Broad Audience Highlights
A new theoretical advance enables us to understand how the magnetic properties of a class of magnets called antiferromagnets respond to a magnetic field. The theory describes the magnetic behaviors of both collinear antiferromagnets, in which adjacent magnetic moments point in opposite directions from atom-to-atom, and noncollinear antiferromagnets, where the magnetic moments rotate from one atom to the next. Advantages of this theory include that it is expressed in quantities that are easily measurable and is useful for polycrystalline samples. Applications of the theory to specific compounds illustrate its general utility to understand the properties of antiferromagnets.This theory will help us to understand the interactions between atomic magnets needed for the development of new magnetic materials for such applications as computers, electric motors and other devices that we extensively use in our everyday lives.
Significant LED performance improvements have been achieved by taking advantage of novel materials.An organic light emitting diode (OLED) requires at least one transparent electrode, which is most commonly indium tin oxide (ITO). While ITO is both transparent and a good electrical conductor, its light transmission differs from the other organic material layers used in the device, leading to internal reflections which reduce efficiency. Researchers replaced ITO with a special highly conductive polymer known as PEDOT:PSS. The new OLEDs have a peak power efficiency and other key properties that are among the highest reported to date. They are 44% more efficient than comparable devices made with ITO. The researchers used computer simulations to show that the enhanced performance is largely an effect of the difference of optical properties between the polymer-based electrode and ITO. Because of the improved efficiency and potentially easier processing of these ITO-free OLEDs, the results pave the way for improved commercial OLEDs at lower cost.
Scientists have discovered that the growth of iron on graphene — a one atom thick layer of carbon — occurs in an unusual way. For other metals the first atoms to arrive form small clusters on the graphene surface, and then the clusters migrate across the surface, seemingly at random. Whenever two clusters encounter each other, they merge to form a larger cluster, which moves a little slower. Growing these larger clusters is important for making electronic connections to graphene for microelectronic applications. Iron is different in that lots of small islands form, but they do not tend to merge together even as the temperature is increased. This was shown by imaging the nano-sized islands as iron was deposited, and following the islands as a function of both time and temperature. Simulations of iron on graphene support the conclusion that the islands actually repel each other. This finding is significant because graphene-based computer data storage and other nanomagnetic applications are possible if magnetic metals, like iron, can be grown controllably on graphene with a high island density.
Capitalizing on the concept that everything proceeds faster with a little cooperation, researchers showed how designing cooperation into solid catalysts leads to enormous benefits.Catalysts attached to a porous solid support are preferred industrially because they are easier to separate from liquid products and reuse. But, these bound catalysts typically do not perform as well and probing their interiors to figure out how to improve them has proved difficult until now. Using new solid-state nuclear magnetic resonance (SSNMR) methods (the equivalent of running an MRI on the catalyst) and innovative synthetic strategies, researchers showed how to probe their inner workings and make optimization possible. Scientists demonstrated this approach on a carbon-carbon bond forming reaction routinely used in chemical manufacturing and biofuel production. Two key insights were revealed. First, access into and out of the pores is blocked by a chemical intermediate. Making the pores a mere 0.8 nanometers wider increased the catalytic activity 20-fold! Knowing the structure of the intermediate, researchers were able to modify the catalyst to eliminate the bottleneck without making the pore wider. This heterogeneous catalyst is significantly more active than the homogeneous catalysts, contrary to expectations. Why? SSNMR showed the support brings the reactants and catalytic groups together, resulting in the enhanced, cooperative activity not possible with the untethered catalyst. This work sets the stage for significant innovations for commonly used catalytic processes.
A new technique simultaneously illuminates the location, orientation and rotation in 3D of individual gold nanorods. Gold nanorods have been used as orientation probes in optical imaging because of their shape-induced anisotropic optical properties and now we can do this even better. Gold nanorods have the benefits of being biocompatible and having optical properties that depend on their orientation. This new development provides full 360° rotational information about these nanorods without sacrificing spatial and time resolution. Previous techniques for tracking nanoprobes in the focal plane could only distinguish from 0 to 90°, so clockwise and counterclockwise movements looked the same. Researchers combined a technique known as differential interference contrast microscopy with image pattern recognition to achieve this breakthrough. Assessing the baseline patterns for each rotational angle involved using static, titled nanorods and a 360° rotating stage. As a first demonstration of the power of the technique, researchers followed functionalized gold nanorods on live cell membranes. Resolving the location, orientation and rotational movements of nanoparticles is important for gaining fundamental information about chemical interactions with nanostructured materials.
Scientists have helped solve an 80-year-old puzzle about a widely used chemical process. The Fenton reaction involves iron and hydrogen peroxide and is used to treat wastewater worldwide. Does the reaction involve a radical intermediate? Or, is it the non-radical, iron species known as Fe(IV)? The exact nature of the intermediate has been debated for decades with data to support both theories. The problem is both intermediates will react to form the same products in most cases making the reaction intermediate hard to pin down. Researchers have now proved that both intermediates can be involved — it just depends on the pH. They carefully studied a reaction for which the two intermediates would form different products. They showed that in an acidic environment, the intermediate is an hydroxyl radical, whereas at near neutral pH the intermediate is Fe(IV). This discovery explains the differences in products formed under certain reaction conditions and clears up a decades-old mystery.
One of the best materials for converting heat to electricity just got 15% better. Adding a small amount of dysprosium to the thermoelectric known as TAGS-85 raises the thermoelectric figure of merit from 1.3 to 1.5. Researchers examined the mechanism by which doping with dysprosium affects the thermopower. The size of dysprosium along with its local magnetic characteristics modifies the interplay between electronic and thermal transport. Dysprosium distorts the local crystalline lattice and enables higher energy carriers to move preferentially through the material. This leads to improved heat conversion. Understanding how doping impacts thermoelectric properties will help researchers design even better thermoelectric materials. An improvement of 0.2 in the figure of merit is a big step toward the goal of 2.0, which is regarded as the requirement for the commercialization of thermoelectric power generation.
Researchers now understand why artificially engineered materials, known as metamaterials, can sometimes perform better than expected. Metamaterials are built from small, engineered structures that manipulate light in ways not found in nature. Unfortunately, energy is typically lost by the conversion of light to heat in the metallic components and typical support materials; this is a key challenge for application development. When a metamaterial is coupled with a support that has a so-called gain material at its surface, the results are unexpected —transmission losses are significantly reduced compared to the support or metamaterial alone. A new approach for simulating the coupling of the metamaterial to the support helps explain why. When these coupled systems are hit with a laser pulse, light absorption and reflection are both affected, albeit differently. If this effect is applied properly the efficiency of the device is improved. For many of the proposed applications of metamaterials, such as perfect lenses, low-loss or even zero-loss materials are required.This new understanding will help scientists explore material designs that will best reduce losses.
Tweaking the chemicals used to form nanorods can be used to control their shape.Controlling a nanorod’s shape is a key to controlling its properties. Researchers used a combined experimental and theoretical approach to show that precursor reactivity determines the relative ease of formation of different nanocrystals. Specifically, photocatalysts made from tiny amounts of cadmium, sulfur and selenium will form selectively into shapes that look like either tadpoles or drumsticks depending on the relative reactivity of the selenium and sulfur precursors. The more strongly bound the selenium or sulfur is to phosphorous in the precursor, the lower the reactivity. The lower the reactivity, the longer the nanorod and the more it is shaped like a tadpole. Purposely altering and modulating chemical reactivity of reactants will contribute to the development of more predictable routes to fabricate nanostructures with highly specific properties.
Researchers may have discovered the key to high temperature superconductivity — quantum criticality. A quantum critical point occurs where a material undergoes a continuous transformation at absolute zero. For superconducting cuprates and iron-arsenides, the curve of the superconducting transition temperature, Tc, versus doping (or pressure) is dome shaped. It wasn’t clear until now if superconductivity prevents a quantum critical point or if quantum critical behavior is hidden beneath the dome. An international team studied a barium-iron arsenic superconductor where arsenic is partially substituted with phosphorous, BaFe2(As1-xPx)2. Phosphorous substitution suppresses magnetism and induces superconductivity leading to a maximum Tc when magnetism is fully suppressed. The team measured the characteristic decay of the magnetic field at the surface, the so-called London penetration depth, and found quantum critical behavior coexists with and may actually be protected by superconductivity. Better understanding what drives high temperature superconductivity will accelerate the search for new, higher temperature superconductors.
A new theory shows that reactivity at catalytic sites inside narrow pores is controlled by how molecules move at the pore openings. Like cars approaching a single lane tunnel from which other cars are emerging, the movement of molecules depends on their distance into the pore; near the ends of the pores, exchange is rapid compared to further into the pores. Dynamics at the openings of these pores controls the penetration of reactants and thus overall conversion to products. Overall, the behavior of catalytic reactions in narrow pores is controlled by a delicate interplay between fluctuations at pore openings, restricted diffusion, and reaction. Until now it has been impossible to reconcile analytical theories with the findings of detailed step-by-step simulations. The new theory enables calculations of reactant and product distributions in minutes compared to the hours or days it takes to do the detailed simulations and yields comparable results. Thus, this new theory is a powerful tool for analyzing the catalytic behavior in these systems.
Glass is often described as being like a liquid, with randomly arranged atoms.New insights are emerging that show some distinct levels of order within the structure of glasses. Our rapidly evolving understanding arises from new structural information made possible because of advanced light sources like the U.S. Department of Energy’s Advanced Photon Source. The new theory fits experimental data better than the widely accepted model based on icosahedral-like clusters. The new model shows many crystal-like polyhedra as well as clustering of polyhedra — features not seen in previous models. Similar clusters group together into nanometer sized regions. The structure emerges by linking short range effects determined from the forces acting on each atom, with medium range information from electron microscopy. After heating for long periods of time to encourage structural relaxation, glasses increasingly conform to the older model suggesting that this represents an ideal glass. Practical glasses that are not heated for so long have more complex structures.This has important implications for designing and manufacturing metallic glasses.
Just like watching boats in the night, seeing movement at the nanoscale is easier when the object you are watching has a beacon.Dynamic three-dimensional tracking with high precision is possible with nanoscale light emitting particles known as quantum dots at better resolution than 10 nanometers in the vertical direction. This opens up the possibility for understanding three dimensional movement in nanoscale structures and biological systems. The quantum dots are followed using the technique known as scanning-angle total internal reflection fluorescence microscopy (SA-TIRFM). Quantum dots hold advantages over other fluorescent probes because they can be tuned to emit various colors of light. Many, however, will spontaneously “blink” meaning the emitted light is suddenly turns off (or on) thus interrupting measurements. Researchers have developed "non-blinking" quantum dots that make them useful for high precision tracking in dynamic environments. This methodology was used to show the potential of motor proteins as components in nanomachines to transport cargo.
Advanced techniques have revealed what happens to the magnetism in an iron-arsenide superconductor when some of the iron atoms are replaced by iridium.Substituting some iron atoms by transition metals (TM) such as cobalt, nickel, platinum and iridium suppresses the magnetic order of the non-superconducting parent phases of the iron pnictides, which promotes superconductivity. The way this happens remains one of the most intriguing puzzles in the field. A team of scientists has used x-ray resonant magnetic scattering at the DOE’s Advanced Photon Source to probe the local magnetic order associated with dilute iridium substitutions for iron in superconducting samples of Ba(Fe1−xIrx)2As2. These measurements show that the individual iridium are magnetically polarized at low temperatures, manifest the same magnetic order as the majority iron moments, and that this magnetically polarized state coexists microscopically with superconductivity in these samples.These findings reveal the interplay between magnetism and superconductivity in doped systems
Researchers have shown that it may be possible to make lasers using single-layer sheets of carbon atoms — the novel material known as graphene. Lasers are made from materials that can absorb ordinary light and then emit photons that have matching waves to provide high intensity.To generate laser power, a material must first undergo a population inversion where an excess of electrons is excited. They must then produce optical gain when one photon is emitted spontaneously causing the excited state electrons to undergo a cascade reaction, each one emitting an additional photon coherent with the first, so a large intensity builds up. Graphene exhibits both of these properties. Very short light pulses, only a few femtoseconds (10-15 seconds) in duration, were used to stimulate the graphene. Almost instantaneously broad population inversions were observed; and the ultrabroad band gain is established at about 10 femtoseconds, producing a much wider tuning range of light (from terahertz to ultraviolet) than in conventional lasing materials. This is remarkable for photonics materials. Comparison of the experiments with newly-developed theoretical approaches neatly explains the findings. This work opens up a wide range of possible uses of graphene in previously-unexplored areas, particularly ultra-fast telecommunications and laser technology.With graphene a little light may go a long way.
Researchers have overcome a fundamental obstacle to realizing the full potential of quantum computing.They developed a method to protect quantum information while simultaneously performing calculations. When a quantum bit (qubit) interacts with the environment its quantum information is quickly destroyed. Until now, methods to decouple individual qubits from the environment isolated the qubits from each other so they could not exchange information. The scientists devised a scheme that seamlessly integrates decoupling from the environment into the quantum computation process. Excellent performance of this method was demonstrated for a hybrid logic device made of an electron qubit and a nucleus qubit in diamond, at room temperature. This development clears the way for the high-fidelity transfer, processing and retrieval of information that is critical for quantum information processing.
Researchers can now analyze how reactions proceed inside porous nanoparticles where the molecules are in such narrow channels that they cannot pass each other. Catalysis within these confined conditions is significantly impacted by restricted transport. Typical pore diameters are in the range of 2 - 10 nm, and with catalyst molecules attached inside them, the pore diameter can be reduced below 2 nm. Traditional computational tools do not capture the evolution of concentrations inside pores so narrow that reactants and products cannot pass each other. The new methods precisely describe this kind of constrained chemical diffusion. Narrow pores with catalytic sites varying in number and location were analyzed. Snapshots of the locations of reactants and products as a function of time show the factors that influence the transient and steady-state behaviors.These studies set the stage for understanding more complex systems and designing new, even better, catalysts.
Designing the building blocks of artificially engineered materials, known as metamaterials, just got easier. Metamaterials are built from small engineered structures that, in some ways, mimic the role of atoms, and can manipulate light in ways not seen in nature. The conducting materials used to make them are central to their efficiency. Energy is lost by conversion of light to heat in the metallic components and the support materials. Gold and silver are known to be relatively good building block materials and now we have a way to predict which other materials could work even better. Materials with a lower optical resistivity at the wavelength of the light are key, but geometry has an effect too. For example, graphene, one atomic layer thick graphite, would work well if not for being unobtainable at greater thicknesses. Superconductors may also have merit, although their properties are entirely different. This work provides a tool to select materials with optimum optical properties for use in metamaterials. The potential impact could be huge, because of the considerable efficiency improvements that are possible.
Scientists have advanced methods to make maps of the locations of molecules within plant materials. Resolution of 10 to 50 microns, less than a quarter the size of a human hair, is routinely possible. The trick with plant materials is to extract the molecules delicately from thin slices with a fine laser moving stepwise across the sample.Many molecules are analyzed at once using a very sensitive mass spectrometer in this technique known as matrix-assisted laser deposition/ionization-mass spectrometry imaging (MALDI-MS). Within cottonseed embryos, which are about 3/16th of an inch in diameter, this method showed a surprisingly non-uniform mixture of lipids whose concentration varies with tissue functionality. These lipids are important for seed development and can affect the chemistry of the cottonseed oil extracted for use in various foods.These findings demonstrate the potential of this technique to provide a new level of understanding of biosynthetic pathways.
Scientists have designed a device to achieve the seeming-impossibility of confining light to a space with dimensions smaller than its wavelength. The deceptively simple device is a pipe with a tiny bore, and walls made of so-called transformation optical materials. To understand how these materials work, consider first what happens when light hits water. Light changes directions, because of the difference in the refractive index of water versus air; it hits the water at one angle and travels through it at a different angle. Transformation optical materials are designed with a continuously changing refractive index, so that light can be made to travel in a circle rather than a straight line. Scientists have demonstrated that this principle can confine light to a space smaller than its own wavelength, while losing little energy. These findings have huge implications for the storage and manipulation of optical information as well as studies of the fundamental properties of light.
The genetic modification of the plant cellulose structure has been demonstrated for the first time.This could be transformative for a bio-based economy. Cellulose is difficult to break down to form the sugars needed to produce biofuels. The goal of genetically altering the plant is to make plant cellulose easier to digest. Using solid-state nuclear magnetic resonance, a relative of magnetic resonance imaging (MRI), researchers identified changes in the crystallinity of the cellulose; and the less crystalline the cellulose, the easier it is to process the plant and form sugars. Developing mutations that produce the cellulose structure that is most compatible with sugar production may lead to plants that are better for biofuel production.
A new material has been made to behave in two distinct ways, helping to break down a significant barrier for understanding the mechanisms of high temperature superconductivity. Known high temperature superconductors fall into two different classes — layered cuprates and iron arsenides. The undoped, parent compounds of the cuprates are insulating, while the parent compounds of iron arsenide superconductors are metallic. Undoped barium–manganese–arsenic (BaMn2As2) has the same crystal structure as BaFe2As2, an iron arsenide superconductor parent compound, but it is a semiconductor and becomes an insulator at low temperatures. Surprisingly, BaMn2As2 becomes metallic rather than superconducting when doped with potassium. The Mn atoms have local magnetic moments in both the undoped and doped material just like the copper in the cuprates. This new material shows that there is a relationship between layered cuprates and arsenides that may be exploited for testing theories of high temperature superconductivity
Using the surface sensitive synchrotron X-ray capabilities at the U.S. Department of Energy’s Advanced Photo Source, researchers were able to figure out that the structure of the vapor/liquid interface of an ionic liquid is actually made up of tiny crystals even 100 °F above the liquid’s melting point.Ionic liquids consist of positive and negative ions. Because of this, they have many interesting properties, like low volatility, that make them potential green alternatives to conventional solvents for commercial syntheses.These liquid salts have many different possible types of short and long-range interactions thus complicating studies to understand their definitive molecular level structure. The X-ray results show that the negative ions of the ionic liquid of 1-butyl-3-methylimidazolium hexafluorophosphate interact with the rings of the molecules in such a way that their 4-carbon chains are forced out towards the vapor side, which leads to the very thin nanocrystalline layer at the interface. This finding has interesting implications for the design of new ionic liquids and for understanding their inhomogeneous structure.
A new iron-based superconductor, calcium-iron-platinum-arsenic, is magnetic but not superconducting at the lowest platinum concentrations and superconducting but not magnetic at the higher platinum levels; most of the other iron-based superconductors are both.This clear separation of magnetism and superconductivity in calcium-iron-platinum-arsenic lets scientists figure out what properties depend on superconductivity alone. In addition, this material exhibits the largest electronic anisotropy, meaning its properties depend on the direction in which they are measured, thus making it very attractive for comparison with high-temperature cuprate superconductors. By measuring the characteristic decay of the magnetic field at the surface of iron-arsenide superconductors, the so-called London penetration depth, researchers can determine the structure of the superconducting gap, which is necessary to understand the mechanism of superconductivity. The synthesis of high quality single crystals enabled the discovery of the universality of the London penetration depth.This research helps answer the basic question “What are the universal characteristics of iron-arsenide superconductors?”.
A layer of lead on clean silicon moves in a surprising way — in waves like a caterpillar. This explains the unexpected ultrafast mass transport observed even at low temperatures for this system. Although solid these single layers of atoms move as fast as molten lead. Computer simulations show that the lead layer forms waves that require almost no energy to keep moving thus explaining the quickness of mass transport. Other metals on surfaces typically move much slower by one atom at a time hopping along the surface. Knowing the critical parameters that give rise to this cooperative liquid-like movement, other systems may be discovered that move the same way and this could have important implications for applications requiring ultrathin films.
Substituting ruthenium for iron in iron-based superconductors tunes their properties in a very unusual way. The substitution of one element for another normally changes the crystal’s electronic structure and induces superconductivity by adding charge carriers and/or altering the size of the crystal lattice. High resolution angle-resolved photoemission experiments showed neither mechanism is responsible in the case of barium–iron–ruthenium–arsenide, Ba(Fe1-xRux)2As2. The researchers speculate that ruthenium dilutes the magnetic characteristics of the material when it substitutes for iron and that this causes the unexpected result. The suppression of the antiferromagnetic phase is associated with the emergence of superconductivity. This finding helps us to understand what controls superconductivity and may lead to the discovery of new superconductors.
Pectins have a previously unsuspected role in holding plant cells together, according to recent research. Cell walls are made up of three major classes of polysaccharides: cellulose, hemicellulose and pectins. The molecular interactions of these polysaccharides walls were studied for the first time within intact plant cells using multidimensional solid state NMR, a technique related to magnetic resonance imaging (MRI). Scientists were able to look at the interactions of both rigid and mobile polysaccharides using solid state NMR by studying plants grown in a solution of water and sugar made with carbon-13 rather than the naturally abundant isotope carbon-12. This study showed that the ends of hemicellulose are deeply embedded into the cellulose microfibril, not just attached to its surface, and that pectin interacts with both the cellulose and the hemicellulose, joining them in a single network. The previously accepted model was based on a cellulose—hemicellulose framework embedded in an independent pectin matrix. This discovery demonstrates the power of combining carbon-13 labeling and solid state NMR to demystify plant cell wall structures in a nearly native environment. It may also help to elucidate how to modify plants cell walls to make it easier to turn plant material into biofuels.
A new group of bacteria has been discovered in Death Valley’s Badwater Basin that makes nanoparticles of both magnetite (Fe3O4) and greigite (Fe3S4). Magnetotactic bacteria use these tiny magnets as part of their navigation system to align themselves along the Earth’s magnetic field. Typical magnetotactic bacteria do not make both magnetite and greigite and the discovery dispels the notion that greigite-producing bacteria live only in marine environments. The researchers found two different gene clusters related to magnet formation, suggesting that one is responsible for making magnetite and the other greigite. The discovery will help provide insight on the chemical conditions under which greigite is formed, which could be important for figuring out how to mass produce this kind of magnetic nanocrystal for nano- and biotechnology applications.
Making superconducting nanocircuits with rounded internal corners will significantly improve performance.Scientists showed this by calculating how circuit geometry impacts current flow. The key is how geometry affects “current crowding”. Crowding can happen when electrical current travels around a sharp corner or hairpin turn much like cars racing on a tight track. The current (like cars) tends to concentrate near the inner edges of sharp turns. By rounding the corners this bottleneck is eased.This work explains reduced critical currents observed in manufactured devices and has important implications for both superconducting and metal film devices, including nanowire single photon detectors and devices that measure extremely small magnetic fields.
Liquids and glasses are often described as having “disordered” structures, but new methods are showing that there are some significant patterns hidden in the seemingly random arrangements of atoms. When applied to a glassy copper–zirconium alloy, “order mining” has revealed an unexpected similarity between metallic glasses and quasicrystals, among other novel features. Previous methods only examined the local arrangements of closely neighbored atoms of these complex materials, but the new approach focuses on larger motifs, rather than getting bogged down in the details, much in the way that an impressionist painting represents a scene without including all of the detail.The new method can be incorporated into existing simulation tools and provides a powerful tool for identifying changes in the chaotic arrangements of atoms as a liquid metal cools.Knowing why some ‘ordered’ arrangements of atoms are preferred over others is one of the most significant challenges in materials science. It is not yet clear whether the method is capable of finding any order in a teenager’s bedroom.
Locating a catalyst and reactants in confined spaces makes catalytic reactions go faster in the desired direction. Of course, the reaction products have to be removed from the confined spaces and researchers have developed a new approach to expelling aqueous reaction products. This works for confinement in nanometer-sized pores in silica particles. By lining the insides of the pores with both catalysts and a fluorinated chemical, like that found in Teflon®, reactions with water as a byproduct proceed much faster. This works because certain chemicals just don’t like each other. Oil and water tend to separate. Water on a Teflon®-coated frying pan balls up to minimize its contact with the Teflon®. Combining state-of-the-art characterization and theory, a structure was designed to maximize this effect inside the catalytic pores. The performance of this catalyst surpasses the commercially available ones for a reaction known as esterification, that yields water as a byproduct. This is the first demonstration of enhancing chemical transformations by expelling the byproducts from porous catalytic materials in this manner and just the beginning of essentially a new class of catalysts.
Iron and copper are both magnetic, but only iron sticks to your refrigerator. That’s because iron is ferromagnetic at room temperature, while copper is paramagnetic. For a material to have two types of magnetism simultaneously at one temperature is uncommon. To have three types simultaneously is exceedingly rare. The alloy NbFe2 is one of those rare materials: at low temperatures it is ferromagnetic, paramagnetic and antiferromagnetic. But why? Researchers have discovered that this odd behavior in a metal is actually caused by a “lucky accident”, rather than a complex interaction between electrons, as had previously been suspected. Just the right ratio of niobium to iron causes the chemistry and structure to produce an interesting electronic feature that explains all of the exotic properties of this material in detail, even the ideal ratio for this to happen. Although the behavior is exotic, the origin is not — but it is exceptional. Knowing about this kind of natural “accident” may lead to the discovery of yet more unusual magnetic materials.
Researchers working to understand high temperature superconductivity in barium-iron-arsenide have discovered that applying pressure affects the material's magnetic and superconducting behavior just as if they had replaced some iron with a little ruthenium. To better quantify and understand the similarities of changing pressure versus ruthenium concentration, they made Ba(Fe1-xRux)2As2 with varying amounts (x) of ruthenium and studied each concentration as a function of pressure and temperature. The temperature-pressure phase diagrams showed this scaling is remarkably simple; every 3 GigaPascals of pressure, 30 times the pressure in the deepest part of the ocean, is roughly equivalent to replacing 10% of the iron with ruthenium.By comparing the various phase diagrams they were also able to show what is required for this material to reach its highest superconducting temperature.
Researchers have used pressure as a tool to study the magnetic behavior of a challenging series of materials, RVSb3, where R is a rare-earth. This series offers a way to study magnetic ordering in materials with a single, unique rare-earth site and it has been studied primarily as a function of temperature. CeVSb3 is the only compound in the family that orders ferromagnetically, that is with all its unpaired electron spins parallel, at low temperatures. Even after applying pressures up to 8 GigaPascals, about 80 times the pressure at the deepest part of the ocean, the cerium-containing material has unique magnetic properties compared to others in the series. Only by going to extreme pressures were they able to show that this material behaves as expected by theoretical models developed to better understand magnetism in rare-earth-containing materials; as pressure increases the magnetic ordering transition temperature and resistance increase, then decrease and finally a new, non-magnetic state emerges wherein the mobile electrons gain weight as other electrons lose their magnetism. Investigating the magnetic phase diagrams of this series may prove to be pivotal for our understanding of magnetism in other rare-earth containing materials.
Scientists have discovered a method to fine-tune the shapes of nanorod photocatalyst particles. These materials accelerate reactions when they are activated by light and their shape affects their behavior. Researchers showed that the photocatalysts, made from tiny amounts of cadmium, sulfur and selenium, will form selectively into shapes that look like either tadpoles or drumsticks depending on the selenium concentration. Their optical properties depend strongly on the relative amounts of sulfur and selenium; the sulfur to selenium ratio changes along the length of each particle causing each end to interact with light differently. These nanomaterials are being studied for their potential as light harvesting antennas, as novel optically driven biomass conversion catalysts and to form more complex nanostructures and light harvesting devices.
A new kind of magnetic order has been observed in barium-cobalt iron arsenide high-temperature superconductors by researchers with expertise in growing large single crystals, conducting x-ray and neutron measurements, and calculating electronic structures. Traditional antiferromagnetic order observed, for example, in the copper oxide high-temperature superconductors is driven by strong electron-electron interactions that can result in insulating behavior. The new structure, called an antiferromagnetic spin-density wave, is also known to form in metals such as chromium when electron-electron interactions are weak. Thus, while all high-temperature superconductors seem to possess antiferromagnetic order of some sort, strong electron-electron interactions do not appear to be a universal requirement. This new discovery changes the constraints on the origin of high-temperature superconductivity.
Using computer simulations, researchers are able to watch how a random mixture of gold nanoparticles with two different DNA strands as linkers assemble right on their computer screen. The magic starts with just a fraction of the nanoparticles forming a large cluster like a plate of spaghetti and meatballs. Within this random cluster, small ordered regions form and eventually lock together into a large uniform array of particles held together with DNA strands. Using computer simulations they also predict previously unseen structures. The computational tools developed for this research can be adapted to a range of other nanoparticles, thus opening up exciting prospects for accelerating material design using DNA to manipulate the structure.
A novel electrode architecture has led to a new way to make transparent electrical contacts. Typical ways of attaching a conductor to a non-metallic material allow you to see the electrode. However, for many applications, like light emitting diode (LED) displays, smart windows and solar cells, transparency to visible light is a requirement that conflicts with electrical conductance. Thinner films are more transparent, but less conductive. The new architecture consists of specially patterned nanoscale-thick metallic ribbons, standing on edge, supported by a polymer matrix. Because the ribbons are only about 40 nanometers wide, light can pass between them, but their height provides enough material to ensure high conductivity. The conductivity and transmittance of the new structure rival currently used indium-tin-oxide transparent electrodes, is less brittle, and will enable flexible, large area applications. It provides an alternative to using indium, which has been identified as an “energy critical element”.
Graphene is supposed to have the potential to replace silicon in electronic devices, making them thinner and faster, but making such devices depends on making electrical contacts. Researchers have deposited two metals onto graphene — a one atom thick layer of carbon — to see what kinds of elements might work best. Metals like lead were predicted to attach weakly, while rare earth metals were predicted to stick strongly giving better results. Scanning tunneling microscopy experiments confirmed the predictions. The experiments show that tiny dysprosium islands can be attached to graphene, and these promise new technologies for information storage based on nanoscale magnetism.
A material that is magnetic, superconducting and behaves nearly like a semiconducting sounds fairly unusual, and it is. Just such a material, made from potassium, iron and selenium, was recently discovered. It has similar superconducting properties to iron-arsenide-based superconductors. However, it is also nearly semiconducting and, most curious, has a very high (antiferro-)magnetic ordering temperature with large (rather than small) magnetic moments. These findings are at odds with the idea that iron-based superconductivity appears when magnetism is suppressed. Researchers have made special single crystals to find out how these characteristics can co-exist in one material. They observed how the structure and the magnetism of this material changes with temperature to understand the unusual aspects of the material’s magnetic properties, including the very strong contributions from its iron atoms. Systematically eliminating the possibilities, the researchers are zeroing in on explaining how magnetism works in this material. Understanding the magnetism and how it interacts with superconductivity in this material may move us a step closer to a room-temperature superconductor.
Thanks to the innovation of “single particle orientation and rotation tracking” (SPORT), we now can watch the distinctive movements of drug delivering nanoparticles in real time. Nanoparticles have the potential to revolutionize drug delivery. When these particles interact with cell membranes they move in all sorts of ways. They spin, they tumble, they move along and through the membrane. At least that’s what we think. But what’s really going on? Until now, we could track how these nanoparticles rotated only by taking a series of still photographs making studies of fast rotations beyond our reach. Nano-sized rods made from gold were modified with drug delivery agents, like transferrin, and watched via SPORT. For the first time, the distinctive rotational behaviors of these modified nanorods were attributed to specific binding sites on the cell membrane. This new technique will lead to a better understanding of nanoparticle-based drug delivery mechanisms and provide guidance on how to improve existing nanoparticle drug delivery technology.
An international team of researchers has discovered a new type of defect in an unconventional material known as a quasicrystal. Mysterious nanodomains observed on the surfaces of quasicrystals led to the discovery. Quasicrystals were already known to have a unique defect type, known as a phason flip, which can form at the surface. The new defect type is related, but unlike the phason flip is not restricted to the surface; it bridges the surface and the bulk. The new defect type serves to balance competing energetic issues and enables higher-energy transition-metal-rich surfaces to be exposed rather than the expected lower-energy aluminum-rich surfaces. If it saves more energy to make the nanodomains than it costs to form higher-energy surfaces and interfaces, then nanodomains are observed. The relationship between surface and bulk defects may be a key to understanding why nanostructures are often unusually strong.
Chemists have synthesized a highly selective and highly efficient zirconium catalyst that makes new carbon-nitrogen bonds by adding a nitrogen-hydrogen bond to a carbon-carbon double bond. Nitrogen-containing chemicals are important as agrichemicals, pharmaceuticals, and specialty chemicals. These zirconium catalysts are expected to show greater tolerance to other functionality than the well-known and highly sensitive rare earth catalysts. The new catalysts are more efficient than previously reported zirconium catalysts, promoting the reaction at room temperature. This high activity may be related to its ability to access a new mechanistic pathway that was proposed based on unique kinetic and selectivity observations. In this mechanism, carbon-nitrogen and carbon-hydrogen bond formation occurs in a concerted fashion.
Researchers have wrestled with the question of whether or not the newest superconducting materials fit within the traditional classifications. Superconductors can be divided into Type 1 and Type 2, depending on how they behave in a magnetic field. However, magnesium diboride was discovered to have characteristics of both categories and has been dubbed “Type 1.5”. Drawing on the fundamentals of Ginzburg-Landau theory, the basis of the two original types, a combination of theory and simulation has determined that a new class is not warranted. Close to the superconducting transition temperature (where the Ginzburg-Landau theory is valid), the energy of the interface between a normal and a superconducting region is described by the same universal function for Type 1 and Type 2 superconductors and also those that behave like both. This makes the idea of Type 1.5 superconductivity unnecessary within the Ginzburg-Landau theory. The behavior of magnesium diboride can be understood in terms of Type 2 superconductivity.
Scientists have discovered a way to improve the energy conversion efficiency of a key green material by 25%. Thermoelectric materials can convert waste heat into electricity, but the low efficiency of existing thermoelectric materials limits their widespread use. Researchers found that by adding a little bit (just 1%) of the rare earths cerium or ytterbium to material made from silver, antimony, germanium and tellerium can make a huge difference. They made this discovery while studying the fundamental science behind the conversion of heat to electricity. Doping with rare earths could affect several possible mechanisms responsible for thermoelectric properties. Using solid-state nuclear magnetic resonance, researchers were able to eliminate some of the proposed mechanisms and gain a deeper understanding of the mechanisms that could lead to improved efficiency.
Ions in water with the same charge, e.g. Fe3+ and La3+, behave dramatically differently at the air/water interface when interacting with a charged surface. This difference violates classic electrostatic theory. The distributions of specific ion types were determined with unprecedented precision using newly developed surface sensitive synchrotron x-ray scattering and spectroscopic techniques. The research team was able to use these results to verify their recently-developed theoretical model that takes into account both classical and effective quantum behavior. This model can be used to predict any distribution of trivalent and divalent ions. Determining the underlying interactions of ions with soft-matter templates in an aqueous solution is a crucial step towards the design of novel functional structures that mimic those of living organisms. This work has the potential to explain the many effects of aqueous environments and also to develop new methods for extracting metal ions from nuclear waste.
Researchers have uncovered what makes bone a naturally nanostructured material. Bone is known to have good mechanical properties because it has strengthening mechanisms on a number of length scales. The smallest of these scales is one of the most important but, so far, one of the least understood: Crystals of apatite (a calcium phosphate), which makes bone rigid, naturally maintain a thickness of only about 3 nanometers, a scale that researchers aim to reproduce in the laboratory. Multinuclear magnetic resonance has been used to show that citrate, found in fruit juices, binds strongly to the surfaces of the apatite nanocrystals in bone. The researchers were able to show that the citrate molecules bind with a spacing and arrangement that effectively prevents the addition of phosphate and thus does not permit crystal growth to larger sizes, as would normally be expected. This maintains the small crystal thickness most favorable for mechanical properties and also for fast calcium phosphate resorption during bone remodeling. This discovery opens up new research directions on materials that mimic bone.
Researchers systematically blocked key chemical reaction pathways to get unambiguous information about how carbon-nitrogen bonds are formed in a catalytic reaction known as hydroamination. Understanding a multi-step catalytic mechanism is like a solving a puzzle where you can’t see the pieces. However, you can add your own “pieces” with known shapes to figure out what other pieces of the puzzle then will (or will not) fit. Hydroamination reactions are catalyzed by several different metal catalysts. The researchers studied magnesium-based hydroamination catalysts because they have stable, potential intermediates in the catalytic process that could be synthesized separately, can be used to understand the catalytic mechanism, and provide alternatives to traditional rare earth catalysts. Blocking the common insertion mechanism showed that a second route for hydroamination is possible, indicating that the catalyst can work in at least two distinct ways. This information is key to understanding this class of catalyst, which is used for carbon-nitrogen bond reforming reactions, and to guiding general strategies for replacing rare earths in catalysts.
Researchers have discovered how the geometry of gold nanoparticles affects their images. Gold nanoparticles can be imaged optically and their movements can be seen using a technique known as differential interference contrast (DIC) microscopy. How gold nanoparticles appear in these images depends upon their environment. This can be used to learn about time-dependent nanoscale processes. However, an outstanding question has been whether or not the geometry of the gold particles affects how they are imaged. Researchers looked at three common nanoparticle geometries: a single rod, two rods stuck together and two rods separated but close to each other, so-called proximate rods. Trapping differently positioned nanoparticles and characterizing them with an electron microscope enabled comparison with the DIC images to see how each geometry is imaged in the optical system. Unlike other techniques, DIC produces images that uniquely distinguish these different geometries to even as they move around. Even at the nanoscale, geometry matters.
Researchers have found evidence of atomic-scale defect formation during crystal growth from the supercooled liquid. Researchers have long speculated that defects incorporate during growth, but until now had no evidence because they heal before they can be observed. Using high energy, high resolution in situ X-ray diffraction at the U.S. Department of Energy’s Advanced Photon Source, researchers overcame accuracy and data collection speed issues to make this discovery. The researchers found evidence of defects that involve swapping of the locations of the elements in Zr2Cu. In these antisite defects, some zirconium atoms swap places with copper. As a result, the length of the crystal unit cell decreases during growth while the width increases. Computer simulations confirmed that, because zirconium atoms are larger than copper atoms, only 1 to 2% of these defects are necessary to cause these distortions in the unit cell during crystal growth; the faster the growth rate, the more defects. This work provides a foundation for better understanding how to control material properties that are dependent upon defects created during solidification.
Until now, watching the detailed spinning motion of nano-objects within living cells has been impossible. Combining an existing technique, known as Differential Interference Contrast (DIC) Microscopy, with nanotechnology, researchers can now see how nanoparticles spin when they move across the interiors of living cells. Nano-sized rods made of gold are non-toxic to living cells and they scatter light differently depending on their orientation. DIC microscopy captures the orientation of gold nanorods in addition to the optical image of the cell. Gold nanorods (25 x 75 nanometers) were used to show particle movement within living cells. Researchers were even able to demonstrate the rotational motions of the host structure using gold nanorod probes. This new technique opens up doors to understanding living nanomachines by revealing their complex internal motions
Researchers have found that two iron arsenide superconductors exhibit novel behavior. When a material is cooled below its superconducting transition temperature in an applied magnetic field, it expels some portion of that field. Exactly how this Meissner effect occurs depends on the physical properties of the sample, the type of superconductivity, and the experimental conditions. However, for all superconductors field expulsion is determined by the strength of the Meissner currents, which peak at some critical field value. The signature of the field dependence is that the magnetization of the superconductor reaches a maximum at the critical field and then starts to decrease. In contrast, for these arsenide materials the degree of Meissner expulsion continues to increase, almost linearly, even when the applied field far exceeds the theoretically estimated critical field value. This suggests that the Cooper pairing in iron arsenide superconductors may recover some strength because the magnetic field suppresses scattering effects. The new findings suggest further experiments that may provide additional clues as to the mechanism of superconductivity in these materials.
Scientists have discovered a way to make strong materials that are also ductile. One of life’s classic problems is that whenever a metal or alloy is altered to make it stronger, it loses its ability to deform – it becomes brittle, so its eventual failure is both unheralded and catastrophic. Nanostructured materials have shown great improvements in strength over their conventional counterparts, but until now, they have also typically been more brittle. Researchers have discovered that conventional levels of ductility can be achieved in high-strength nanostructured cobalt, when a planar-defect deformation mechanism called “twinning” is active. This was determined using real-time in-situ x-ray scattering during mechanical testing and post-deformation high-resolution electron microscopy. High-strength allows structures to be made with less material, and the consequent weight saving provides energy-efficiency improvements in transportation systems. This new discovery suggests a path forward to using high-strength materials in applications of this type, where brittleness is unacceptable.
Quantum critical transitions take place at absolute zero and their occurrence can provide fundamental information about the onset of magnetism. Studying quantum criticality is challenging, however, because we cannot make measurements at absolute zero, and we must rely on less distinct changes in the state of a solid, that occur at slightly higher temperatures. Using an unusually broad set of precise physical property measurements very close to absolute zero, researchers have found strong evidence of quantum critical transitions in YbAgGe and mapped out its response to small applied magnetic fields. The results provide profound new understanding about magnetism and superconductivity in systems where the electrons interact strongly. They have observed the very birth of local moment behavior, providing insights into how magnetism comes into being.
