Broad Audience Highlights
A new ultra-fast laser technique has yielded insights into how iron arsenide materials evolve to form a superconducting state. This transformation involves complex changes in magnetism, structural order, and electronic order that appear to be going on simultaneously — not simply competing with each other. Only by looking at very fast time scales (10 thousandths of a billionth of a second) and using the highest quality single crystals could these transformational changes be separated and analyzed; ultra-fast spectroscopy enabled scientists to study the superconducting transformation dynamics to unravel what happens where and when. The results demonstrated that the crossover involves an independent electronically-driven order (so-called nematic order), previously proposed. These findings will motivate the development of new microscopic theories to further understand this emerging behavior and its influence on superconductivity in these complex materials.
A new metallic material — based on the substitution of manganese for iron in an iron–arsenide superconductor — has been discovered in which the microscopic magnets of the electron current carriers provided by the potassium atoms all line up in the same direction at low temperatures whereas the neighboring microscopic magnets of the manganese atoms line up in opposite directions to each other.This material, Ba0.6K0.4Mn2As2, thus exhibits a novel magnetic behavior with ferromagnetic and antiferromagnetic behavior coexisting. Interestingly, the ordered moments in the two magnetic substructures are aligned perpendicular to each other.Understanding what causes this unique magnetic structure may help researchers to understand the mechanism of high temperature superconductivity and to design materials for the new field of spintronics, where electron magnets are used in electronic devices for information processing rather than their charge.
Crowding controls whether carbon chains or a hydrogen atom will transfer from transition metal molecular complexes to acceptor molecules.To gain this new insight into the factors governing the onset of hydrogen abstraction from metal alkyls, researchers carefully designed experiments involving series of cobalt and chromium alkyls. The results show that when the alkyl chain is only one carbon long, the alkyl group will transfer to a rhodium acceptor molecule. But, if the chain is made up of two or more carbons, crowding makes it hard for the alkyl group to transfer. In these cases the reaction finds another pathway, one in which a single hydrogen atom, rather than the entire alkyl group, is transferred. The transfer of hydrogen and carbon chains from transition metal molecular complexes to an acceptor molecule is involved in many catalytic reactions of industrial and biological significance. These results provide key fundamental information about these processes.
Graphene — a one layer thick sheet of carbon atoms — has special properties that make it a desirable material for manipulating terahertz waves.Terahertz applications operate at frequencies between microwave and far infrared. Some metamaterials, which are engineered structures that can manipulate light in ways not seen in conventional materials, could benefit by replacing the metals currently used to build them with graphene. Indeed, graphene provides a number of advantages over metals including that its properties offer the unprecedented ability to tune the electrical response for a given application. Graphene also offers the advantage of a potential enhancement of terahertz wave confinement. Experimental data have shown significantly higher electrical losses than has been estimated by theoretical work, showing there is more research that needs to be done to make metamaterial devices from graphene.
The propagation of a novel magnetic excitation in the superconducting state, called a spin resonance, has been observed in iron arsenide superconductorsfor the first time. How the resonance disperses depends upon the direction probed within the single crystals studied. Propagation of the spin resonance reveals details about the superconducting state and highlights qualitative differences between iron arsenide and cuprate superconducting materials. The magnetic excitation appears in the superconducting state with upwards dispersion in iron arsenide superconductors. By contrast, in cuprate superconductors the dispersion is downwards. The neutron scattering measurements designed to study the spin resonance were performed on a single crystal of a nickel-doped barium–iron–arsenide superconductor [Ba(Fe0.963Ni0.037)2As2] at the Spallation Neutron Source and the High Flux Isotope Reactor, U.S. Department of Energy user facilities. Neutron spin resonance is considered to be a hallmark of unconventional superconductivity, thus a detailed understanding is important to future developments of superconducting materials.
A recent discovery suggested to be a universal behavior of superconductors does not require a fancy new explanation; it elegantly falls out from the BCS theory of superconductivity, first published in 1957.The universal behavior is scaling relationship, known as Homes scaling, that relates the penetration depth of the magnetic field to the superconducting transition temperature and conductivity. It is valid over many orders of magnitude from the so-called “dirty”, short mean-free path, superconductors up to as clean materials as one can synthesize. Thus, Homes scaling can be considered another confirmation of BCS theory, if any is still needed.
Scientists have discovered that the rare earth element dysprosium grown on graphene — a one atom thick layer of carbon — forms triangular-shaped islands, whereas other magnetic metals form hexagonal-shaped islands. Based on the hexagonal closed packed (hcp) bulk crystal structure of dysprosium, hexagonal islands would also have been expected. Researchers used scanning tunneling microscopy to identify the crystal structure of dysprosium on graphene. The results indicate that dysprosium grows as face centered cubic (fcc) crystals on graphene rather than hcp. The triangular shape arises, in part, from unequal energy barriers for dysprosium atoms to move around the corners of the islands. This difference in growth structure compared to the bulk suggests that these islands may have different magnetic properties from that observed in the bulk. Understanding the growth of magnetic materials on graphene is important for furthering the development of graphene-based electronic devices that take advantage of the spin properties of materials, so-called spintronic devices. Rare earth metals have large bulk magnetic moments that make them likely elements of these spintronic devices.
A new series of catalysts is able to selectively make “left-handed” or “right-handed” nitrogen-containing compounds known as amines. Left-handed and right-handed molecules contain the same components, but are mirror images of each other. Researchers were able to take strings of nitrogen-containing molecules and make five-, six- and seven-membered rings with enantiomeric excess of 90%. Most other catalysts produce a mixture of both enantiomers. Researchers studied hafnium, titanium and zirconium (Group 4) containing catalysts and found the zirconium catalysts to be the best at producing one enantiomer in high yield over the other. The new zirconium catalysts do what no other Group 4 catalyst has done before — they can operate at room temperature and down to minus 20 °F. All other zirconium catalysts operate at 300 °F. These new catalysts are also tolerant of various functional groups attached to the amines. Researchers performed detailed studies of the structure, activity and selectivity of this system of catalysts and were able to characterize the reaction pathways. The pursuit of these optically active amines is important for improved syntheses of commodity and specialty chemicals.
Scientists have discovered a fascinating secret about praseodymium aluminide. When PrAl2 is cooled, its crystal structure changes from high symmetry cubic to low symmetry tetragonal below -400 °F (32 K). However, when the cooling is done in a high magnetic field, the material retains the cubic structure. This change is not observed in other rare-earth aluminides. Furthermore, PrAl2 has an anomalous heat capacity per unit mass at low temperatures. It is 10x higher than pure praseodymium. (Heat capacity is the amount of heat needed to raise the temperature of a material by one degree.) First principle calculations confirm the mysterious behaviors originate from an unexpectedly strong competition between the interactions of magnetic (4f) moments mediated by the sea of conduction electrons in PrAl2 and the splitting of the energy levels of the 4f electrons of the rare-earth atoms in both the cubic and tetragonal lattices. The structure’s flexibility may yield practical applications, including magnetostrictive transducers and magnetoresistive sensors, and may serve as a foundation for energy efficient and environmentally friendly magnetic cooling for consumer use.
A study of thermoelectrics, materials that convert heat to electricity, demonstrates the importance of characterizing materials using several different methods. According to Vegard’s Law of Alloys, the size of a crystalline lattice (lattice parameter) changes linearly with composition. It is actually not a law, but an empirical observation that has been found to hold true for a majority of alloys. Researchers studied thermoelectrics made from lead, tellurium, and sulfur and varied the tellurium and sulfur content to answer the question “If a thermoelectric follows Vegard’s Law, does that make it an alloy?” The answer is: not necessarily. High-resolution synchrotron X-ray diffraction data taken at the Department of Energy’s Advanced Photon Source, probing long-range order, shows that this material adheres to Vegard’s Law. Solid-state tellurium (125Te) nuclear magnetic resonance (similar to magnetic resonance imaging) was used to determine the short-range structure and composition of the samples with high accuracy; sulfur shows strong effects on the resulting NMR spectra, with the Te peaks shifted depending on the number and arrangement of sulfur atoms near Te. Combined NMR and infrared spectroscopy results show that above 16% sulfur the samples are not alloys. Additionally, inhomogeneities improve the material’s ability to convert heat to electricity. These findings suggest that there may be many thermoelectric systems that are not alloys as reported, but instead are nanostructured. The work also demonstrates the importance of using probes of both long range and local chemical structure to differentiate between alloys and composites.
For the first time, researchers can now both explain and predict the behavior of different materials while they are being pulled apart. Some materials are ductile, meaning they will deform without losing their toughness, and others are brittle. The results explain even the unexpected ductility of a material within a class of rare-earth-containing materials that are otherwise known to be brittle. To predict the behavior requires two maps. The first map reveals whether a system has the ability to slip in a particular direction and form stable defects (a necessary condition for ductility), while the second map reveals if the required defects have multiple, active slip planes (a sufficient condition). With both conditions satisfied, the material will be exhibit unusual ductility. The maps are accurate, with observed ductile to brittle transitions reproduced, too. Similarly criteria can be formulated for a variety of systems — a direct form of computational materials discovery.
A distinct anomaly exists within a series of iron arsenic superconductors, possibly indicating a new form of iron-based superconductivity. This is seen in plots of the temperature at which the material becomes superconducting (Tc) compared to the change in heat capacity at the superconducting temperature. (Heat capacity is the amount of heat needed to raise the temperature of a material by one degree.) Typically, newly discovered Fe-base superconductors follow a simple empirical rule: the change in the sample’s heat capacity when it becomes superconducting varies with the cube of the temperature at which the sample becomes superconducting. Researchers carefully examined high purity, single phase, polycrystalline samples of barium and potassium-containing iron arsenic superconductors of 8 different compositions ranging from x = 0.2 to x = 1.0. Below x = 0.7, Ba1-xKxFe2As2 follows the same trend seen for other iron arsenic superconductors. At 0.7 the material starts to deviate from the expected results. The findings suggest that there is a significant modification in the superconducting state around x = 0.7, suggesting a new type of iron-based superconductivity. Further measurements of materials with composition around this concentration are needed to understand the cause of the anomaly.
Researchers have discovered a new family of stable quasicrystals made from only two elements, a rare earth and cadmium. The family includes the first magnetic binary quasicrystals. Quasicrystals are metallic alloys that lack the periodic order seen in conventional crystals. Instead, they exhibit aperiodic, long-range order and have “forbidden” rotational symmetries (for example, five-fold). Most are made from three or four elements. Binary quasicrystals are rare and sought by scientists because they offer a cleaner way to investigate the relationships between the unusual structures of quasicrystals and their properties. For this family, there also are crystalline approximants — materials that are periodic but with atomic decorations similar to those of quasicrystals — that are close in composition allowing comparisons with related crystalline materials. The Department of Energy’s Advanced Photon Source was used to confirm their structure. The discovery of this family enables researchers to study the nature of magnetic interactions in quasicrystals. Thus far, studies show the magnetic behavior of these quasicrystals and related approximants is vastly different.
Nanoscale twin boundaries — where one side of the boundary is a mirror image of the other — are not straight as thought, but instead have "kinks". Researchers used a newly developed transmission electron microscopy technique to resolve the orientation of features along these boundaries with 1 nanometer resolution. Twin boundaries that appear straight at lower resolution, actually contain many kink-like steps. These kinks are distributed non-uniformly from twin boundary to twin boundary. By correlating the new results and additional experiments with large-scale simulations, researchers found that these tiny kinks can control how a material deforms when a force is applied. The results also reveal an unexpected mechanism for "de-twinning". These findings demonstrate a need to develop realistic models of nanotwinned structures for designing new high strength materials that exploit the unique properties of twin boundaries.
Researchers have discovered that barium–iron–nickel–arsenic superconductors clearly deviate from the famous Ginzburg-Landau Theory developed in the 1960’s. According to this theory, superconductors should show a linear relationship between the magnetic field at which superconductivity is suppressed (known as the upper critical field) and the direction of the magnetic field. Using single crystals, researchers performed detailed experiments of the upper critical field as a function of temperature and the direction of the magnetic field. They were able to study the dependence on magnetic field direction to less than 0.1°. Plotting the inverse of the square of the upper critical field versus sin2θ should have given a straight line according to the Ginzburg-Landau theory. It clearly did not. The non-linear dependence is speculated to reflect a variation in the superconducting gap along one axis of this superconductor. The upper critical fields of iron arsenic superconductors are very high leading to possible uses in high magnetic field applications. This work provides additional insight needed to find the best materials for such applications.
For the first time, researchers can keep multiple nanoparticles in focus while tracking their 3D orientations on a surface with unprecedented angular resolution. The new technique can accurately track anisotropic gold particles that are tilted out of the horizontal plane and has the advantage of not relying on particle interactions with the surface to keep track of them. This technique takes advantage of the optical properties of gold; at certain frequencies of light electrons in the gold are stimulated to collectively oscillate, known as surface plasmon resonance. For gold nanorods the frequency of light needed to induce surface plasmon resonance differs for the long axis of the rod compared to the short axis. This technique takes advantage of this difference. The power of the technique was demonstrated by studying modified gold nanoparticles landing on lipid membrane bilayers. While a significant fraction is "frozen" on the surface, many particles are not. The capability to follow the 3D movements of nanoparticles will greatly enhance our understanding of the way nanoparticles interact with surfaces.
Physicists have devised a material that allows them to study the birth and evolution of magnetism. This is analogous to understanding how a caterpillar becomes a butterfly. Without understanding the key transitional point in its lifecycle, a butterfly just seems to appear fully formed. For butterflies, we need to discover and study a chrysalis. Taking this cue from nature, scientists have developed ytterbium—bismuth—platinum (YbBiPt) and used a magnetic field to suppress a non-magnetic, metallic state and study how a non‐magnetic metal develops magnetic properties. This can be done with minimal interference from other effects by forcing this event to happen as close to absolute zero temperature as possible. Even though this birth takes place at extremely low temperature, the fluctuations (ripples) associated with it are thought to be key to high temperature events, such as superconducting transitions at 50 or 100 K in copper or iron based systems. Researchers performed careful, systematic measurements of the temperature and magnetic field dependencies of the properties of YbBiPt to map out the birth and evolution of magnetism. This work clearly identifies where and how this rare event takes place.
A new technique makes it possible to track not only the location of moving particles to within 10 nanometers, but also their rotation and orientation. This is like watching a football game from the ionosphere and knowing where the football is at anytime within 1.5 inches, how the ball is spinning, and what direction it is moving. This is made possible by inserting an additional arm into the optical path of the specialized instrument known as a differential interference contrast microscope used for visualizing motion. This addition makes it possible to operate in two modes simultaneously; one mode focuses on particle location while the other focuses on rotation and orientation. Images can be taken every 75 milliseconds, 4 times faster than a blink of an eye. The new technique will aid researchers looking to understanding particle movement in nanoengineered environments and within cells involved in important processes such as carbon fixation.
Researchers have discovered an unusual temperature behavior of the electrons in iron arsenic superconductors that may play a crucial role in the emergence of high temperature superconductivity. The electrons in solids occupy areas called pockets. In regular metals the sizes of these pockets remain constant as a function of temperature and are proportional to number of electrons that conduct current. Surprisingly, in iron arsenic high temperature superconductors, the pocket associated with empty electron states, known as holes, decreases and ones associated with electrons expand, as if there were more conduction electrons. More importantly, at low temperatures, the hole and electron pockets are about the same size and shape. This is a condition called “nesting”, which often leads to special magnetic state called “antiferromagnetic order” in which the magnetic moments of adjacent ions point in opposite directions. This state is believed to be a key to the emergence of high temperature superconductivity in iron arsenic superconductors. Knowledge of how the temperature affects nesting and the destruction of antiferromagnetic order may be key to enhancing the critical superconducting temperature and searching for new superconducting materials.
A new material made from three elements — yttrium, manganese and gold — woven together in an unusual crystalline lattice shows surprisingly diverse characteristics for each element. The manganese electrons are localized at the manganese sites, whereas the yttrium and gold electrons are delocalized. Yttrium in this rhombohedral lattice tends to give up electrons and thus be positively charged whereas the gold prefers to take on electrons. The magnetic characteristics are also unusual with electron spins strongly aligned only at the manganese sites. This research is an excellent demonstration of exploratory synthesis and chemical intuition leading to fascinating new materials. This work adds to our understanding of what stabilizes unusual structures and influence their properties, moving us a step closer to being able to predict what happens when you put any three metallic elements together.
How current flows through iron-based superconductors is very sensitive to composition. Iron-based superconductors provide a unique window into the role magnetism plays in superconductivity, because their magnetism and superconductivity coexist, whereas in conventional superconductors they do not. Researchers studied current flow by measuring the resistivity along various directions of barium–potassium–iron–arsenide superconductors with differing amounts of potassium. Samples with lower amounts of potassium exhibit electric current flow that is easier in one direction, and as the amount of potassium increases the current flow becomes harder. Surprisingly, above a certain amount of potassium, the electric current flow actually becomes easier again, but in a direction perpendicular to that for the lower amounts of potassium. This change can only be explained if the underlying magnetic behavior is intimately tied to superconductivity. Understanding the fundamental science behind high temperature superconductors lays the foundation for producing new energy saving materials.
The self-organization of lead on silicon stands out for its remarkable efficiency and surprising new results suggest why. Most atoms sitting on surfaces like to go about their business by themselves. Alone they walk in random directions. Rarely do they move together, so when a billion atoms collectively decide to move 0.05 mm within 1 second below room temperature, it is exceptional. Researchers have found evidence of this 'superdiffusion' for lead on silicon using a technique known as low energy electron microscopy. The mass transport mechanism involves layers of lead atoms sliding across the surface all at once; the motion is believed to involve the correlated, instantaneous movements of single atoms. This superdiffusion mechanism is orders of magnitude faster than classical diffusion. Atomic and molecular diffusion on solid surfaces is critical to many physical and chemical phenomena including catalysis, surface supported nanoclusters and the formation of patterned structures. The discovery of this new mechanism provides important insight for designing other self-organizing systems.
A new recipe for storing hydrogen involves taking magnesium diboride, putting it in a container, adding some hydrogen and ball bearings, and shaking vigorously. Unlike other methods, heating the mixture is not required. Creating a room-temperature-stable, solid-stored hydrogen fuel has been described as one of the grand challenges of science. MgB2 (previously known primarily for its high-temperature superconductivity) has a hexagonal structure of boron sheets separated by layers of Mg. High-energy reactive ball milling breaks down this structure, creates trapping sites for hydrogen, and forms boron-hydrogen bonds. Hydrogen uptake is relatively easy, as is its release and regeneration of the starting material. Hydrogen is released by heating above 200 °C; complete release is achieved by 390 °C. Using x-ray diffraction and solid-state nuclear magnetic resonance (the equivalent of a MRI machine) researchers were able to understand the unusual mechanism for hydrogen uptake by MgB2 and release from the borohydride material prepared by reactive milling. The formation of nanoscale particles during milling appears critical to the reversibility of the hydrogen uptake since stable intermediates no longer form during the release process. The results have important implications for the design of room temperature hydrogen storage devices and hydrogen-fueled machines of all types.
Scientists have made air-stable compounds that should not be stable in air. Custom-designed carbon chains bonded to zinc control the rate and selectivity of reactions with oxygen and lead to the formation of novel stable zinc peroxides. New metal peroxides are desirable because they are highly effective oxidants that are useful for a variety of chemical transformations. Most metal peroxides are made using transition metals and lack the stability necessary to directly study their role in important catalytic industrial processes. Zinc is not a transition metal, but some of its properties are similar to those of transition metals, and this was crucial to the discovery. Common transition metal peroxide decomposition pathways are blocked when zinc is used. Zinc—oxygen and oxygen—oxygen bond cleavage, for example, are not fast whereas they are if transition metals are used. The lack of decomposition pathways make these zinc peroxides remarkably robust compounds. And, zinc has the added benefit of being inexpensive. These new air-stable organozinc compounds may be useful for a variety of catalytic conversions and also provide new routes to metal alkylperoxides.
Working to use sunlight to convert biomass to biofuels, researchers have found a pathway toward reducing the energy costs associated with making renewable biofuels. To achieve this, they designed semiconducting nanorods that act as light harvesting antennas, and attached metal nanoparticles that are activated by energy from the sun. This nanostructured photocatalyst converted bioderived alcohols to benzaldehyde, toluene, and the zero-emission biofuel hydrogen. Benzaldehyde is used as an almond-flavoring agent in foods and as a precursor for many pharmaceuticals, and toluene is a common industrial solvent. The metal nanoparticles, made from platinum or palladium, not only provided photocatalytic activity, but also prevented etching and degradation of the nanorods. By tuning the composition of the nanorods and the amount of metal attached (less being better!) the production of hydrogen could be increased relative to that of benzaldehyde and toluene. Further tuning and new designs of these photocatalytic nanocomposites are expected to lead to additional ways to produce lower-cost biofuels from sunlight.
Researchers have found a trick that could make writing data to a hard disk as much as a thousand times faster. Recording information in today’s magnetic memory and magneto-optical drives uses an external magnetic field and/or a laser that heats up tiny spots, one at a time, to the point where the magnetic field can switch the magnetic ordering, to store single binary digits. The speed of the magnetic switching is limited by how long it takes the laser to heat the spot close to its Curie point and the external field to reverse the magnetic region. With this approach it is extremely difficult, if not impossible, to push data transfer rates beyond the gigahertz range (109 cycles per second). In the new technique, short laser pulses, interact with so-called colossal magnetoresistive materials to create ultra-fast changes in the magnetic structure, from anti-ferromagnetic to ferromagnetic ordering — anti-parallel to parallel magnetic alignment associated with huge magnetization change. These materials are highly responsive to the external magnetic fields used to write data into memory, but do not require heat, to trigger magnetic switching. The newly discovered mechanism switches magnetic ordering much faster and potentially opens the door to terahertz (1012 hertz) memory speeds.
Gold atoms can be the key to making new materials with fascinating and frequently beautiful arrangements of atoms. For example, materials made from gold, sodium and gallium contain gold atoms arranged into tetrahedra, rods of hexagonal stars, or diamond-like three-dimensional frameworks. For certain gold concentrations, gold interacts in a novel way with sodium and stabilizes the formation of icosahedra. Icosahedral atomic arrangements are seen in many quasicrystalline materials — materials that lack the periodic long-range order of conventional crystals and exhibit crystallographically-forbidden rotational symmetries — so this prompted the idea that further tuning might lead to new quasicrystalline materials, and the discovery of the world’s first sodium-containing quasicrystals. The surprising structural versatility of gold is opening up whole new insights into structure-bonding relationships involving clusters of atoms and bulk solids.
Researchers have found a way to enhance the force of light on matter. Most of the time the momentum of light and the associated forces are too small to notice, but at the nanoscale the effect can be quite large, and researchers have used these forces to dynamically manipulate optical waveguides at the nanoscale. Optical forces decay significantly, however, as the distance between the waveguides increases and become too small for all-optical device actuation at larger separations. The new method amplifies the optical forces and thus extends them to larger separations between the waveguides by using a novel way to alter the perceived distance between them. This is done with thin layers of engineered structures known as metamaterials, which can manipulate light in ways not seen in conventional materials, extending its influence to greater distances from the surfaces of the waveguide. This work paves the way for the production of optical forces with unprecedented amplitude and eventually the design of mechanical devices activated entirely by light.
Researchers have developed a new way to track gold nanorods as they move around and re-orient themselves on metal surfaces, with significantly improved spatial resolution and speed compared with existing methods. Fluorescent dyes are commonly attached to molecules to make it possible to study their orientation and rotation. However this approach has drawbacks, because of limited signal stability and long observation times. One solution is to replace the fluorescent molecules with gold nanoparticles, providing better stability but making it harder to get detailed orientation data. Focused orientation and position imaging (FOPI) overcomes a key limitation of older methods — not being able to distinguish the full 360° orientation of the nanorods. This technique is capable of faster, higher throughput detection of the position and the 3D orientation of the gold nanoparticles, a key step towards allowing researchers to follow the motion of gold-tagged molecules as they move, interact and react on metal surfaces. This could impact a number of technologies ranging from catalysis to corrosion protection.
Designing methods to slow down electromagnetic signals just got easier with a new model that predicts how light will absorb and scatter from devices made from metamaterials. Metamaterials are built from small, engineered structures that, in some ways, mimic the role of atoms, yet can manipulate light in ways not seen in conventional materials. Slowing down light can arise in metamaterials through a process known as electromagnetically induced transparency, when destructive coupling occurs between a bright resonator and a dark resonator. The model shows what microscopic parameters will lead to the largest slowdown and also predicts an interesting phenomenon related to this — a classical analogue of electromagnetically induced absorption. This effect provides enhanced light absorption in a very narrow frequency band, and such a device has been demonstrated experimentally. The new model and associated findings have potential applications for building sensing devices and new spectroscopic tools.
Stepping on a sample, particularly in high heels, is not something most researchers would do. However, by applying pressure comparable to that generated under a stiletto heel, researchers discovered that the low temperature properties of an iron-arsenide-based superconductor where cobalt is substituted for some of the iron, Ca(Fe1-xCox)2As2, can be changed from antiferromagnetic, to superconducting to non-magnetic in a highly controllable manner. This remarkable finding is a consequence of the extreme sensitivity of the various types of order to changes in pressure. This is a unique case where the main states associated with iron-based superconductivity can all be accessed in a single sample by applying modest pressure. This discovery allows for the careful study of how these states interact and may shed light on why and how the iron arsenide materials have such remarkable superconducting properties.
The Helfand and Werthamer theory developed in 1960s predicts the magnetic field at which a superconductor turns into a normal metal if certain details of the electronic structure are known. When new superconductors are discovered, their upper critical field is usually analyzed using this theory, even though it has a well-known shortcoming — it assumes that the electronic properties of the superconductor are the same in all directions and lead to an isotropic upper critical field. A new generalization and expansion of the Helfand and Werthamer theory includes anisotropy and allows for more than one electron energy band. Researchers used the new theory to look at recently discovered anisotropic superconductors, including magnesium diboride and iron–based compounds. Previously it was thought that the cause of the temperature dependence found for the anisotropy of the upper critical field was multiple bands.However, the new scheme rules out this commonly held belief. This improved analysis method greatly advances our understanding of new superconductors.
Minute chemical substitutions are used to induce superconductivity in many materials, but the precise role of these dopants in iron-pnictide superconductors is an ongoing debate. In semiconductors, doping allows charge carrier concentrations to be controlled enabling electronic devices to be created. However, dopants in iron-arsenide superconductors do not simply impact charge carrier concentrations. Recent studies show that Co or Ni substitution for iron in BaFe2As2 produces superconducting samples, but Cu substitution does not lead to superconductivity at any doping level. Neutron scattering measurements have revealed that the magnetic order in Cu-doped samples is different and, combining experimental and theoretical results, these differences can be attributed to the stronger electron scattering effect of Cu, than for Co and Ni.This new understanding of the doping effects of transition metals may lead to the discovery of new, even higher temperature superconductors.
Magnetism behaves very strangely in compounds of lanthanum, strontium, cobalt and oxygen, and researchers have just attained new insight into the decades-old question of why. Pure LaCoO3 is a non-magnetic, narrow-gap semiconductor at low temperatures, but it acquires magnetic properties as the temperature is raised – in contrast with most materials, which tend to lose magnetism at higher temperatures. With strontium doping the magnetic properties become more prominent until, at 18% Sr, the compound becomes metallic and ferromagnetic, like iron. This behavior has often been explained hypothetically by narrow electronic states associated with the d-electrons of cobalt; however, a new study shows that the oxygen becomes partially magnetic, too. X-ray absorption measurements performed at DOE's Advanced Photon Source agree quantitatively with non-localized electronic structure calculations, which showed how the bonding between the oxygen p-orbitals and cobalt d-orbitals formed broad bands of electronic states. The calculated magnetic properties also matched the experiments. This new understanding may help the further development of these materials for applications in high temperature fuel cells and as catalysts for oxygen separation cells for batteries.
Light, combined with a novel rhodium catalyst, enables greener production of chemical feedstocks from biorenewables. A key challenge in the utilization of biomass for fuels and fine chemical applications is the control of oxygen and nitrogen-containing functional groups.Unfortunately, current routes such as gasification also generate unwanted by-products such as carbon dioxide and carbonaceous material. Other processes require additional, sacrificial chemicals, increasing costs and decreasing sustainability. Researchers developed a new process for the conversion of primary alcohols into hydrocarbons through tandem catalytic reactions. One reaction removes hydrogen from the alcohol producing valuable H2, while the other removes an atom of carbon and oxygen and produces carbon monoxide. Typically carbon monoxide inhibits the removal of hydrogen, but the use of light and a new rhodium catalyst created specially for this process prevents the inhibition. Interestingly, the catalyst was also found to be useful for controlling reactions with primary amines.The tandem reactions run at room temperature are highly selective and high yielding so these findings have great potential for enabling new industrial biorenewable-based processes.
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.