Wednesday, November 23, 2011

Why solar wind is rhombic-shaped: Temperature and energy equipartition in cosmic plasmas explained

 Why the temperatures in the solar wind are almost the same in certain directions, and why different energy densities are practically identical, was until now not clear. With a new approach to calculating instability criteria for plasmas, Bochum researchers led by Prof. Dr. Reinhard Schlickeiser (Chair for Theoretical Physics IV) have solved both problems at once. They were the first to incorporate the effects of collisions of the solar wind particles in their model. This explains experimental data significantly better than previous calculations and can also be transferred to cosmic plasmas outside our solar system.

The scientists report on their findings in Physical Review Letters.

Temperatures and pressures in the cosmic plasma

The solar wind consists of charged particles and is permeated by a magnetic field. In the analysis of this plasma, researchers investigate two types of pressure: the magnetic pressure describes the tendency of the magnetic field lines to repel each other, the kinetic pressure results from the momentum of the particles. The ratio of kinetic to magnetic pressure is called plasma beta and is a measure of whether more energy per volume is stored in magnetic fields or in particle motion. In many cosmic sources, the plasma beta is around the value one, which is the same as energy equipartition. Moreover, in cosmic plasmas near temperature isotropy prevails, i.e. the temperature parallel and perpendicular to the magnetic field lines of the plasma is the same.

Explaining satellite data

For over a decade, the instruments of the near-earth WIND satellite have gathered various solar wind data. When the plasma beta measured is plotted against the temperature anisotropy (the ratio of the perpendicular to the parallel temperature), the data points form a rhombic area around the value one. "If the values move out of the rhombic configuration, the plasma is unstable and the temperature anisotropy and the plasma beta quickly return to the stable region within the rhombus" says Prof. Schlickeiser. However, a specific, detailed explanation of this rhombic shape has, until now, been lacking, especially for low plasma beta.

Collisions in the solar wind

In previous models it was assumed that, due to the low density, the solar wind particles do not directly collide, but only interact via electromagnetic fields. "Such assumptions are, however, no longer justified for small plasma beta, since the damping due to particle collisions needs to be taken into account" explains Dipl.-Phys. Michal Michno. Prof. Schlickeiser's group included this additional damping in their model, which led to new rhombic thresholds i.e. new stability conditions. The Bochum model explains the solar wind data measured significantly better than previous theories.

Universally valid solution

The new model can be applied to other dilute cosmic plasmas which have densities, temperatures and magnetic field strengths similar to the solar wind. Even if the diagram of temperature anisotropy and plasma beta does not have exactly the rhombic shape that the researchers found for the solar wind, the newly discovered mechanism predicts that the values are always close to one. In this way, the theory also makes an important contribution to the explanation of the energy equipartition in cosmic plasmas outside of our solar system.

Journal Reference:

R. Schlickeiser, M. Michno, D. Ibscher, M. Lazar, T. Skoda. Modified Temperature-Anisotropy Instability Thresholds in the Solar Wind. Physical Review Letters, 2011; 107 (20) DOI: 10.1103/PhysRevLett.107.201102

Weird world of water gets a little weirder

Strange, stranger, strangest! To the weird nature of one of the simplest chemical compounds -- the stuff so familiar that even non-scientists know its chemical formula -- add another odd twist. Scientists are reporting that good old H2O, when chilled below the freezing point, can shift into a new type of liquid.

The report appears in ACS' Journal of Physical Chemistry B.

Pradeep Kumar and H. Eugene Stanley explain that water is one weird substance, exhibiting more than 80 unusual properties, by one count, including some that scientists still struggle to understand. For example, water can exist in all three states of matter (solid, liquid,gas) at the same time. And the forces at its surface enable insects to walk on water and water to rise up from the roots into the leaves of trees and other plants.

In another strange turn, scientists have proposed that water can go from being one type of liquid into another in a so-called "liquid-liquid" phase transition, but it is impossible to test this with today's laboratory equipment because these things happen so fast. That's why Kumar and Stanley used computer simulations to check it out.

They found that when they chilled liquid water in their simulation, its propensity to conduct heat decreases, as expected for an ordinary liquid. But, when they lowered the temperature to about 54 degrees below zero Fahrenheit, the liquid water started to conduct heat even better in the simulation. Their studies suggest that below this temperature, liquid water undergoes sharp but continuous structural changes whereas the local structure of liquid becomes extremely ordered -- very much like ice. These structural changes in liquid water lead to increase of heat conduction at lower temperatures.

The researchers say that this surprising result supports the idea that water has a liquid-liquid phase transition.

Journal Reference:

Pradeep Kumar, H. Eugene Stanley. Thermal Conductivity Minimum: A New Water Anomaly. The Journal of Physical Chemistry B, 2011; 111013123335006 DOI: 10.1021/jp2051867

Note: If no author is given, the source is cited instead.

Novel nanocrystals with advanced optical properties developed for use as luminescent biomarkers

 Upconversion emission materials are ideal for bioimaging due to its effectiveness as contrast agents for the detection of cancer cells, more so when the background emission of non-cancerous tissues can be minimised. These materials could be used as biomarkers for luminescent labeling of cancerous cells. Opaque tissues can be turned into glassy, transparent substances by using these biomarkers which rely on near-infrared excitation.

The Singapore research team led by Associate Professor Xiaogang Liu and its co-researchers from Saudi Arabia and China succeeded in developing an efficient upconversion process in nanoparticles, ensuring a broad tunability of light emission that could be used in imaging applications. They found a chemical structure that can exhibit efficient upconversion properties through a special arrangement of energy levels. Their synthesis of lanthanide-doped core-shell nanocrystals which resulted in advanced optical properties that can control light, proved to be a novel approach.

For sensing applications, separating optical signals from the background can be tricky when the signal and noise occur at the same wavelength. This problem can be solved with upconversion -- a nonlinear optical process -- where two low-energy photons of an incident beam can be converted into a single photon of higher energy, which can then be easily distinguished from the background.

The ability to convert light using these nanomaterials for heating also offers promising applications in photodynamic therapy and drug delivery.

The work of Assoc Prof Liu and team was reported in Nature Materials on 23 October 2011. His team comprises research fellow Dr Feng Wang and graduate students Renren Deng and Juan Wang from the National University of Singapore's (NUS) Department of Chemistry. They worked alongside researchers from King Abdullah University of Science and Technology and Fujian Institute of Research on the Structure of Matter. Assoc Prof Liu and Dr Feng Wang are also scientists at the Institute of Materials Research and Engineering (IMRE), a research institute of Singapore's Agency for Science, Technology and Research (A*STAR).

The published research work was funded by Singapore's A*STAR and Ministry of Education.

A novel approach to cancer detection

The team of researchers focuses on controlling the optical properties of nanomaterials by doping rare-earth metals in confined layer-by-layer structures. The nanoparticle shell can be doped with different rare earth metals, resulting in a broad tunability of the upconverted emission.

By producing nanoparticles with tunable emission which should also have a low toxicity, the researchers have made a great leap in the development of upconverting materials.

Their novel approach involves the designing of core-shell nanoparticles that separates the upconversion process from that of light emission. Photons are absorbed in the core of the nanoparticles and turned into excited electrons, after which they cascade from the core of the nanoparticles into the excited state of rare earth dopants in the shell. While there, these electrons relax and emit light.

Although such sequential energy transfer has been investigated for certain semiconductor nanoparticles and nanowires for solar energy applications, it has not been done so before for rare earth-doped nanoparticles.

Assoc Prof Liu pointed out that effort to find upconverting ions that emit in a wide-ranging spectral region has been unsuccessful until now. This is because an efficient photon upconversion has generally been restricted to a small number of lanthanide ions with emitted light signal detectable by the naked eye.

Explaining his successful approach, Assoc Prof Liu said: "We perform photon upconversion on an array of rare-earth metals. Photon upconversion turns low energy near-infrared light into higher energy made visible with the rational design and chemical synthesis of a core-shell nanostructure."

Assoc Prof Liu and team prepared nanoparticles which could demonstrate an upconversion emission ranging from violet, blue, green to red yellow, with significantly longer infrared excitation wavelengths of up to 980 nm. An important aspect of using light with 980 nm wavelength is such that the transparency of living tissues is high in infrared. This enhances the opportunity for the use of these nanoparticles for cancer detection. Furthermore, the multiple emission colours demonstrated in this research can potentially be used for a more reliable biological diagnostics application, for instance, multiple cell markers.

Opportunities for wider use

The ability to convert low energy near-infrared light into higher energy visible emission, along with low levels of toxicity to cells, and ease of processing, will turn nanometer-sized lanthanide-doped crystals into ideal materials for numerous applications.

According to the group from NUS, the results indicate that a large "library" of luminescent upconversion nanocrystals with distinguishable spectroscopic fingerprints can now be established. When coupled with biological molecules, these nanomaterials would provide a platform for a rapid and reliable route to multiplex detection of cancer or other diseases. The ability of these nanomaterials to induce light-control release of drugs for site-specific delivery also bodes well for future medicine -- fewer or reduced side effects can be expected as lanthanide-doped crystals have been tested to be non toxic.

"This work made me confident that we will see exciting new applications for these particles soon," says Thomas Nann, a research professor from the University of South Australia whose research is in this same field. Prof Nann adds that "Up-converting nanoparticles are materials with a tremendous potential for application. However, due to the need for a rigorous selection of usable up-converting ions, Science appeared not to have made any headway for some time prior to this discovery."

Assoc Prof Liu and co-researchers noted the uniqueness of their design, which is the use of core-shell nanostructures and gadolinium ions for energy migration that enhances the ability to produce a wide range of lanthanide-doped nanocrystals to yield upconverted luminescence.

"Benefiting from the sub-lattice of gadolinium ions as a network for energy migration, these judiciously-designed nanoparticles light up those less commonly used lanthanide ions like terbium, europium, and samarium under near-infrared excitation," explains Professor Chun-Hua Yan, a chemistry professor and well known scientist in the same field in Peking University, China. Adding, Prof Yan says "I do believe that this model, with its uniqueness and versatility, will vastly enrich the currently available upconversion materials, and will have impact on relevant fields such as luminescent biolabelling, multiplexed data storage and display."

The Singapore group has recently filed a related patent for their ground-breaking discovery. Currently, they are working with clinicians to develop clinical diagnostic models for use in a practical manner.

Story Source:

The above story is reprinted from materials provided by National University of Singapore, via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Feng Wang, Renren Deng, Juan Wang, Qingxiao Wang, Yu Han, Haomiao Zhu, Xueyuan Chen, Xiaogang Liu. Tuning upconversion through energy migration in core–shell nanoparticles. Nature Materials, 2011; DOI: 10.1038/NMAT3149

Note: If no author is given, the source is cited instead.

Incredible shrinking material: Engineers reveal how scandium trifluoride contracts with heat

They shrink when you heat 'em. Most materials expand when heated, but a few contract. Now engineers at the California Institute of Technology (Caltech) have figured out how one of these curious materials, scandium trifluoride (ScF3), does the trick -- a finding, they say, that will lead to a deeper understanding of all kinds of materials.

The researchers, led by graduate student Chen Li, published their results in the November 4 issue of Physical Review Letters (PRL).

Materials that don't expand under heat aren't just an oddity. They're useful in a variety of applications -- in mechanical machines such as clocks, for example, that have to be extremely precise. Materials that contract could counteract the expansion of more conventional ones, helping devices remain stable even when the heat is on.

"When you heat a solid, most of the heat goes into the vibrations of the atoms," explains Brent Fultz, professor of materials science and applied physics and a coauthor of the paper. In normal materials, this vibration causes atoms to move apart and the material to expand. A few of the known shrinking materials, however, have unique crystal structures that cause them to contract when heated, a property called negative thermal expansion. But because these crystal structures are complicated, scientists have not been able to clearly see how heat -- in the form of atomic vibrations -- could lead to contraction.

But in 2010 researchers discovered negative thermal expansion in ScF3, a powdery substance with a relatively simple crystal structure. To figure out how its atoms vibrated under heat, Li, Fultz, and their colleagues used a computer to simulate each atom's quantum behavior. The team also probed the material's properties by blasting it with neutrons at the Spallation Neutron Source at Oak Ridge National Laboratory (ORNL) in Tennessee; by measuring the angles and speeds with which the neutrons scattered off the atoms in the crystal lattice, the team could study the atoms' vibrations. The more the material is heated the more it contracts, so by doing this scattering experiment at increasing temperatures, the team learned how the vibrations changed as the material shrank.

The results paint a clear picture of how the material shrinks, the researchers say. You can imagine the bound scandium and fluorine atoms as balls attached to one another with springs. The lighter fluorine atom is linked to two heavier scandium atoms on opposite sides. As the temperature is cranked up, all the atoms jiggle in many directions. But because of the linear arrangement of the fluorine and two scandiums, the fluorine vibrates more in directions perpendicular to the springs. With every shake, the fluorine pulls the scandium atoms toward each other. Since this happens throughout the material, the entire structure shrinks.

The surprise, the researchers say, was that in the large fluorine vibrations, the energy in the springs is proportional to the atom's displacement -- how far the atom moves while shaking -- raised to the fourth power, a behavior known as a quartic oscillation. Most materials are dominated by quadratic (or harmonic) oscillations -- characteristic of the typical back-and-forth motion of springs and pendulums -- in which the stored energy is proportional to the square of the displacement.

"A nearly pure quantum quartic oscillator has never been seen in atom vibrations in crystals," Fultz says. Many materials have a little bit of quartic behavior, he explains, but their quartic tendencies are pretty small. In the case of ScF3, however, the team observed the quartic behavior very clearly. "A pure quartic oscillator is a lot of fun," he says. "Now that we've found a case that's very pure, I think we know where to look for it in many other materials." Understanding quartic oscillator behavior will help engineers design materials with unusual thermal properties. "In my opinion," Fultz says, "that will be the biggest long-term impact of this work."

The other authors of the PRL paper, "The structural relationship between negative thermal expansion and quartic anharmonicity of cubic ScF3," are former Caltech postdoctoral scholars Xiaoli Tang and J. Brandon Keith; Caltech graduate students Jorge Munoz and Sally Tracy; and Doug Abernathy of ORNL. The research was supported by the Department of Energy.

Story Source:

The above story is reprinted from materials provided by California Institute of Technology. The original article was written by Marcus Woo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Note: If no author is given, the source is cited instead.