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2015-04-25

On the road to spin-orbitronics: New way to manipulate magnetic domain walls -- ScienceDaily

Gong Chen and Andreas Schmid, experts in electron microscopy with Berkeley Lab's Materials Sciences Division, led the discovery of a technique by which the so-called "spin textures" of magnetic domain walls in ultrathin magnets can be switched between left-handed, right-handed, cycloidal, helical and mixed structures. Given that the "handedness" or chirality of spin texture determines the movement of a magnetic domain wall in response to an electric current, this technique, which involves the strategic application of uniaxial strain, should lend itself to the creation of domains walls designed for desired electronic memory and logic functions.

"The information sloshing around today's Internet is essentially a cacophony of magnetic domain walls being pushed around within the magnetic films of memory devices," says Schmid. "Writing and reading information today involves mechanical processes that limit reliability and speed. Our findings pave the way to use the spin-orbit forces that act upon electrons in a current to propel magnetic domain walls either in the same direction as the current, or in the opposite direction, or even sideways, opening up a rich new smorgasbord of possibilities in the field of spin-orbitronics."

The study was carried out at at the National Center for Electron Microscopy (NCEM), which is part of the Molecular Foundry, a DOE Office of Science User Facility. The results have been reported in a Nature Communications paper titled "Unlocking Bloch-type chirality in ultrathin magnets through uniaxial strain." Chen and Schmid are the corresponding authors. Other co-authors are Alpha N'Diaye, Sang Pyo Kang, Hee Young Kwon, Changyeon Won, Yizheng Wu and Z.Q. Qiu.

In addition to carrying a negative electrical charge, electrons also carry a quantum mechanical property known as "spin," which arises from tiny magnetic fields created by their rotational momentum. For the sake of simplicity, spin is assigned a direction of either "up" or "down." Because of these two properties, a flow of electrons creates both charge and spin currents. Charge currents are well understood and serve as the basis for today's electronic devices. Spin currents are just beginning to be explored as the basis for the emerging new field of spintronics. Coupling the flows of charge and spin currents together opens the door to yet another new field in electronics called "spin-orbitronics." The promise of spin-orbitronics is smaller, faster and far more energy efficient devices through solid-state magnetic memory.

The key to coupling charge and spin currents lies within magnetic domains, regions in a magnetic material in which all of the spins of the electrons are aligned with one another and point in the same direction - up or down. In a magnetic material containing multiple magnetic domains, individual domains are separated from one another by narrow zones or "walls" that feature rapidly changing spin directions. There are two types of magnetic domain walls known to exist in magnetic thin films: Bloch, in which electron spin rotates like a helical spiral around an axis; and Neel, in which electron spin rotates like a cycloidal spiral. Both types of walls can have either right-handed or left-handed chirality.

Applying a technique called "SPLEEM," for Spin-Polarized Low Energy Electron Microscopy, to a thin-film of iron/nickel bilayers on tungsten, Chen and Schmid and their collaborators were able to stabilize domain walls that were a mixture of Bloch and Neel types. They also showed how the chirality of domain walls can be switched between left-and right-handedness. This was accomplished by controlling uniaxial strain on the thin films in the presence of an asymmetric magnetic exchange interaction between neighbouring electron spins.

"Depending on their handedness, Neel-type walls are propelled with or against the current direction, while Bloch-type walls are propelled to the left or to the right across the current," Chen says. "Our findings introduce Bloch-type chirality as a new spin texture and might allow us to tailor the spin structure of chiral domain walls. This would present new opportunities to design spin-orbitronic devices."

A key to the success of Chen, Schmid and their colleagues was their SPLEEM imaging technique, which in this country could only be carried out at the Molecular Foundry's NCEM.

"Magnetization is a 3D vector, not just a scalar property and in order to see spin textures, the three Cartesian components of the magnetization must be resolved," Schmid says. "Berkeley Lab's SPLEEM instrument is one of a mere handful of instruments worldwide that permit imaging all three Cartesian components of magnetization. It was the unique SPLEEM experimental capability that made this spin-orbitronics research possible."

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Quantum Hall effect: Quantization of 'surface Dirac states' could lead to exotic applications -- ScienceDaily

Topological insulators are an unusual type of material, which do not conduct electricity in the inside but only on the surfaces. Their surfaces are populated by massless electrons and electron holes--known as Dirac fermions--which can conduct electricity in a nearly dissipationless fashion, like a superconductor. As a result, their properties are being studied in an intense way with the hope of creating low-power consumption electronic devices. However, impurities in the crystal structures of these topological conductors have, up to now, made it difficult to realize this potential.

In the current research, published in Nature Communications, the group was able to overcome these limitations through careful engineering of the material. The group fabricated a 3D topological conductor made from bismuth, antimony, and tellurium, successfully eliminating the impurities that have plagued previous efforts. By fixing the material on an indium phosphide semiconductor substrate and then placing an insulating oxide film and electrodes on top, they transformed the films into electric gating devices known as "field effect transistors," and measured the Hall resistance, a type of electric resistance, while tuning the strength of the electric field, using a constant magnetic field. By doing this, they were able to show that the resistance became constant at certain plateaus, demonstrating the presence of the quantum Hall effect in the material.

In addition, by tuning the external voltage placed on the films, they were able to show that the Dirac states could be switched between the integer quantum Hall state and insulating state by changing the electrical current.

According to Ryutaro Yoshimi of the Strong Correlation Physics Research Group, who led the research, "It was very exciting to see this exotic effect in a 3D topological insulator, and we plan to continue our work to show how materials can be finely tuned to have various electronic properties. In the future, these results could I hope be used for the creation of high-speed and low-power-consumption electronic elements."

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Method for printing write-once-read-many (WORM) memories developed -- ScienceDaily

The write-once-read-many (WORM) memories can be fabricated directly on the product or packaging using flexographic or inkjet printing machines, common in the packaging industry.

The memory is based on sintering of metallic nanoparticles using an electric current. A low voltage of a few volts is sufficient for the writing of the memory. Because the sintering of nanoparticles is non-reversible, the data content of the memory can only be written once.

VTT researchers envisage uses for inexpensive WORM memories in sensor applications, product originality marking, playing cards, interactive packaging and product information cards (e.g. user surveys).

"We expect massive growth from printed electronics, printed memories being a part of this sector. There are many applications for printed memories in Internet of Things and gaming fields to link the virtual and physical worlds. At VTT, we are developing methods that are tested and ready for industrialization," says Raimo Korhonen, Head of Research Area at VTT.

VTT is searching for a partner to commercialize the method patented by VTT.

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Sputtering start for flat materials -- ScienceDaily

Two-dimensional materials have a whole host of exotic properties because they are just one atom thick. A*STAR researchers have now developed a method for creating large areas of atom-thin material for use in electronic devices [1].

Graphene, a single layer of carbon atoms arranged into a honeycomb-like pattern, is the most famous example of a two-dimensional material. It is stronger than steel, has excellent electrical properties, and could be used to make two-dimensional devices that are much smaller than those currently made from bulk or thin-film silicon. However, it is not a semiconductor. And so scientists are turning to other materials that have this essential property for creating transistors.

Shijie Wang from the A*STAR Institute of Materials Research and Engineering and his collaborators have now demonstrated a technique for creating a single atomic layer of molybdenum disulfide -- a two-dimensional semiconductor.

Molybdenum disulfide belongs to a family of materials called transition-metal dichalcogenides. They have two chalcogenide atoms (such as sulfur, selenium or tellurium) for every transition-metal atom (molybdenum and tungsten are examples). These materials and their wide range of electrical properties provide an excellent platform material system for versatile electronics. But creating high-quality material over areas large enough for industrial-scale production is difficult.

"Traditional mechanical exfoliation methods for obtaining two-dimensional materials have limited usefulness in commercial applications, and all previous chemical methods are incompatible for integration with device fabrication," says Wang. "Our technique is a one-step process that can grow good-quality monolayer films, or few layers of molybdenum disulfide films, at wafer scale on various substrates using magnetron sputtering."

The team fired a beam of argon ions at a molybdenum target in a vacuum chamber. This ejected molybdenum atoms from the surface where they reacted with a nearby sulfur vapor. These atoms then assembled onto a heated substrate of either sapphire or silicon. The team found that they could grow monolayer, bilayer, trilayer or thicker samples by altering the power of the argon-ion beam or the deposition time.

They confirmed the quality of their material using a number of common characterization tools including Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The researchers also demonstrated the excellent electrical properties of their molybdenum disulfide films by creating a working transistor (see image).

"Our next step in this work will focus on the application of this technique to synthesize other two-dimensional materials and integrate them with different materials for various device applications," says Wang.

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Engineer improves rechargeable batteries with nano 'sandwich' -- ScienceDaily

Gurpreet Singh, assistant professor of mechanical and nuclear engineering, and his research team are improving rechargeable lithium-ion batteries. The team has focused on the lithium cycling of molybdenum disulfide, or MoS2, sheets, which Singh describes as a "sandwich" of one molybdenum atom between two sulfur atoms.

In the latest research, the team has found that silicon carbonitride-wrapped molybdenum disulfide sheets show improved stability as a battery electrode with little capacity fading.

The findings appear in Nature's Scientific Reports in the article "Polymer-Derived Ceramic Functionalized MoS2 Composite Paper as a Stable Lithium-Ion Battery Electrode." Other Kansas State University researchers involved include Lamuel David, doctoral student in mechanical engineering, India; Uriel Barrera, senior in mechanical engineering, Olathe; and Romil Bhandavat, 2013 doctoral graduate in mechanical engineering.

In this latest publication, Singh's team observed that molybdenum disulfide sheets store more than twice as much lithium -- or charge -- than bulk molybdenum disulfide reported in previous studies. The researchers also found that the high lithium capacity of these sheets does not last long and drops after five charging cycles.

"This kind of behavior is similar to a lithium-sulfur type of battery, which uses sulfur as one of its electrodes," Singh said. "Sulfur is notoriously famous for forming intermediate polysulfides that dissolve in the organic electrolyte of the battery, which leads to capacity fading. We believe that the capacity drop observed in molybdenum disulfide sheets is also due to loss of sulfur into the electrolyte."

To reduce the dissolution of sulfur-based products into the electrolyte, the researchers wrapped the molybdenum disulfide sheets with a few layers of a ceramic called silicon carbonitride, or SiCN. The ceramic is a high-temperature, glassy material prepared by heating liquid silicon-based polymers and has much higher chemical resistance toward the liquid electrolyte, Singh said.

"The silicon carbonitride-wrapped molybdenum disulfide sheets show stable cycling of lithium-ions irrespective of whether the battery electrode is on copper foil-traditional method or as a self-supporting flexible paper as in bendable batteries," Singh said.

After the reactions, the research team also dissembled and observed the cells under the electron microscope, which provided evidence that the silicon carbonitride protected against mechanical and chemical degradation with liquid organic electrolyte.

Singh and his team now want to better understand how the molybdenum disulfide cells might behave in an everyday electronic device -- such as a cellphone -- that is recharged hundreds of times. The researchers will continue to test the molybdenum disulfide cells during recharging cycles to have more data to analyze and to better understand how to improve rechargeable batteries.

Other research by Singh's team may help improve high temperature coatings for aerospace and defense. The engineers are developing a coating material to protect electrode materials against harsh conditions, such as turbine blades and metals subjected to intense heat.

The research appears in the Journal of Physical Chemistry. The researchers showed that when silicon carbonitride and boron nitride nanosheets are combined, they have high temperature stability and improved electrical conductivity. Additionally, these silicon carbonitride/boron nitride nanosheets are better battery electrodes, Singh said.

"This was quite surprising because both silicon carbonitride and boron nitride are insulators and have little reversible capacity for lithium-ions," Singh said. "Further analysis showed that the electrical conductivity improved because of the formation of a percolation network of carbon atoms known as 'free carbon' that is present in the silicon carbonitride ceramic phase. This occurs only when boron nitride sheets are added to silicon carbonitride precursor in its liquid polymeric phase before curing is achieved."

For both projects, the researchers have received support from the National Science Foundation.

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How to maximize the superconducting critical temperature in a molecular superconductor -- ScienceDaily

In results published in the American scientific journal Science Advances, the team was able to demonstrate the guiding influence of the molecular electronic structure in controlling superconductivity and achieving the maximum Tc, opening the way to new routes in the search of new molecular superconductors with enhanced figures of merit.

Background

Metals are used for electricity transmission, but energy is lost as heat because of electrical resistance. Superconductors have no electrical resistance and can carry electricity without losing energy, so it is important to find superconductors which can work at the highest possible temperature.

Most superconductors have simple structures built from atoms. But recently, superconductors made from molecules arranged in regular solid structures have been found.

Work by members of the team on molecular fulleride-based systems has previously led to the discovery of the highest working temperature (at 38 K) for a molecular superconductor (Nature Materials 7, p. 367, 2008).

The electronic ground state, which is in competition with superconductivity, was found to be magnetically ordered (Science 323, p. 1585, 2009). And the zero-resistance superconducting state could be switched on by tuning the exact arrangement of the C60 molecules in the solid by external pressure (Nature 466, p. 221, 2010).

The controlling role of the molecular electronic structure was then identified by demonstrating that the parent insulating state involves Jahn-Teller distortion*3 of the molecular anions that produces the magnetism from which the superconductivity emerges (Nature Communications 3, 912, 2012).

Breakthrough

The research team has addressed for the first time the relationship between the parent insulator, the normal metallic state above Tc and the superconducting pairing mechanism in a new family of chemically-pressurized fullerene materials. This is a key question in understanding all unconventional superconductors including the high-Tc cuprates, the iron pnictides and the heavy fermion systems.

Their work unveiled a new state of matter -- the Jahn-Teller metal -- and showed that when the balance between molecular and extended lattice characteristics of the electrons at the Fermi level is optimized, the highest achievable temperature for the onset of superconductivity is attained.

As synthetic chemistry allows the creation of new molecular electronic structures distinct from those in the atoms and ions that dominate most known superconductors, there is now strong motivation to search for new molecular superconducting materials.

Notes

(*1) Fullerenes

Fullerenes are molecules consisting of an even number of carbon atoms arranged over the surface of a closed hollow cage. C60 (buckminsterfullerene) which has a soccer-ball shape is the archetypal member of the fullerene family and can be considered as the third allotrope of carbon after graphite and diamond. British and American scientists won the Nobel Prize in Chemistry in 1996 for their discovery of the fullerenes.

(*2) Superconductivity

Superconductors have no electrical resistance and can carry electricity without losing energy. The temperature at which the resistance becomes zero is called the critical temperature for the onset of superconductivity, Tc. In superconducting materials, a strong attractive force acts between the electrons, which pair up and can move throughout the material without resistance.

(*3) Jahn-Teller effect

The Jahn-Teller theorem states that for any degenerate electronic state associated with a molecular electronic configuration, there will be some electron-vibrational interaction which lifts the electronic degeneracy and leads to a molecular distortion. A negatively-charged C60 molecular ion can undergo a Jahn-Teller distortion by reshaping its molecular structure away from perfect icosahedral symmetry.

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Efficient method of producing metallic nanoparticles -- ScienceDaily

VTT's aerosol technology reactor for nanoparticle production can generate a variety of pure metal particles, particles of various alloys and carbon-coated particles. The reactor can efficiently produce hundreds of grammes or even kilogrammes of nanoparticles per day.

"Demand has outstripped supply in the nanoparticle markets. This has been an obstacle to the development of product applications; nano-metal composites are scarce and often available in small quantities only. We wanted to demonstrate that it was possible to produce nanomaterials in considerable quantities cost-effectively," comments Ari Auvinen of VTT, head of the research team.

When developing the reactor, the aim was to achieve a production figure of 200-3,000 grammes per day. This has already been clearly exceeded. Due to the extremely small material wastage incurred when using this equipment, remote-control production can be maintained for several days. In most cases, industrial production of metallic nanoparticles involves chemical reduction in liquid solutions, which requires the design of product-specific solutions. Plasma synthesis, which consumes large amounts of energy and involves significant material wastage, is another generally used method.

In the design of the reactor developed by VTT, the scalability and cost-effectiveness of the synthesis process were key criteria. For this reason, synthesis is performed under air pressure at a comparatively low temperature. This means that the equipment can be built from materials commonly used in industry and energy consumption is low. The process generates an extremely high particle concentration, enabling a high production speed but with low gas consumption. In addition, even impure metallic salts can be used as a raw material, which keeps the price low.

VTT has demonstrated the practical functionality of its reactor by testing the production of various nanometals, metallic compounds and carbon-coated materials. Materials such as carbon-coated magnets, which can be used as catalysts in biorefineries -- say, in the production of biofuels -- have been produced in the reactor. Following synthesis, magnets used as catalysts can be efficiently gathered in and recycled back into the process.

Nanoparticles have also been tested in the manufacture of magnetic inks and inks that conduct electricity in printed electronics. For example, VTT succeeded in using a permalloy ink to print a magnetically anisotropic material, which can be used in the manufacture of magnetic field sensors.

VTT's third application trial involved the prevention of microwave reflection. The tests showed that reflection can be reduced by even 10,000 times in polymers, by adding particles which increase radar wave attenuation.

VTT's researchers believe that the reactor has many applications in addition to those already mentioned. The silicon nanoparticles it produces may even enable lithium battery capacity to be boosted by a factor of 10. Other possible applications, all of which require further investigation, include high permeability polymers, nanomagnets for medical diagnostics applications, materials for the 3D printing of metal articles, and silicon-based materials for thermoelectric and solar power components.

VTT is currently seeking a party interested in commercializing the technique.

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New path to loss-free electricity -- ScienceDaily

This discovery of substitution-induced charge redistribution demonstrates the prominent role of bonding (and the associated electron fluctuations) in the emergence of superconductivity in iron-based alloys. It suggests a new route for finding higher performance superconductors through engineering and optimization of the electron density among the atoms in the material.

The flow of current in ordinary metals and other materials that conduct electricity is composed of electrons; however, the charge carriers are scattered when they conduct electricity, resulting in dissipation and energy loss, typically in the form of heat. In a superconductor, the electrons form into pairs that allow them to move through the material without resistance, eliminating the energy lost and thus increasing the efficient use of electricity.

The challenge in creating such electron pairs is overcoming the natural tendency for electrons to repel each other. One solution is to utilize electronic polarizability that can yield an attractive interaction between electrons, thus allowing pair formation and the potential for loss-free current flow. Such a mechanism was proposed almost five decades ago, but it was never experimentally verified. Using electron diffraction with subatomic precision, scientists at Brookhaven National Laboratory have mapped out the redistribution of orbital electrons in barium iron arsenide, with and without cobalt substitution.

The results reveal a remarkable increase in charge distribution around the iron and arsenic atoms as cobalt is incorporated into the material. Electron energy loss spectroscopy was carried out to determine the charge carrier-injection effect of cobalt substitution, while density functional theory was used to model electronic and atomic behavior. The induced charge redistribution around the iron and arsenic atoms after cobalt substitution suggests that the strongly coupled bond-electron fluctuation and charge separation may provide a new mechanism for high-temperature superconductivity. These results may guide the design of new superconductors.

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Theoretical framework for graphene physics established -- ScienceDaily

Graphene, a single-atom-thick sheet of carbon atoms arranged in a honeycomb-like lattice, is one of the simplest materials with unrivalled mechanical and electronic properties. The material has been hailed by scientists as an extremely good conductor of electrons due to its strength and its light weight. In 2013, researchers from the Massachusetts Institute of Technology (MIT) discovered that placing graphene on top of hexagonal boron nitride, another one-atom-thick material with similar properties will create a hybrid material that shares graphene's amazing ability to conduct electrons, while adding the band gap necessary to form transistors and other semiconductor devices. Semiconductors, which can switch between conducting and insulating states, are the basis for modern electronics. The reasons behind why the hybrid material performed as such were unexplained until this new theoretical framework was created by researchers from Yale-NUS, NUS and UT Austin.

To fully harness the hybrid material's properties for the creation of viable semiconductors, a robust band gap without any degradation in the electronic properties is a necessary requirement. The researchers concluded that it is necessary to use a theoretical framework that treats electronic and mechanical properties equally in order to make reliable predictions for these new hybrid materials.

Shaffique Adam, Assistant Professor at Yale-NUS College and NUS Department of Physics, said," This theoretical framework is the first of its kind and can be generally applied to various two dimensional materials. Prior to our work, it was commonly assumed that when one 2D material is placed on top of another, they each remain planar and rigid. Our work showed that their electronic coupling induces significant mechanical strain, stretching and shrinking bonds in three dimensions, and that these distortions change the electronic properties. We find that the band gap depends on several factors including the angle between the two sheets and their mechanical stiffness. Going forward, we will continue to theoretically explore the optimal parameters to create larger bandgaps that can be used for a wide range of technologies. "

Pablo Jarillo-Herrero, the Mitsui Career Development Associate Professor of Physics at MIT, whose research team first reported band gaps in this new graphene hybrid material said, "This theory work has increased the accuracy and predictability of calculating the induced band gap in graphene, which may enable applications of graphene in digital electronics and optoelectronics. If we are able to increase the magnitude of the band gap through new research, this could pave the way to novel flexible and wearable nanoelectronic and optoelectronic devices."

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Connecting three atomic layers puts semiconducting science on its edge -- ScienceDaily

Layered materials at the atomic limit, where electrons are constrained to two dimensions, can be engineered into electronic structures with unique optical, electronic, and magnetic properties. The new structures are a test bed for theories of low dimensional materials physics, but more practically, the optical manipulation of electron charge and magnetic order can lead to new modes of solar energy conversion and flexible, transparent computation devices.

A new ultra-thin semiconducting material consists of three atomic layers in an "atomic sandwich" configuration with a heavy metal atom layer between two selenium layers. The material is quasi-two dimensional, just three atoms thick, and exhibits unique properties at junctions. When the material is exposed to light and absorbs photons, excited electrons are created which remain coherently coupled, in unique ways, with the charge "hole" they left behind. Stacking up two "atomic sandwiches" yields coupled excited charge states across the planar interface with the magnetic direction or "spin state" becoming aligned for a large population of electrons.

A companion result by the same research group demonstrated a method to make the edge of one metal layer match up with the edge of a second, different metal layer -- a linear boundary or "hetero-junction" rather than the more typical planar boundary. Engineered electron spin and charge polarization, as well as transport across or along the interface, might be possible as evidenced by the enhanced photoluminescence signals at these positions.

Primary support by the DOE Office of Science, Basic Energy Sciences. Some students were supported by graduate student fellowships and doctoral training grants (NSF, EPSRC, HEFCE, and Cottrell Scholar Award). Some researchers were supported by the Hong Kong Research Grant Council, the Croucher Foundation, the Science City Research Alliance and the University of Washington Clean Energy Institute.

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Electrons move like light in three-dimensional solid -- ScienceDaily

A stable bulk material has been discovered that shows the same physics found in graphene, which illuminated the detailed interactions of electron's orbital motion and its intrinsic magnetic orientation. The new material will be a test ground for theories on how electron interactions in solids shape exotic electron behavior, including the highest electron mobility in bulk materials.

Investigations of electronic behavior have expanded beyond familiar systems of metals, insulators, and semi-conductors into the realm of strongly interacting electrons, which exhibit exotic relationships between the allowed electron velocities and their energy states. A key feature for the new materials is behavior in which the electron velocity does not depend on its energy. Such a relationship is a hallmark of photons, the energetic particles that make up a beam of light. This property is found in the new class of materials exhibiting a strong interaction between the electron trajectory and the electron spin alignment (called "spin-orbit coupling"). Two-dimensional versions of such systems (for example, grapheme) have been recently explored, but the systems are hard to work with because of their ultra-thin film nature.

This work establishes graphene-like electronic behavior in the bulk three-dimensional materials Na₃Bi and Cd₃As₂ and explains their exceptionally high electronic mobility. The required advances in electron spectroscopy techniques, used to investigate the electronic structure, employed an energy tunable bright x-ray source and a high-resolution spectrometer.

Funded by DOE Office of Science, Basic Energy Sciences, including support for the Advanced Light Source. Researchers from foreign institutions were supported by the Engineering and Physical Sciences Research Council (UK), the National Science Foundation of China, the National Basic Research Program of China, the International Science and Technology Cooperation Program of China, the China Scholarship Council, and Defense Advanced Research Projects Agency (USA).

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