency range--including the full human audible range--with high sound pressure level, low total harmonic distortion, and no magnets. The team created the film by drawing nanotubes from a so-called superaligned array grown on a wafer, a technique the group introduced six years ago (see also PHYSICS TODAY, October 2005, page 23). The resulting film, only tens of nanometers thick but up to 10 cm wide, is transparent and has a nearly purely resistive impedance. When electrodes are placed along its ends and an alternating current is applied, the film produces clear tones that can be as loud as a conventional speaker. Moreover, since the film is flexible, the nanotube speaker can be configured into arbitrary shapes or mounted onto curved substrates; the figure shows an omnidirectional cylindrical loudspeaker 9 cm in diameter and 8.5 cm high. The film can even be stretched with essentially no degradation of the sound reproduction. The researchers attribute the sound generation not to vibration but to a thermoacoustic effect first proposed nearly a century ago: Thanks to the nanotube film's extremely low heat capacity per unit area, changes in the current flowing through the film are reflected in the film's temperature, and those temperature changes excite pressure waves in the surrounding air. The mechanism is independent of the sign of the current, which leads to a frequency doubling of the input signal, but that drawback can be overcome by applying a constant current bias. The movie shows a nanotube loudspeaker being periodically stretched with almost no noticeable effect on the sound intensity. (L. Xiao et al., Nano Lett., in press, doi:10.1021/nl802750z.) -- Richard J. Fitzgerald

The Vitthala Temple in the south Indian city of Hampi is more than 500 years old. The image here shows its most curious feature—numerous pillars, each of which includes separate columns that sound musical notes when struck with a finger. Different columns in a pillar produce sounds of different frequencies. Moreover, several multi-columned pillars make sounds similar to specific Indian musical instruments such as the ghanta (bell), mridanga (percussion), or veena (strings). Well known for centuries, the musical pillars are only now beginning to be studied scientifically. Anish Kumar of the Indira Gandhi Center for Atomic Research in Kalpakkam, India, and colleagues took the first steps to characterize the columns: The physicists applied three techniques to learn about the structure of the columns and also analyzed recordings of generated sound. In situ metallography showed the granite to have typical microstructures; both low-frequency ultrasound and impact-echo testing revealed all the columns to be solid shafts. From those studies and spectral analyses, the researchers conclude that the pillars' sounds arise from the flexural mode of vibrations. Next on their agenda is to study how the columns can be excited by just the tap of a finger. (A. Kumar et al., J. Acoust. Soc. Amer. 124, 911, 2008.) — Stephen G. Benka

Silicon, silicon dioxide, and other materials typically used to make electronic components are intrinsically rigid and brittle, problematic properties if you want to make a compact device that’s tough or flexible. In principle, you could make such devices from stretchy organic materials. The trouble is that suitable insulators and semiconductors exist, but not conductors. The University of Tokyo’s Takao Someya and colleagues have now solved the conductor problem by embedding single-wall carbon nanotubes in a flexible polymer. Prior efforts to dope a polymer with nanotubes had produced composites that were unstretchable due to the tubes’ natural tendency to clump because of attractive van der Waals interactions. Someya’s team mixed ultralong nanotubes with an ionic liquid that disperses them uniformly in the polymer without sacrificing the composite’s flexibility. The result was an elastic compound with a conductivity of 57 siemens per centimeter, two orders of magnitude higher than the most conductive elastomers. As proof of principle that the material is suitable for skinlike circuitry, the group fashioned the compound into thin fishnet-like strips, stretchable up to 134%. Those strips were then used to link an array of semirigid thin-film transistors in the two-dimensional elastic matrix shown here. The black polymer strips are visible through a layer of white silicone rubber added for stability, and the device can be stretched up to 70% with negligible effect on its electrical behavior. (T. Sekitani et al., Science 321, 1468, 2008.) — R. Mark Wilson

The loops and folds that result when a sheet, tape, or wire crumples are of practical and theoretical interest. Engineers want to predict how structures deform under stress; physicists want to reduce diverse crumpling behavior to a few simple principles. Toward that second aim, Norbert Stoop, Falk Wittel, and Hans Herrmann of ETH Zürich have conducted an experimental study of one elementary system: a length of metal wire stuffed from two opposing directions into a cylindrical container so shallow that the crumpling is two-dimensional. At the start of each run, the wire spanned the container in a straight line. Two counterrotating drums then pushed more and more of the wire into the container until, having bent to form a loop, the wire touched the side. What happened next, the researchers found, depended on the wire's elasticity and on the friction between the wire and the container. When friction is high, the wire adopted near-symmetric looping patterns, which the researchers termed classical. When friction is low and the wires are stiff and springy (the researchers used steel), the wire adopted spiral patterns. Floppy, soft wires (solder) adopted messy, asymmetric patterns, which the researchers termed plastic. By adjusting the elasticity and friction in their experiment, the researchers could delineate the three regimes in a morphological phase diagram. And, as the figure shows, they could reproduce the three phases with a simple continuum model. The ETH team anticipates their phase diagram could prove useful in characterizing the packing of DNA inside viral capsids and other crumpling systems. (N. Stoop, F. K. Wittel, H. J. Herrmann, Phys. Rev. Lett., in press.) ― Charles Day