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

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

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

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

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Robert Singer explains super-registration microscopy

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

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

Just as accelerating charge produces electromagnetic radiation, accelerating mass is predicted to produce gravitational radiation. The effect of a gravitational wave’s alternating distortions of space could be detected by a Michelson interferometer, but gravity’s weakness means that extraordinary sensitivity is needed to observe even a relatively intense wave. The Laser Interferometer Space Antenna (LISA) is a proposed mission to achieve that sensitivity with an interferometer 5 million km on a side, its vertices located on three spacecraft orbiting the Sun. But the inevitable fluctuations in laser frequencies introduce noise a billion times more intense than the signal from the gravitational waves researchers hope to see. The solution, a technique called time-delay interferometry (TDI), is to reduce noise not through better stabilization of the physical components but by signal processing. In essence, the phase gained by light traversing each arm is subtracted from the phase from the same arm offset by the round-trip time for the other arm; subtracting those two differences then yields a quantity unmarred by laser frequency fluctuations. In two more steps, noise due to clock error can likewise be eliminated. Now, researchers at NASA’s Jet Propulsion Laboratory have demonstrated TDI in a laboratory experiment designed to mimic LISA’s noise environment. They’ve shown that the technique can indeed reduce laser frequency noise and clock noise by the necessary nine orders of magnitude. (G. de Vine et al., Phys. Rev. Lett. 104, 211103, 2010.)—Johanna Miller

In the presence of a suitable nucleating agent, a liquid in a metastable state below its thermodynamically defined melting point freezes. That’s what happens when atmospheric aerosol particles cause supercooled water droplets in clouds to form snowflakes. Researchers have suspected that the atomic surface structure of such seeding particles acts as a template, inducing local order in the disordered liquid and catalyzing its crystallization. Conversely, a solid with a different structure can inhibit crystallization, as has now been observed at the European Synchrotron Radiation Facility in Grenoble, France, by Tobias Schülli and his colleagues. The researchers coupled x-ray scattering data with molecular-dynamics simulations to study supercooled gold-silicon droplets on a silicon substrate, a system that is used to grow Si nanowires. Surprising results emerged when they heated the AuSi alloy above 676 K: As it cooled, the Si atoms leached onto the substrate and, as the figure shows, rearranged its surface atoms into pentagonal clusters. The alloy’s atoms near the interface mimicked the substrate’s surface structure (see inset), but the resulting local order did not promote crystallization in the droplets, which froze at 513 K, about 120 K below the freezing temperature for the AuSi alloy. Apparently, the pentagonal geometry inhibits freezing because it is not conducive to crystal packing. That finding suggests that substrates with such atomic structures offer a simpler method of maintaining and observing the supercooling process than such techniques as magnetically levitating or otherwise suspending the liquid droplet. (T. Schülli et al., Nature 464, 1174, 2010.)—Jermey N. A. Matthews

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

In a combustion engine, work is produced from heat liberated by burning the fuel. In hydrocarbon fuel cells, the fuel is directly converted into electricity. Both types of engines, however, waste heat and emit gas byproducts that are considered useless—or even pernicious, as in the case of the greenhouse gas carbon dioxide. But Martin Gellender, an environmental officer for the state government of Queensland, Australia, makes the case for exhaust gases as an energy source: In a conceptual paper, he explores the overlooked entropy increase that occurs when concentrated gases isothermally mix with air. As illustrated in the schematic, if an exhaust gas mixture containing, for example, CO₂ at a high concentration is separated from air by a piston-membrane barrier that selectively blocks CO₂ passage, the concentration gradient performs work on the piston until the CO₂ concentrations on both sides are equal. According to Gellender’s calculations, a secondary entropy engine could theoretically recover up to 7% of the fuel’s energy and could provide a power boost to the primary engine: up to 1.5% for combustion engines and up to 3.5% for fuel cells. He says that new fuel-cell designs and material advances could lead to a practical entropy engine that reduces the fuel consumption of power plants. (M. Gellender, J. Renew. Sust. Energy, in press.)—Jermey N. A. Matthews

Designers of transportation networks have to weigh the cost of serving customers against the need for an efficient, robust system. Natural organisms, too, confront tasks in which they need to balance competing desiderata. As it forages for food, for example, a slime mold must balance cost (that is, the amount of protoplasm it extrudes), efficiency, and the ability to withstand injury. Remarkably, as recently reported by Atsushi Tero and colleagues from Japan and the UK, the molds do as well as transportation engineers in balancing their analogous competing needs. Panel a of the figure re-creates a 17-cm-wide map of the principal cities served by the Tokyo railway system with a slime mold (yellow) at the location of Tokyo and food flakes (white) representing other cities. In about a day’s time, the slime mold finds where the nourishment is and generates a protoplasm network with the food flakes as nodes. Standard metrics for analyzing transportation networks reveal that the mold’s foraging network and the Tokyo railway system perform similarly. Perhaps more significantly, Tero and company imitated slime-mold networks in numerical simulations that don’t incorporate detailed biochemistry. Instead, they include a feedback step in which tubular links carrying a large protoplasm flux grow wider and flux-poor links contract. By tweaking their simulation parameters, the researchers could nudge the network toward, for example, greater cost efficiency. With optimal parameters, they could even improve upon the work of slime molds and human engineers. (A. Tero et al., Science 237, 439, 2010.) —Steven K. Blau

The phenomenon of dynamic stabilization can be demonstrated with an inverted pendulum: If the pivot point vibrates fast enough and strongly enough, the pendulum aligns with the vibration direction and can stably stand upside down, even at an angle, seeming to defy gravity. Physicists Greg Swift and Scott Backhaus (Los Alamos National Laboratory) looked at an analogous situation with gas in a so-called pulse tube that has one end much hotter than the other. Colder gas is denser and therefore sinks below the hotter gas; a vertical tube with the cold end down is like an undisturbed pendulum with the heavy bob at the bottom. However, raise the cold end above the hot end and convection sets in—the cold gas falls due to gravity and the hot gas rises in a natural convective flow. Such orientation-dependent effects are undesirable for cryogenic thermoacoustic pulse-tube refrigerators, like the commercial one shown here, in which the gas is used to transmit acoustic power but not heat. (For more on thermoacoustics, see Physics Today, July 1995, page 22.) Swift and Backhaus found that suppression of convection when these refrigerators run at high enough frequency and amplitude is related to the well-understood stabilization of the inverted pendulum. Although their experiments and theoretical analysis are beginning to unravel the essentially nonlinear physics at the core of the system, many mysteries remain, including the actual role of the oscillating pressure. (G. W. Swift, S. Backhaus, J. Acoust. Soc. Am. 126, 2273, 2009.) —Stephen G. Benka

In principle, setting a droplet in motion inside a microfluidic channel is straightforward: Apply pressure and the liquid flows. In practice, however, precise control of droplet flow simultaneously along multiple channels is technically challenging; conventional pressure pumps are not feasible for microfluidic systems. Inspired by the potential of finely tunable acoustic-pressure generators, a group of engineers at the University of Michigan set out to control droplet motion with music. First, they composed a computer-synthesized sequence of single notes and chords. That signal was then sent to four resonance cavities that were tuned according to their lengths to extract and amplify narrow, non-overlapping frequency bands from the input tones. As shown in the figure and the movie, unidirectional droplet flow was generated from the difference between positive air pressure in the oscillating cavity and relative negative pressure at vent ports near the cavity's outlet. Although the relatively high frequencies of the selected tones produced steady flow, the researchers adjusted the relative amplitudes of the input tones as needed to compensate for variations in average flow velocity. Maybe someday, conducting complex lab-on-chip microfluidic operations will be as simple as stringing together a few musical notes. (S. M. Langelier et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0900043106.) — Jermey N. A. Matthews

Familiar software-based “random number generators” rely on deterministic algorithms, so their outputs are not actually random. For some applications, such as Monte Carlo simulations or randomized music playlists, that’s not a problem. But for others, such as secure communications or online gaming, it’s important to use numbers that are truly unguessable, such as can be generated from measurements of stochastic physical processes. One candidate is the chaotic output of a semiconductor laser: When a certain fraction of laser light is fed back into the laser, small intensity fluctuations (thought to be quantum in origin) are amplified into large, irregular, subnanosecond oscillations, as shown in the figure. Last year, Atsushi Uchida (Takushoku University, Tokyo) and colleagues used digitized measurements of two semiconductor lasers to generate random binary sequences at a rate of 1.7 billion bits per second—much faster than any previous method based on a physical system (A. Uchida et al., Nat. Photonics 2, 728, 2008). Small drifts in the lasers’ average intensities can throw off the balance of ones and zeros, but with the lasers suitably tuned, the sequences meet statistical criteria for randomness. Now, Michael Rosenbluh, Ido Kanter, and their students at Bar-Ilan University in Ramat-Gan, Israel, have developed a new method, using just one laser, that avoids the problem of intensity drift. From the differences between successive intensity measurements, they can generate 12.5 billion random bits per second. (I. Reidler et al., Phys. Rev. Lett., in press.) —Johanna Miller

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

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

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

To map molecules in cells and tissue, researchers prefer biomedical imaging techniques that rely solely on the intrinsic responses of chemical bonds to optical stimulation. Although fluorescence microscopy and other chemical tagging methods yield high-resolution images, they also introduce foreign species or synthetic derivatives that can alter the dynamics of intracellular processes. Spontaneous Raman scattering, which uses a single laser beam to excite the vibrational and rotational modes in chemical bonds, requires no chemical labels but generates a weak signal that gets muddled by Rayleigh scattering. A more sensitive technique known as coherent anti-Stokes Raman scattering uses multiple laser beams to generate coherent optical signals that enhance resonant frequencies in the sample; that method, however, also produces nonresonant background noise. Recently a team led by Harvard University chemist Sunney Xie demonstrated a new technique based on stimulated Raman scattering that tunes the difference between the frequencies of two laser beams to match a desired molecule's resonant frequency, thus amplifying the Raman signal. The measurable intensities of the transmitted beams change only when a match occurs; nonresonant signals are not picked up. The images show the top view (a) and the depth profile (b) of an acne medication (blue) that penetrated a mouse's skin, thus demonstrating the potential of the new technique to monitor drug delivery. (C. W. Freudiger et al., Science 322, 1857, 2008.)
— Jermey N. A. Matthews

With their ability to manipulate microliter to nanoliter volumes of liquids, microfluidic devices have found increasing application in a variety of fields, from ink-jet technology to proteomics and DNA analysis. Most current microfluidic devices are made from glass or polymers, and advances in design and fabrication have opened the realm of three-dimensional, complex flow paths. George Whitesides and colleagues at Harvard University have recently demonstrated 3D devices made from stacked layers of ordinary paper and tape. Thanks to paper's wicking ability, the devices don't require external pumps to drive the liquids through. Indeed, the wicking property of paper is routinely exploited in medical tests such as those for blood glucose, pregnancy, and HIV. To define the microfluidic pathways in the paper-based microfluidic device, the team impregnated each paper layer with a common photoresist, a hydrophobic polymer that could be patterned with UV light. With their channels thus established, the layers of paper were alternated with layers of double-sided tape; holes cut in the tape connected channels in adjacent paper layers. The figure illustrates the complex routing that can be achieved: Four differently colored liquids deposited on the top of a 5 cm × 5 cm, nine-layer stack (left) are, within 5 minutes, wicked through horizontally and vertically to the array of 1024 detection zones on the bottom (right). With reagents or antibodies placed in detection zones prior to assembly, such devices would provide highly parallel, independent assays. The Harvard team sees particular potential for their paper-based devices in medical diagnostics in developing countries. (A. W. Martinez, S. T. Phillips, G. M. Whitesides, Proc. Natl. Acad. Sci. USA, in press.) — Richard J. Fitzgerald

Through its influence on evaporation rates, humidity levels, and other factors, the moisture content of soil has a significant impact on weather. Accurate measurements of that content, though important for meteorological, hydrological, and ecological forecasting, are difficult to make. Extrapolating point measurements to larger areas is inaccurate, and satellite-based remote-sensing methods are hindered by ground cover, surface roughness, and other limitations. A team from the University of Arizona and the Southwest Watershed Research Center in Tucson has shown that just above the ground surface, so-called fast neutrons with energies on the order of an MeV are quantifiably correlated with soil moisture and thus provide a noninvasive means for measuring the average moisture levels over regions several hundred meters wide and tens of centimeters deep. The neutrons are generated by cosmic rays. Upon collision with atmospheric nuclei, cosmic rays create showers of high-energy secondary particles, and those that reach Earth's surface can penetrate it, collide with nuclei there, and produce among their debris fast neutrons, some of which escape back into the atmosphere. Marek Zreda and colleagues discovered that hydrogen, mostly found in water, dominates soil's ability to moderate fast neutrons and that a strong inverse correlation, independent of soil chemistry, exists between moisture content and the intensity of the fast neutrons that escape out of the ground. The team demonstrated that with an independent measurement of the moisture content for calibration, a neutron detector a few meters above the ground can give precise measurements of soil moisture on the time scale of minutes to a few hours. As the figure shows, the hourly soil moisture determined by a cosmic-ray neutron detector (top) agrees with that determined by time-domain reflectometry probes (middle) and with the monitored daily precipitation (bottom). (M. Zreda et al., Geophys. Res. Lett. 35, L21402, 2008, doi:10.1029/2008GL035655.) — Richard J. Fitzgerald

Catalysts are ubiquitous in today's chemical industry, but there remains much to be learned about the specific mechanisms by which many of them work. Though such knowledge could lead to improved or new catalysts, obtaining atomic-scale information about in situ chemical changes in a hot environment at atmospheric pressure has presented a difficult challenge. A Dutch team led by Frank de Groot and Bert Weckhuysen of Utrecht University has recently demonstrated the potential of a new approach to imaging catalysts at work: scanning transmission x-ray microscopy. As a catalyst and reactants interact, the valence states and chemical bonding of the participating atoms evolve. STXM detects those changes by looking at the absorption of x rays by the atoms' inner electron shells. The researchers demonstrated the technique by looking at the iron-based catalyst for the Fischer–Tropsch process, in which hydrogen and carbon monoxide are converted to hydrocarbon chains. Soft x rays used in STXM are strongly attenuated in matter, so the research team used a nanoreactor of thickness 50 μm; the reactor was connected to gas lines and mounted on an adapter that was scanned in 35-nm steps through the focus of a monochromatic x-ray beam. In that way, two-dimensional absorption maps at various x-ray energies could be recorded. The researchers paid particular attention to energies near the absorption edges of carbon, oxygen, and iron. Analyzing the maps they obtained, the researchers could extract the carbon hybridization states and determine the extent to which the iron atoms, which started off in iron oxide, had been reduced, formed other oxides, or reacted with the silicon dioxide substrate or with carbon. The figure maps the distribution of the inferred iron compounds, each represented by a different color. With better optics and detection techniques, the team hopes to improve on its current 40-nm resolution and perhaps provide time-resolved and 3D imaging of complex chemical reactions in situ. (E. de Smit et al., Nature 456, 222, 2008.) — 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

Overuse of antibiotics has spawned strains of bacteria whose cell walls are impervious to the crippling blows once delivered by penicillin and its derivatives. One such so-called superbug, methicillin-resistant staphylococcus aureus, although found primarily in prisons and hospitals, has now spread beyond those confines. Despite the controlled use of the drug vancomycin, a last line of defense against MRSA, the latest threat are vancomycin-resistant bacteria, which mutate by deleting a key hydrogen bond that allows the drug to bind and inhibit cell wall growth, thereby mechanically weakening the bacteria . Rachel McKendry at University College London and her collaborators recently demonstrated a nanoscale cantilever system that is sensitive enough to detect the difference between the native drug-sensitive bacteria and the mutated resistant form with the missing hydrogen bond. The researchers coated silicon cantilevers with vancomycin-resistant (DLac in the schematic) and vancomycin-sensitive (DAla ) bacterial cell-wall analogues, then immersed them in a solution containing free vancomycin molecules. As expected, the molecules preferentially bound to the cantilevers coated with the drug-sensitive analogue; those cantilevers experienced a marked deflection—as measured by an optical detector—that equated to an 800-fold difference in binding compared with the cantilevers coated with the drug-resistant analogue. The researchers believe their system will lead to sensitive, nondestructive, and rapid nanomechanical biosensors for high-throughput drug– target interaction studies and will aid in the design of more effective drugs. (J.W. Ndieyira et al., Nat. Nanotechnol., doi: 10.1038/nnano.2008.275 .) — Jermey N.A. Matthews

The recently elucidated crystal structure of a promising class of inorganic polymer salts reveals why these materials generate strong second-harmonic generation (SHG) responses to optical stimulation. In general, asymmetric inorganic polymer thin films with highly polarizable bonds exhibit strong nonlinear optical behavior, and are used in some tunable, coherent IR lasers to probe the electronic or structural properties of molecules or surfaces. A team from Northwestern University and Argonne National Laboratory used Argonne’s Advanced Photon Source to study the quaternary salts formed from the zirconium selenophosphate (ZrPSe₆^-) polyanion and its complementary metal cation (K⁺, Rb⁺, or Cs⁺)—this class of salt tends to crystallize as microneedles (see figure 1). The crystal structure (see schematic in figure 2) revealed a distortion in the molecular backbone from its ideal geometry, which contributes to the salt's high polarity. The second harmonic—a beam generated in the crystal and emitted at half the wavelength and twice the frequency of incident light—for the sample with the largest cation, Cs⁺, had an intensity 15 times that produced by a typical commercial nonlinear optical material. Even the smallest cation, K⁺, mixed with Cs⁺, produced about double the SHG response of the commercial benchmark material. The new salts exhibit strong photoluminescence in solution; they are also optically transparent from the mid- to the near-IR region, which gives them potential for use in a range of applications, from broadband communication to medical devices. (S. Banerjee et al., J. Amer. Chem. Soc. 130, 12270, 2008.) — Jermey N. A. Matthews

Symmetry abounds in nature, but the beauty of proteins and other biological polymers lies in their asymmetry. Chirality, or handedness, of DNA and other biopolymers plays a critical role in their biochemical pathways: The binding properties of a left-handed DNA double helix differ significantly from those of the right-handed counterpart. Mimicking nature has not proven easy for scientists attempting to model, study, and engineer asymmetric polymers. Colloids—nanometer- to micron-sized particles—have emerged as molecular building-block candidates (see Physics Today, June 2006, page 15). Left alone, however, they spontaneously clump together or form ordered crystals. A key to making helical structures is steric hindrance, in which the size of the building blocks—atoms, molecules, or colloids—constricts the resulting bond angles. A team of scientists from the Paris Institute of Technology in France and New York University have discovered that binary silica microspheres, joined into dumbbell shapes and with an iron-oxide ring around the joining bond, align and extend into long asymmetric polymer chains in the presence of a magnetic field. The researchers showed that steric repulsion causes either left- or right-handed helices to form when the particles have different diameters. The figure shows an optical microscope image (left) of such a helical structure and its corresponding schematic (right). (D. Zerrouki et al., Nature 455, 380, 2008.) — Jermey N.A. Matthews

Asymmetric dumbbell-shaped magnetic colloids in this real-time optical microscope video cluster and extend into either left- or right-handed chiral polymers.

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

One difficulty with finding extrasolar planets is that a planet’s light is overwhelmed by that of its parent star. To block that light, astronomers typically occult the star with a disk in an instrument called a coronagraph. At the University of Arizona, however, Grover Swartzlander’s group has developed another method: They focus the primary star onto the very center of a so-called optical vortex lens, which acts like a helical phase mask, and the on-axis source of light is removed from the optical path while the off-axis source of light passes through. Shown here is the central region of a 2-mm square OVL. The instrumentalists put the OVL into a coronagraph, incorporated some adaptive optics to eliminate the twinkling caused by atmospheric turbulence, and mounted the entire package on an 8-inch telescope that they pointed at the binary star system Cor Caroli in the constellation Canes Venatici. The false-color image on the left is what they obtained without the optical vortex coronagraph: Only the primary star, labeled a2, with its 12-fold more light flux than the secondary, can be seen. With the OVL in place, the secondary star, a1, became visible. The primary’s light was suppressed by 97%, but not over its entire disk because the optics were not optimally aligned. Next on the researchers’ agenda is to fabricate higher-quality OVLs and more advanced adaptive-optics and optomechanical alignment systems. (G. A. Swartzlander et al., Opt. Express 16, 10200, 2008.) — Stephen G. Benka

Biology, dauntingly complex as it is, nevertheless is slowly becoming more quantitative and thus more amenable to testable models and predictions. For example, an embryo's various organs and body parts develop at different times and at different rates. How can one come up with a rigorous model for the process? James Sharpe (Centre for Genomic Regulation, Barcelona, Spain) and his colleagues are beginning to address that question with a new imaging technique: time-lapse optical projection tomography. Their setup involves taking live tissue from a mouse embryo and transferring it on tungsten pins to a nutrient- and oxygen-rich chamber. The pins are on a mount that is magnetically attached to a micromanipulator, which rotates the tissue through 360° in 100–200 steps. Labeling gene activity within the tissue with green fluorescent protein and using deep-penetrating 800-nm light, the researchers acquired a full set of images every 15 minutes. The images here of three-dimensional surface renderings show the dynamic activity of a gene involved in controlling development of the limb, as it buds out from abdominal tissue, at 0, 13, and 19 hours. The researchers quantified the global dynamics by measuring the surface expansion through tissue velocity vector fields. Surprisingly, the limb buds didn't simply expand radially but twisted and showed other spatial variations as they grew. In other experiments, Sharpe and company imaged dynamic changes in spatial gene-expression patterns in growing limbs and studied the early development of embryonic mouse eyes. (M. J. Boot et al., Nat. Methods, advance online publication, doi:10.1038/nmeth.1219, 30 May 2008.)     — Stephen G. Benka

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OTP Microscopy

At the May Conference on Lasers and Electro-Optics in San Jose, California, University of Colorado graduate student Mark Siemens reported on studying how tiny parcels of heat, called phonons, spread in a crystal. He and his colleagues used a near-IR laser to heat a grating of nickel lines—each 20 nm high and 1 µm
wide—grown on a sapphire substrate that acted as a heat sink. Then, by recording the transient diffraction of 10-fs pulses of coherent soft x rays from the sample, the researchers could monitor with picometer (10^-12 m) precision the displacement of the heated nickel nanostructure. The transport of heat is considered "ballistic" if the characteristic distance over which a phonon moves—about a micron in this case—is smaller than its mean free path before scattering off another phonon. At room temperature a typical phonon's mean free path in sapphire is a mere 150 nm but grows to more than a micron when the sample is cooled below 130 K. At that temperature the data show a clear transition from thermally diffusive to ballistic behavior. One reason for trying to understand how heat moves away from a nanoscale interface, says Siemens, is to manage the thermal environment of future advanced high-speed transistors. — Phillip F. Schewe

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Nanoscale Probes of Materials

A new experiment, conducted by scientists from France, Switzerland, and the UK, has recorded the largest-ever change brought about in a bulk material's electrical resistance by straining the material at room temperature. Called piezoresistance, the phenomenon is often exploited in sensors. In simple metal-foil piezoresistors, the kind used to examine the integrity of a concrete wall or to monitor a prosthetic limb, the change in resistance per unit of strain (a ratio referred to as the gage factor) has a typical value of about 2. For silicon-based piezoresistors, the kind used in cell phones and airbag accelerometers,
the gage factor is usually about 100. The new experiment uses a silicon–aluminum hybrid material in which the arrangement of the components, not their composition, is of paramount importance. The metal—in this case aluminum—is effectively a current shunt; applying a mechanical stress to the device deflects current toward or away from the shunt and thereby alters the device's resistance. For appropriate geometric configurations, the researchers, led by Alistair Rowe of the École Polytechnique in Palaiseau, France, measured a gage factor of nearly 900, the largest ever seen at room temperature in a bulk material. Giant piezoresistive structures could be good news for the designers of microelectromechanical devices in which the measurement of ultra-small accelerations or atomic-scale deflections is important. Alternatively, higher sensitivity to movement can be translated into lower power requirements when battery energy is at a premium,
as in cell phones. (A. C. H. Rowe et al., Phys. Rev. Lett. 100, 145501, 2008 [SPIN].)    — Phillip F. Schewe

In the pursuit of a quantum computer, the photon is a leading candidate for the quantum bit, or qubit. Working models of photonic circuits, however, have been unscalable arrangements of bulky mirrors and beamsplitters sitting atop a square-meter-sized table. Now scientists at the Center for Quantum Photonics at the University of Bristol in the UK have printed several dozen photonic circuits onto a silicon wafer. The research team created waveguides by first depositing a doped layer of silica onto the wafer, then patterning 3.5-micron-wide ridges into the silica. Two waveguides are coupled when they approach each other and then diverge, as shown in the figure, allowing evanescent waves to overlap. Using such directional couplers, the researchers not only fabricated on-chip beamsplitters, interferometers, and even a controlled-NOT gate, but combined those devices into photonic circuits. Among their demonstrated results is a high-fidelity, path entangled state of two photons, an important element for quantum computation. The silica-on-silicon photonic circuits may also be applied to quantum metrology and communication technologies. (A. Politi et al., Science 320, 646, 2008 [MEDLINE].)   — Jermey N.A. Matthews