The 500 phases of matter: New system successfully classifies symmetry-protected phases

Dec. 21, 2012 — Forget solid, liquid, and gas: there are in fact more than 500 phases of matter. In a major paper in a recent issue of Science, Perimeter Faculty member Xiao-Gang Wen reveals a modern reclassification of all of them.

Condensed matter physics -- the branch of physics responsible for discovering and describing most of these phases -- has traditionally classified phases by the way their fundamental building blocks -- usually atoms -- are arranged. The key is something called symmetry.

To understand symmetry, imagine flying through liquid water in an impossibly tiny ship: the atoms would swirl randomly around you and every direction -- whether up, down, or sideways -- would be the same. The technical term for this is "symmetry" -- and liquids are highly symmetric. Crystal ice, another phase of water, is less symmetric. If you flew through ice in the same way, you would see the straight rows of crystalline structures passing as regularly as the girders of an unfinished skyscraper. Certain angles would give you different views. Certain paths would be blocked, others wide open. Ice has many symmetries -- every "floor" and every "room" would look the same, for instance -- but physicists would say that the high symmetry of liquid water is broken.

Classifying the phases of matter by describing their symmetries and where and how those symmetries break is known as the Landau paradigm. More than simply a way of arranging the phases of matter into a chart, Landau's theory is a powerful tool which both guides scientists in discovering new phases of matter and helps them grapple with the behaviours of the known phases. Physicists were so pleased with Landau's theory that for a long time they believed that all phases of matter could be described by symmetries. That's why it was such an eye-opening experience when they discovered a handful of phases that Landau couldn't describe.

Beginning in the 1980s, condensed matter researchers, including Xiao-Gang Wen -- now a faculty member at Perimeter Institute -- investigated new quantum systems where numerous ground states existed with the same symmetry. Wen pointed out that those new states contain a new kind of order: topological order. Topological order is a quantum mechanical phenomenon: it is not related to the symmetry of the ground state, but instead to the global properties of the ground state's wave function. Therefore, it transcends the Landau paradigm, which is based on classical physics concepts.

Topological order is a more general understanding of quantum phases and the transitions between them. In the new framework, the phases of matter were described not by the patterns of symmetry in the ground state, but by the patterns of a decidedly quantum property -- entanglement. When two particles are entangled, certain measurements performed on one of them immediately affect the other, no matter how far apart the particles are. The patterns of such quantum effects, unlike the patterns of the atomic positions, could not be described by their symmetries. If you were to describe a city as a topologically ordered state from the cockpit of your impossibly tiny ship, you'd no longer be describing the girders and buildings of the crystals you passed, but rather invisible connections between them -- rather like describing a city based on the information flow in its telephone system.

This more general description of matter developed by Wen and collaborators was powerful -- but there were still a few phases that didn't fit. Specifically, there were a set of short-range entangled phases that did not break the symmetry, the so-called symmetry-protected topological phases. Examples of symmetry-protected phases include some topological superconductors and topological insulators, which are of widespread immediate interest because they show promise for use in the coming first generation of quantum electronics.

In the paper featured in Science, Wen and collaborators reveal a new system which can, at last, successfully classify these symmetry-protected phases.

Using modern mathematics -- specifically group cohomology theory and group super-cohomology theory -- the researchers have constructed and classified the symmetry-protected phases in any number of dimensions and for any symmetries. Their new classification system will provide insight about these quantum phases of matter, which may in turn increase our ability to design states of matter for use in superconductors or quantum computers.

This paper is a revealing look at the intricate and fascinating world of quantum entanglement, and an important step toward a modern reclassification of all phases of matter.

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Journal Reference:

X. Chen, Z.-C. Gu, Z.-X. Liu, X.-G. Wen. Symmetry-Protected Topological Orders in Interacting Bosonic Systems. Science, 2012; 338 (6114): 1604 DOI: 10.1126/science.1227224

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Science's breakthrough of the year: Discovery of the Higgs boson

Dec. 20, 2012 — The observation of an elusive sub-atomic particle, known as the Higgs boson, has been heralded by the journal Science as the most important scientific discovery of 2012. This particle, which was first hypothesized more than 40 years ago, holds the key to explaining how other elementary particles (those that aren't made up of smaller particles), such as electrons and quarks, get their mass.

In addition to recognizing the detection of this particle as the 2012 Breakthrough of the Year, Science and its international nonprofit publisher, AAAS, have identified nine other groundbreaking scientific achievements from the past year and compiled them into a top 10 list that will appear in the 21 December issue.

Researchers unveiled evidence of the Higgs boson on 4 July, fitting into place the last missing piece of a puzzle that physicists call the standard model of particle physics. This theory explains how particles interact via electromagnetic forces, weak nuclear forces and strong nuclear forces in order to make up matter in the universe. However, until this year, researchers could not explain how the elementary particles involved got their mass.

"Simply assigning masses to the particles makes the theory go haywire mathematically," explained Science news correspondent Adrian Cho, who wrote about the discovery for the journal's Breakthrough of the Year feature. "So, mass must somehow emerge from interactions of the otherwise mass-less particles themselves. That's where the Higgs comes in."

As Cho explains, physicists assume that space is filled by a "Higgs field," which is similar to an electric field. Particles interact with this Higgs field to obtain energy and -- thanks to Einstein's famous mass-energy equivalence -- mass as well. "Just as an electric field consists of particles called photons, the Higgs field consists of Higgs bosons woven into the vacuum," he explains. "Physicists have now blasted them out of the vacuum and into brief existence."

But, a view to the Higgs boson did not come easy -- or cheap. Thousands of researchers working with a 5.5-billion-dollar atom-smasher at a particle physics laboratory near Geneva, Switzerland, called CERN, used two gargantuan particle detectors, known as ATLAS and CMS, to spot the long-sought boson.

It is unclear where this discovery will lead the field of particle physics in the future but its impact on the physics community this year has been undeniable, which is why Science calls the detection of the Higgs boson the 2012 Breakthrough of the Year. The special 21 December issue of the journal includes three articles written by researchers at CERN, which help to explain how this breakthrough was achieved.

Science's list of nine other pioneering scientific achievements from 2012 follows.

The Denisovan Genome: A new technique that binds special molecules to single strands of DNA allowed researchers to sequence the complete Denisovan genome from just a fragment of bone from an ancient pinky finger. The genomic sequence has allowed researchers to compare Denisovans -- archaic humans closely related to Neandertals -- with modern humans. It also revealed that the finger bone belonged to a girl with brown eyes, brown hair and brown skin who died in Siberia between 74,000 and 82,000 years ago.

Making Eggs From Stem Cells: Japanese researchers showed that embryonic stem cells from mice could be coaxed into becoming viable egg cells. They clinched the case when the cells, fertilized by sperm in the laboratory, developed into live mouse pups born of surrogate mothers. The method requires female mice to host the developing eggs in their bodies for a time, so it falls short of scientists' ultimate goal: deriving egg cells entirely in the laboratory. But, it provides a powerful tool for studying genes and other factors that influence fertility and egg cell development.

Curiosity's Landing System: Though unable to test their rover's entire landing system under Martian conditions, mission engineers at NASA's Jet Propulsion Laboratory in Pasadena, California, safely and precisely placed the Curiosity rover on the surface of Mars. The 3.3-ton rover entry vehicle was too massive for traditional landings, so the team took inspiration from cranes and helicopters to create a "sky crane" landing system that dangled Curiosity, wheels deployed, at the end of three cables. The flawless landing reassured planners that NASA could someday land a second mission near an earlier rover to pick up samples the rover collected and return them to Earth.

X-ray Laser Provides Protein Structure: Researchers used an X-ray laser, which shines a billion times brighter than traditional synchrotron sources, to determine the structure of an enzyme required by the Trypanosoma brucei parasite, the cause of African sleeping sickness. The advance demonstrated the potential of X-ray lasers to decipher proteins that conventional X-ray sources cannot.

Precision Engineering of Genomes: The revision and deletion of DNA in higher organisms has generally been a hit-or-miss proposition. But, in 2012, a tool known as TALENs, which stands for "transcription activator-like effector nucleases," gave researchers the ability to alter or inactivate specific genes in zebrafish, toads, livestock and other animals -- even cells from patients with disease. This technology, along with others that are emerging, is proving to be just as effective as (and cheaper than) established gene-targeting techniques, and it may allow researchers to determine specific roles for genes and mutations in both healthy and diseased individuals.

Majorana Fermions: The existence of Majorana fermions, particles that (among other properties) act as their own antimatter and annihilate themselves, has been debated for more than seven decades. This year, a team of physicists and chemists in The Netherlands provided the first solid evidence that such exotic matter exists, in the form of quasi-particles: groups of interacting electrons that behave like single particles. The discovery has already prompted efforts to incorporate Majorana fermions into quantum computing, as scientists think "qubits" made of these mysterious particles could be more efficient at storing and processing data than the bits currently used in digital computers.

The ENCODE Project: A decade-long study that was reported this year in more than 30 papers revealed that the human genome is more "functional" than researchers had believed. Although only two percent of the genome codes for actual proteins, the Encyclopedia of DNA Elements, or ENCODE, project indicated that about 80 percent of the genome is active, helping to turn genes on or off, for example. These new details should help researchers to understand the ways in which genes are controlled and to clarify some genetic risk factors for diseases.

Brain-Machine Interfaces: The same team that had previously demonstrated how neural recordings from the brain could be used to move a cursor on a computer screen showed in 2012 that paralyzed human patients could move a mechanical arm with their minds and perform complex movements in three dimensions. The technology is still experimental -- and extraordinarily expensive -- but scientists are hopeful that more advanced algorithms could improve these neural prosthetics to help patients paralyzed by strokes, spinal injuries and other conditions.

Neutrino Mixing Angle: Hundreds of researchers working on the Daya Bay Reactor Neutrino Experiment in China reported the last unknown parameter of a model that describes how elusive particles, known as neutrinos, morph from one type or "flavor" to another as they travel at near-light speed. The results show that neutrinos and anti-neutrinos could possibly change flavors differently and suggest that neutrino physics may someday help researchers to explain why the universe contains so much matter and so little antimatter. If physicists cannot identify new particles beyond the Higgs boson, neutrino physics could represent the future of particle physics.

Science's 2012 Breakthrough of the Year feature, along with a related editorial by Bruce Alberts, Science's Editor-in-Chief, and three related articles about the Higgs boson, a podcast interview and other multimedia, will be available for free after the embargo lifts with registration at www.sciencemag.org/special/btoy2012.

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Production of 5-aminovaleric and glutaric acid by metabolically engineered microorganism

Dec. 20, 2012 — We use many different types of chemicals and plastics for the convenience of our everyday life. The current sources of these materials are provided from petrochemical industry, using fossil oil as a raw material. Due to our increased concerns on the environmental problems and fossil resource availability, there has been much interest in producing those chemicals and materials from renewable non-food biomass through biorefineries.

For the development of biorefinery process, microorganisms have successfully been employed as the key biocatalysts to produce a wide range of chemicals, plastics, and fuels from renewable resources. However, the natural microorganisms without modification are not suitable for the efficient production of target products at industrial scale due to their poor metabolic performance. Thus, metabolic capacities of microorganisms have been improved to efficiently produce desired products, the performance of which is suitable for industrial production of such products. Optimization of microorganism for the efficient production of target bioproducts has been achieved by systems metabolic engineering, which allows metabolic engineering at the systems-level.

5-aminovalic acid (5AVA) is the precursor of valerolactam, a potential building block for producing nylon 5, and can potentially be used as a C5 platform chemical for synthesizing 5-hydroxyvaleric acid, glutaric acid, and 1,5-pentanediol. It has been reported that a small amount of 5AVA is accumulated in Pseudomonas putida that has impaired L-lysine catabolism since 5AVA is a natural metabolite of L-lysine catabolism in P. putida. However, direct fermentative production of 5AVA has not yet been demonstrated, which might have great potential to open market for C5 chemicals and plastics.

In the paper published in Metabolic Engineering, a Korean research team led by Distinguished Professor Sang Yup Lee at the Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), a premier science and engineering university in Korea, together with Dr. Seung Hwan Lee at Korea Research Institute of Chemical Technology (KRICT), a government supported research institute in Korea, and Prof. Si Jae Park at Myongji University in Korea, applied systems metabolic engineering approach to develop recombinant Escherichia coli for the production of 5-aminovaleric acid and glutaric acid, the promising C5 platform chemicals, by fermentation.

Firstly, they constructed metabolic pathway to produce 5-aminovaleric acid (5AVA) using L-lysine as a direct precursor by employing two enzymes lysine 2-monooxygenase (DavB) and delta-aminovaleramidase (DavA). Secondly, metabolic pathway for the further conversion of 5AVA into glutaric acid was constructed by employing two more enzymes 5AVA aminotransferase (GabT) and glutarate semialdehyde dehydrogenase (GabD). Recombinant E. coli expressing DavB and DavA produced 5AVA using L-lysine as a direct precursor, and recombinant E. coli expressing DavB, DavA, GabT, and GabD produced glutaric acid from L-lysine. Finally, the L-lysine biosynthetic pathway of E. coli was systematically engineered to produce 5AVA from glucose. As a proof-of-concept demonstration, fermentation of this metabolically engineered E. coli strain successfully produced 5AVA from glucose. This study showcases the first microbial process for the production of 5AVA and glutatic acid as C5 platform chemicals by developing microbial strain through systems metabolic engineering.

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Journal Reference:

Si Jae Park, Eun Young Kim, Won Noh, Hye Min Park, Young Hoon Oh, Seung Hwan Lee, Bong Keun Song, Jonggeon Jegal, Sang Yup Lee. Metabolic engineering of Escherichia coli for the production of 5-aminovalerate and glutarate as C5 platform chemicals. Metabolic Engineering, 2012; DOI: 10.1016/j.ymben.2012.11.011

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Unlocking new talents in nature: Protein engineers create new biocatalysts

Dec. 20, 2012 — Protein engineers at the California Institute of Technology (Caltech) have tapped into a hidden talent of one of nature's most versatile catalysts. The enzyme cytochrome P450 is nature's premier oxidation catalyst -- a protein that typically promotes reactions that add oxygen atoms to other chemicals. Now the Caltech researchers have engineered new versions of the enzyme, unlocking its ability to drive a completely different and synthetically useful reaction that does not take place in nature.

The new biocatalysts can be used to make natural products -- such as hormones, pheromones, and insecticides -- as well as pharmaceutical drugs, like antibiotics, in a "greener" way.

"Using the power of protein engineering and evolution, we can convince enzymes to take what they do poorly and do it really well," says Frances Arnold, the Dick and Barbara Dickinson Professor of Chemical Engineering, Bioengineering and Biochemistry at Caltech and principal investigator on a paper about the enzymes that appears online in Science. "Here, we've asked a natural enzyme to catalyze a reaction that had been devised by chemists but that nature could never do."

Arnold's lab has been working for years with a bacterial cytochrome P450. In nature, enzymes in this family insert oxygen into a variety of molecules that contain either a carbon-carbon double bond or a carbon-hydrogen single bond. Most of these insertions require the formation of a highly reactive intermediate called an oxene.

Arnold and her colleagues Pedro Coelho and Eric Brustad noted that this reaction has a lot in common with another reaction that synthetic chemists came up with to create products that incorporate a cyclopropane -- a chemical group containing three carbon atoms arranged in a triangle. Cyclopropanes are a necessary part of many natural-product intermediates and pharmaceuticals, but nature forms them through a complicated series of steps that no chemist would want to replicate.

"Nature has a limited chemical repertoire," Brustad says. "But as chemists, we can create conditions and use reagents and substrates that are not available to the biological world."

The cyclopropanation reaction that the synthetic chemists came up with inserts carbon using intermediates called carbenes, which have an electronic structure similar to oxenes. This reaction provides a direct route to the formation of diverse cyclopropane-containing products that would not be accessible by natural pathways. However, even this reaction is not a perfect solution because some of the solvents needed to run the reaction are toxic, and it is typically driven by catalysts based on expensive transition metals, such as copper and rhodium. Furthermore, tweaking these catalysts to predictably make specific products remains a significant challenge -- one the researchers hoped nature could overcome with evolution's help.

Given the similarities between the two reaction systems -- cytochrome P450's natural oxidation reactions and the synthetic chemists' cyclopropanation reaction -- Arnold and her colleagues argued that it might be possible to convince the bacterial cytochrome P450 to create cyclopropane-bearing compounds through this more direct route. Their experiments showed that the natural enzyme (cytochrome P450) could in fact catalyze the reaction, but only very poorly; it generated a low yield of products, didn't make the specific mix of products desired, and catalyzed the reaction only a few times. In comparison, transition-metal catalysts can be used hundreds of times.

That's where protein engineering came in. Over the years, Arnold's lab has created thousands of cytochrome P450 variants by mutating the enzyme's natural sequence of amino acids, using a process called directed evolution. The researchers tested variants from their collections to see how well they catalyzed the cyclopropane-forming reaction. A handful ended up being hits, driving the reaction hundreds of times.

Being able to catalyze a reaction is a crucial first step, but for a chemical process to be truly useful it has to generate high yields of specific products. Many chemical compounds exist in more than one form, so although the chemical formulas of various products may be identical, they might, for example, be mirror images of each other or have slightly different bonding structures, leading to dissimilar behavior. Therefore, being able to control what forms are produced and in what ratio -- a quality called selectivity -- is especially important.

Controlling selectivity is difficult. It is something that chemists struggle to do, while nature excels at it. That was another reason Arnold and her team wanted to investigate cytochrome P450's ability to catalyze the reaction.

"We should be able to marry the impressive repertoire of catalysts that chemists have invented with the power of nature to do highly selective chemistry under green conditions," Arnold says.

So the researchers further "evolved" enzyme variants that had worked well in the cyclopropanation reaction, to come up with a spectrum of new enzymes. And those enzymes worked -- they were able to drive the reaction many times and produced many of the selectivities a chemist could desire for various substrates.

Coelho says this work highlights the utility of synthetic chemistry in expanding nature's catalytic potential. "This field is still in its infancy," he says. "There are many more reactions out there waiting to be installed in the biological world."

The paper, "Olefin cyclopropanation via carbene insertion catalyzed by engineered cytochrome P450 enzymes," was also coauthored by Arvind Kannan, now a Churchill Scholar at Cambridge University; Brustad is now an assistant professor at the University of North Carolina at Chapel Hill. The work was supported by a grant from the U.S. Department of Energy and startup funds from UNC Chapel Hill.

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

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Journal Reference:

P. S. Coelho, E. M. Brustad, A. Kannan, F. H. Arnold. Olefin Cyclopropanation via Carbene Transfer Catalyzed by Engineered Cytochrome P450 Enzymes. Science, 2012; DOI: 10.1126/science.1231434

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Data storage: A fast and loose approach improves memory

Dec. 20, 2012 — An unconventional design for a nanoscale memory device uses a freely moving mechanical shuttle to improve performance.

A loose and rattling part in your cell phone is generally a cause for concern. Like most other electronic devices, your phone works by moving electrons through fixed circuit pathways. If electrons are not sufficiently contained within these pathways, the efficiency and speed of a device decrease. However, as the miniature components inside electronic devices shrink with each generation, electrons become harder to contain. Now, a research team led by Vincent Pott at the A*STAR Institute of Microelectronics, Singapore, has designed a memory device using a loose and moving part that actually enhances performance.

The loose part is a tiny metal disk, or shuttle, about 300 nanometers thick and 2 micrometers long, and lies inside a roughly cylindrical metal cage. Because the shuttle is so small, gravity has little effect on it. Instead, the forces of adhesion between the shuttle and its metal cage determine its position. When stuck to the top of its cage, the shuttle completes an electrical circuit between two electrodes, causing current to flow. When it is at the bottom of the cage, the circuit is broken and no current flows. The shuttle can be moved from top to bottom by applying a voltage to a third electrode, known as a gate, underneath the cage.

Pott and co-workers suggested using this binary positioning to encode digital information. They predicted that the forces of adhesion would keep the shuttle in place even when the power is off, allowing the memory device to retain information for long periods of time. In fact, the researchers found that high temperature -- one of the classic causes of electronic memory loss -- should actually increase the duration of data retention by softening the metal that makes up the shuttle memory's disk and cage, thereby strengthening adhesion. The ability to operate in hot environments is a key requirement for military and aerospace applications.

The untethered shuttle also takes up less area than other designs and is not expected to suffer from mechanical fatigue because it avoids the use of components that need to bend or flex -- such as the cantilevers used in competing mechanical memory approaches. In a simulation, Pott and co-workers found that the shuttle memory should be able to switch at speeds in excess of 1 megahertz.

The next steps, the researchers say, include designing arrays of the devices and analyzing fabrication parameters in detail. If all goes well, their novel device could compete head-to-head with the industry-standard FLASH memory.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Microelectronics/

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Vincent Pott, Geng Li Chua, Ramesh Vaddi, Julius Ming-Lin Tsai, Tony T. Kim. The Shuttle Nanoelectromechanical Nonvolatile Memory. IEEE Transactions on Electron Devices, 2012; 59 (4): 1137 DOI: 10.1109/TED.2011.2181517

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Nanotechnology: Spotting a molecular mix-up

Dec. 20, 2012 — Information within the bonds of molecules known as super benzene oligomers pave the way for new types of quantum computers.

Scanning tunneling microscopy (STM) is routinely employed by physicists and chemists to capture atomic-scale images of molecules on surfaces. Now, an international team led by Christian Joachim and co-workers from the A*STAR Institute of Materials Research and Engineering has taken STM a step further: using it to identify the quantum states within 'super benzene' compounds using STM conductance measurements1. Their results provide a roadmap for developing new types of quantum computers based on information localized inside molecular bonds.

To gain access to the quantum states of hexabenzocoronene (HBC) -- a flat aromatic molecule made of interlocked benzene rings -- the researchers deposited it onto a gold substrate. According to team member We-Hyo Soe, the weak electronic interaction between HBC and gold is crucial to measuring the system's 'differential conductance' -- an instantaneous rate of current charge with voltage that can be directly linked to electron densities within certain quantum states.

After cooling to near-absolute zero temperatures, the team maneuvered its STM tip to a fixed location above the HBC target. Then, they scanned for differential conductance resonance signals at particular voltages. After detecting these voltages, they mapped out the electron density around the entire HBC framework using STM. This technique provided real-space pictures of the compound's molecular orbitals -- quantized states that control chemical bonding.

When Joachim and co-workers tried mapping a molecule containing two HBC units, a dimer, they noticed something puzzling. They detected two quantum states from STM measurements taken near the dimer's middle, but only one state when they moved the STM tip to the dimer's edge (see image). To understand why, the researchers collaborated with theoreticians who used high-level quantum mechanics calculations to identify which molecular orbitals best reproduced the experimental maps.

Traditional theory suggests that STM differential conductance signals can be assigned to single, unique molecular orbitals. The researchers' calculations, however, show that this view is flawed. Instead, they found that observed quantum states contained mixtures of several molecular orbitals, with the exact ratio dependent upon the position of the ultra-sharp STM tip.

Soe notes that these findings could have a big impact in the field of quantum computing. "Each measured resonance corresponds to a quantum state of the system, and can be used to transfer information through a simple energy shift. This operation could also fulfill some logic functions." However, he adds that advanced, many-body theories will be necessary to identify the exact composition and nature of molecular orbitals due to the location-dependent tip effect.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering

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We-Hyo Soe, Hon Seng Wong, Carlos Manzano, Maricarmen Grisolia, Mohamed Hliwa, Xinliang Feng, Klaus Müllen, Christian Joachim. Mapping the Excited States of Single Hexa-peri-benzocoronene Oligomers. ACS Nano, 2012; 6 (4): 3230 DOI: 10.1021/nn300110k

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Physicists take photonic topological insulators to the next level

Dec. 21, 2012 — Researchers at The University of Texas at Austin have designed a simulation that for the first time emulates key properties of electronic topological insulators.

Their simulation, which was described this week in Nature Materials, is part of a rapidly moving scientific race to understand and exploit the potential of topological insulators, which are a state of matter that was only discovered in the past decade. These insulators may enable dramatic advances in quantum computing and spintronics.

"The discovery of these materials, which are insulators in their volume while capable of conducting current on their surface, was a bit of a surprise to the condensed matter community," said Gennady Shvets, professor of physics in the College of Natural Sciences. "Before that, we classified solid materials into three categories, based on their ability to conduct electric current: insulators, conductors, and semiconductors. Topological insulators fall somewhere in between."

Shvets co-authored the article with his physics department colleagues Alexander Khanikaev, S. Hossein Mousavi, Wang-Kong Tse, Mehdi Kargarian, and Professor Allan MacDonald.

He said that what's particularly exciting about topological insulators is that they can conduct electrons -- or in the case of photonic ones, photons -- in a way that protects them from scattering or reflecting when they encounter obstacles.

"Usually when photons run into an obstacle, they reflect," said Shvets. "We are basically designing interfaces in such a way that they lock photons into one spin state. So in one direction they're in one spin state, and when they're going in another direction they're locked into another spin state. In that configuration they cannot reflect without changing their spin, which is forbidden by the design of the photonic crystal. They flow around defects and can be routed along arbitrarily shaped paths defined by the interface."

If this property could be achieved with electrons it would be particularly relevant to quantum computers, which are likely to require their electrons to maintain coherence for a much longer time than digital computers.

Over the past decade scientists have had modest success making or finding electronic topological insulators. But these substances are limited both in what they can do and in what they can reveal about the potential of this new state of matter.

"Those systems are very difficult to study systematically, because when you have a real material it is what it is. You're limited to studying its properties," said Shvets. "Nature doesn't give you the knobs to increase or decrease various aspects of it, so it's very difficult to benchmark the existing theories against what's observed."

Shvets and his physics department colleagues expect that their simulated photonic insulator will be a much more powerful and flexible tool for studying the general properties of topological insulators.

"With these purely artificial photonic crystals, we can study these systems in a more systematic way," he said.

In order for their insights from the photonic system to be applicable to electronic systems, Shvets and his colleagues had to make their simulated photons behave sufficiently like electrons. To do that, they designed simulated "metamaterials." These are artificial electromagnetic materials that can be tuned to influence photons in ways that are otherwise impossible. Other metamaterials are being used to develop invisibility cloaks.

Shvets and his colleagues designed what they've called SPINDOMs (spin-degenerate optically-active metamaterials). When arranged periodically, the resulting meta-crystals are the first demonstration that it's possible to control the spin of photons in a way that emulates what can be done with electrons.

This is significant on a few fronts. Even as a computer simulation it allows researchers to explore the properties of topological insulators. When these photonic topological insulators are physically built, as Shvets and his colleagues hope will be done soon, they'll allow more exploration. And there's great promise that such insulators may eventually be used to reduce interference in wireless communications systems.

"Right now if you put multiple emitting or receiving antennas in close proximity to each other, whether on a semiconductor chip or on top of a cellular base station, the radiation from each antenna is affected by the others," he said. "To deal with this you have to design around it. What would be better is if all cross talk between emitting/receiving sources could just be eliminated. That's what we believe could be done by photonic topological insulators, which can directionally guide electromagnetic waves."

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Journal Reference:

Alexander B. Khanikaev, S. Hossein Mousavi, Wang-Kong Tse, Mehdi Kargarian, Allan H. MacDonald, Gennady Shvets. Photonic topological insulators. Nature Materials, 2012; DOI: 10.1038/nmat3520

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May the force be with the atomic probe

Dec. 21, 2012 — A new models suggest devising means of probing a surface at a sub-micrometric level as this will help us understand how electrons' diffusion affects long-range attractive forces.

Theoretical physicist Elad Eizner from Ben Gurion University, Israel, and colleagues created models to study the attractive forces affecting atoms located at a wide range of distances from a surface, in the hundreds of nanometers range. Their results, about to be published in EPJ D, show that these forces depend on electron diffusion, regardless of whether the surface is conducting or not. Ultimately, these findings could contribute to designing minimally invasive surface probes.

Bombarding a surface with atoms helps us understand the distribution of its electrons and the structural arrangement of the surface atoms. The authors focused on understanding how a long-range force- referred to as the van der Waals-Casimir-Polder (vdW-CP) force - present between an atom and a surface allow us to distinguish surface characteristics on the basis of their conductivity.

A key factor in understanding the behaviour of the force, they realised, is the size of the electron cloud surrounding an impurity charge in the system. The latter depends both on the electrons' conductivity and their capability to diffuse in and along the surface.

They devised one model for the diffusion of the electronic charge in the bulk of the material and another one in the near-surface region. They tested their models on both conducting and non-conducting surfaces. They were thus able to explain why the atom-surface force shows a continuous transition in terms of conductivity between both types of surfaces.

For distances comparable to the size of the electron cloud spread, the strength of the vdW-CP attraction force, they found, can help distinguish between bulk and surface electrons diffusion. It could therefore be used as a probe. Potential applications exist, for example, in quantum computer hardware architectures focusing on the interface between different carriers of quantum bits of information.

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E.Eizner, B. Horovitz, and C. Henkel. Van der Waals-Casimir-Polder interaction of an atom with a composite surface. European Physical Journal D, 2012 DOI: 10.1140/epjd/e2012-30294-x

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Suspend the crystals, and they grow better

Dec. 21, 2012 — The idea is so simple you wonder why no one thought of it before. Crystals growing near the bottom of a beaker are subject to convection, but it is much quieter near the top of the beaker. In that case, why not just let them grow hanging in the beaker? Well, the idea was there for the taking, and that is exactly what Elias Vlieg and his team from Radboud University Nijmegen have done.

Their work will be published in this month's edition of Crystal Growth & Design.

Proteins are the building blocks of life and incredibly complicated. Because their structure determines the way that they work, many researchers are attempting to define the structure of protein molecules as accurately as possible. No easy task, as they first need crystals -- pure crystals. The best way to obtain such pure crystals is to let them grow slowly, with no vibrations or other disturbances. You cannot therefore just leave them in a beaker in a quiet corner of the room; the density differential means that the solution flows under the influence of gravity and the crystal grows too quickly.

Into space...

If gravity is the problem, then into space with them -- crystal growth experiments have been carried out in weightless conditions in space for 25 years already. Elias Vlieg, Professor of Solid State Chemistry at Radboud University Nijmegen, also once sent crystals into space to grow in an unmanned satellite, though with disappointing results -- a situation that has not much improved. 'Scientifically speaking, it is a dead end.'

... or suspended in a magnet

The university in Nijmegen has got a magnet sitting in its back garden that is so strong it can work against gravity: the HFML. In 2007, Vlieg successfully crystallised proteins in the magnetic field produced by the HFML. In doing so, all the problems involved with growing crystals under the effect of gravity were avoided: no lower density solution flowed above the crystal. And then came the Eureka moment: why not just do away with 'above'? Hang the crystals seeds high up in the solution and make sure that nothing else can flow above them -- no more need for spaceships or magnets!

Developed and investigated

The idea was thoroughly tested by the Egyptian PhD student Alaa Adawy, with remarkably good results. The hanging crystals grow more perfectly than the 'old' type and therefore diffract X-rays to much higher resolution limits (X-ray diffraction is the standard method for determining protein structures). Vlieg's idea has been well received and he hopes that other researchers will soon start growing crystals using this method. In his laboratory, all the hanging crystals have done better than the others. 'The crystallisation of proteins is high precision work; everything in the solution has to be exactly right -- the salt concentration, the temperature -- it in an industry in itself. Achieving such an improvement in results just by changing one factor is spectacular.'

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Alaa Adawy, Etienne Rebuffet, Susanna Tornroth-Horsefield, Willem J. DeGrip, Willem J. P. van Enckevort, Elias Vlieg. High Resolution Protein Crystals Using an Efficient Convection-Free Geometry. Crystal Growth & Design, 2012; : 121205124039002 DOI: 10.1021/cg301497t

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Engineers seek ways to convert methane into useful chemicals

Dec. 20, 2012 — Little more than a decade ago, the United States imported much of its natural gas. Today, the nation is tapping into its own natural gas reserves and producing enough to support most of its current needs for heating and power generation, and is beginning to export natural gas to other countries.

The trend is expected to continue, as new methods are developed to extract natural gas from vast unrecovered reserves embedded in shale. Natural gas can be used to generate electricity, and it burns cleaner than coal.

"With petroleum reserves in decline, natural gas production is destined to increase to help meet worldwide energy demands," said Matthew Neurock, a chemical engineering professor in the University of Virginia's School of Engineering and Applied Science. "But petroleum -- in addition to being used to make fuels -- is also used to make ethylene, propylene and other building blocks used in the production of a wide range of other chemicals. We need to develop innovative processes that can readily make these chemical intermediates from natural gas." The problem is, there currently are no cost-effective ways to do this. Methane, the principal component of natural gas, is rather inert and requires high temperatures to activate its strong chemical bonds; therefore the practical and successful conversion of methane to useful chemical intermediates has thus far eluded chemists and engineers.

Neurock is working with colleagues at Northwestern University to invent novel ways and catalytic materials to activate methane to produce ethylene. This week the collaborators published a paper in the online edition of the journal Nature Chemistry detailing the use of sulfur as a possible "soft" oxidant for catalytically converting methane into ethylene, a key "intermediate" for making chemicals, polymers, fuels and, ultimately, products such as films, surfactants, detergents, antifreeze, textiles and others.

"We show, through both theory -- using quantum mechanical calculations -- and laboratory experiments, that sulfur can be used together with novel sulfide catalysts to convert methane to ethylene, an important intermediate in the production of a wide range of materials," Neurock said.

Chemists and engineers have attempted to develop catalysts and catalytic processes that use oxygen to make ethylene, methanol and other intermediates, but have had little success as oxygen is too reactive and tends to over-oxidize methane to common carbon dioxide.

Neurock said that sulfur or other "softer" oxidants that have weaker affinities for hydrogen may be the answer, in that they can help to limit the over-reaction of methane to carbon disulfide. In the team's process, methane is reacted with sulfur over sulfide catalysts used in petroleum processes. Sulfur is used to remove hydrogen from the methane to form hydrocarbon fragments, which subsequently react together on the catalyst to form ethylene.

Theoretical and experimental results indicate that the conversion of methane and the selectivity to produce ethylene are controlled by how strong the sulfur bonds to the catalyst. Using these concepts, the team explored different metal sulfide catalysts to ultimately tune the metal-sulfur bond strength in order to control the conversion of methane to ethylene. Chemical companies consider methane a particularly attractive raw material because of the large reserves of natural gas in the U.S. and other parts of the world.

In 2007, Dow issued a "Methane Challenge," seeking revolutionary chemical processes to facilitate the conversion of methane to ethylene and other useful chemicals. The company received about 100 proposals from universities, institutes and companies around the world. In 2008, the company awarded major research grants to Cardiff University and Northwestern University to advance the quest. Neurock is a member of the Northwestern University team. He is using theoretical methods and high-performance computing to understand the processes that control catalysis and to guide the experimental research at Northwestern.

"The abundance of natural gas, along with the development of new methods to extract it from hidden reserves, offers unique opportunities for the development of catalytic processes that can convert methane to chemicals," Neurock said. "Our finding -- of using sulfur to catalyze the conversion of methane to ethylene -- shows initial promise for the development of new catalytic processes that can potentially take full advantage of these reserves. The research, however, is really just in its infancy."

Neurock's co-investigators on the Nature Chemistry paper are Qingjun Zhu, Staci Wegener, Chao Xie and Tobin Marks of Northwestern University, and U.Va. colleague Obioma Uche. 

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Qingjun Zhu, Staci L. Wegener, Chao Xie, Obioma Uche, Matthew Neurock, Tobin J. Marks. Sulfur as a selective ‘soft’ oxidant for catalytic methane conversion probed by experiment and theory. Nature Chemistry, 2012; DOI: 10.1038/nchem.1527

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Steering stem cells to become two different building blocks for new blood vessels

Dec. 20, 2012 — Growing new blood vessels in the lab is a tough challenge, but a Johns Hopkins engineering team has solved a major stumbling block: how to prod stem cells to become two different types of tissue that are needed to build tiny networks of veins and arteries.

The team's solution is detailed in an article appearing in the January 2013 print edition of the journal Cardiovascular Research. The article also was published recently in the journal's online edition. The work is important because networks of new blood vessels, assembled in the lab for transplanting into patients, could be a boon to people whose circulatory systems have been damaged by heart disease, diabetes and other illnesses.

"That's our long-term goal: to give doctors a new tool to treat patients who have problems in the pipelines that carry blood through their bodies," said Sharon Gerecht, an assistant professor of chemical and biomolecular engineering who led the research team. "Finding out how to steer these stem cells into becoming critical building blocks to make these blood vessel networks is an important step."

In the new research paper, the Gerecht team focused on vascular smooth muscle cells, which are found within the walls of blood vessels. Two types have been identified: synthetic smooth muscle cells, which migrate through the surrounding tissue, continue to divide and help support the newly formed blood vessels; and contractile smooth muscles cells, which remain in place, stabilize the growth of new blood vessels and help them maintain proper blood pressure.

To produce these smooth muscle cells, Gerecht's lab has been experimenting with both National Institutes of Health-approved human embryonic stem cells and induced pluripotent stem cells. The induced pluripotent stem cells are adult cells that have been genetically reprogrammed to act like embryonic stem cells. Stem cells are used in this research because they possess the potential to transform into specific types of cells needed by particular organs within the body.

In an earlier study supervised by Gerecht, her team was able to coax stem cells to become a type of tissue that resembled smooth muscle cells but didn't quite behave properly. In the new experiments, the researchers tried adding various concentrations of growth factor and serum to the previous cells. Growth factor is the "food' that the cells consume; serum is a liquid component that contains blood cells.

"When we added more of the growth factor and serum, the stem cells turned into synthetic smooth muscle cells," Gerecht said. "When we provided a much smaller amount of these materials, they became contractile smooth muscles cells."

This ability to control the type of smooth muscle cells formed in the lab could be critical in developing new blood vessel networks, she said. "When we're building a pipeline to carry blood, you need the contractile cells to provide structure and stability," she added. "But in working with very small blood vessels, the migrating synthetic cells can be more useful."

In cancer, small blood vessels are formed to nourish the growing tumor. The current work could also help researchers understand how blood vessels are stabilized in tumors, which could be useful in the treatment of cancer.

"We still have a lot more research to do before we can build complete new blood vessel networks in the lab," Gerecht said, "but our progress in controlling the fate of these stem cells appears to be a big step in the right direction."

In addition to her faculty appointment with Johns Hopkins' Whiting School of Engineering, Gerecht is affiliated with the university's Institute for NanoBioTechnolgy (INBT) and the Johns Hopkins Engineering in Oncology Center.

The lead author of the new Cardiovascular Research paper is Maureen Wanjare, a doctoral student in Gerecht's lab who is supported both by the INBT, through a National Science Foundation Integrative Graduate Education and Research Traineeship, and by the NIH. Coauthors of the study are Gerecht and Frederick Kuo, who participated in the research as an undergraduate majoring in chemical and biomolecular engineering. The human induced pluripotent stem cells used in the study were provided by Linzhao Cheng, a hematology professor in the Johns Hopkins School of Medicine.

This research was supported by an American Heart Association Scientist Development Grant and NIH grant R01HL107938.

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M. Wanjare, F. Kuo, S. Gerecht. Derivation and maturation of synthetic and contractile vascular smooth muscle cells from human pluripotent stem cells. Cardiovascular Research, 2012; DOI: 10.1093/cvr/cvs315

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Super-fine sound beam could one day be an invisible scalpel

Dec. 19, 2012 — A carbon-nanotube-coated lens that converts light to sound can focus high-pressure sound waves to finer points than ever before. The University of Michigan engineering researchers who developed the new therapeutic ultrasound approach say it could lead to an invisible knife for noninvasive surgery.

Today's ultrasound technology enables far more than glimpses into the womb. Doctors routinely use focused sound waves to blast apart kidney stones and prostate tumors, for example. The tools work primarily by focusing sound waves tightly enough to generate heat, says Jay Guo, a professor of electrical engineering and computer science, mechanical engineering, and macromolecular science and engineering. Guo is a co-author of a paper on the new technique published in the current issue of Nature's journal Scientific Reports.

The beams that today's technology produces can be unwieldy, says Hyoung Won Baac, a research fellow at Harvard Medical School who worked on this project as a doctoral student in Guo's lab.

"A major drawback of current strongly focused ultrasound technology is a bulky focal spot, which is on the order of several millimeters," Baac said. "A few centimeters is typical. Therefore, it can be difficult to treat tissue objects in a high-precision manner, for targeting delicate vasculature, thin tissue layer and cellular texture. We can enhance the focal accuracy 100-fold."

The team was able to concentrate high-amplitude sound waves to a speck just 75 by 400 micrometers (a micrometer is one-thousandth of a millimeter). Their beam can blast and cut with pressure, rather than heat. Guo speculates that it might be able to operate painlessly because its beam is so finely focused it could avoid nerve fibers. The device hasn't been tested in animals or humans yet, though.

"We believe this could be used as an invisible knife for noninvasive surgery," Guo said. "Nothing pokes into your body, just the ultrasound beam. And it is so tightly focused, you can disrupt individual cells."

To achieve this superfine beam, Guo's team took an optoacoustic approach that converts light from a pulsed laser to high-amplitude sound waves through a specially designed lens. The general technique has been around since Thomas Edison's time. It has advanced over the centuries, but for medical applications today, the process doesn't normally generate a sound signal strong enough to be useful.

The U-M researchers' system is unique because it performs three functions: it converts the light to sound, focuses it to a tiny spot and amplifies the sound waves. To achieve the amplification, the researchers coated their lens with a layer of carbon nanotubes and a layer of a rubbery material called polydimethylsiloxane. The carbon nanotube layer absorbs the light and generates heat from it. Then the rubbery layer, which expands when exposed to heat, drastically boosts the signal by the rapid thermal expansion.

The resulting sound waves are 10,000 times higher frequency than humans can hear. They work in tissues by creating shockwaves and microbubbles that exert pressure toward the target, which Guo envisions could be tiny cancerous tumors, artery-clogging plaques or single cells to deliver drugs. The technique might also have applications in cosmetic surgery.

In experiments, the researchers demonstrated micro ultrasonic surgery, accurately detaching a single ovarian cancer cell and blasting a hole less than 150 micrometers in an artificial kidney stone in less than a minute.

"This is just the beginning," Guo said. "This work opens a way to probe cells or tissues in much smaller scale."

The researchers will present the work at the SPIE Photonics West meeting in San Francisco. The research was funded by the National Science Foundation and the National Institutes of Health.

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Hyoung Won Baac, Jong G. Ok, Adam Maxwell, Kyu-Tae Lee, Yu-Chih Chen, A. John Hart, Zhen Xu, Euisik Yoon, L. Jay Guo. Carbon-Nanotube Optoacoustic Lens for Focused Ultrasound Generation and High-Precision Targeted Therapy. Scientific Reports, 2012; 2 DOI: 10.1038/srep00989

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Traffic congestion can be alleviated throughout a metropolitan area by altering trips in specific neighborhoods, model shows

Dec. 20, 2012 — In most cities, traffic growth has outpaced road capacity, leading to increased congestion, particularly during the morning and evening commutes. In 2007, congestion on U.S. roads was responsible for 4.2 billion hours of additional travel time, as well as 2.8 billion gallons of fuel consumption and an accompanying increase in air pollution.

One way to prevent traffic tie-ups is to have fewer cars on the road by encouraging alternatives such as public transportation, carpooling, flex time and working from home. But a new study -- by researchers at MIT, Central South University in China, the University of California at Berkeley and the Austrian Institute of Technology -- incorporates data from drivers' cellphones to show that the adoption of these alternatives by a small percentage of people across a metropolitan area might not be very effective. However, if the same number of people, but from a carefully selected segment of the driving population, chooses not to drive at rush hour, this could reduce congestion significantly.

The study, published in the Dec. 20 issue of the journal Scientific Reports, demonstrates that canceling or delaying the trips of 1 percent of all drivers across a road network would reduce delays caused by congestion by only about 3 percent. But canceling the trips of 1 percent of drivers from carefully selected neighborhoods would reduce the extra travel time for all other drivers in a metropolitan area by as much as 18 percent.

"This has an analogy in many other flows in networks," says lead research Marta González, the Gilbert W. Winslow Career Development Assistant Professor in MIT's Department of Civil and Environmental Engineering. "Being able to detect and then release the congestion in the most affected arteries improves the functioning of the entire coronary system."

The study, designed by González and former MIT postdoc Pu Wang, now a professor at Central South University, is the first large-scale traffic study to track travel using anonymous cellphone data rather than survey data or information obtained from U.S. Census Bureau travel diaries. Both of these are prone to error because of the time lag between gathering and releasing data and the reliance on self-reporting.

González and Wang used three weeks of cellphone data to obtain information about anonymous drivers' routes and the estimated traffic volume and speed on those routes. They inferred a driver's home neighborhood from the regularity of the route traveled and from the locations of cell towers that handled calls made between 9 p.m. and 6 a.m. They combined this with information about population densities and the location and capacity of roads in the networks of two metropolitan areas -- Boston and San Francisco -- to determine which neighborhoods are the largest sources of drivers on each road segment, and which roads these drivers use to connect from home to highways and other major roadways.

In the Boston area, they found that canceling 1 percent of trips by select drivers in the Massachusetts municipalities of Everett, Marlborough, Lawrence, Lowell and Waltham would cut all drivers' additional commuting time caused by traffic congestion by 18 percent. In the San Francisco area, canceling trips by drivers from Dublin, Hayward, San Jose, San Rafael and parts of San Ramon would cut 14 percent from the travel time of other drivers.

"These percentages are averages based on a one-hour commute with additional minutes caused by congestion," Wang says. "The drivers stuck in the roads with worst congestion would see the greatest percentage of time savings, because the selective strategy can more efficiently decrease the traffic flows in congested roads."

To validate the study's methodology, Alexandre Bayen, an associate professor of systems engineering at Berkeley, and graduate student Timothy Hunter compared González and Wang's estimations of travel time based on cellphone data with their own data obtained from GPS sensors in taxis in the San Francisco area. Using GPS data, Bayen and Hunter computed taxis' speed based on travel time from one location to another; from that speed of travel, they then determined congestion levels. Their findings agreed with those of González and Wang.

Because the new methodology requires only three types of data -- population density, topological information about a road network, and cellphone data -- it can be used for almost any urban area.

"In many cities in the developing world, traffic congestion is a major problem and travel surveys don't exist," González says. "So the detailed methodology we developed for using cellphone data to accurately characterize road network use could help traffic managers control congestion and allow planners to create road networks that fit a population's needs."

González and Wang are currently studying road use in the Dominican Republic, France, Portugal, Rwanda and Spain. They treat the anonymous cellphone data with the privacy-protection measures required for the treatment of human subjects under an institutional review board.

Katja Schechtner, head of the Dynamic Transportation Systems group at the Austrian Institute of Technology and a visiting scholar at the MIT Media Lab, is a co-author on the Scientific Reports paper with González, Wang, Bayen and Hunter.

"We are now at a time where it is less difficult to get mobility data, thanks to mobile phones and other devices, and the main problem we have is how to extract useful information from all these data," says Marc Barthelemy, a senior researcher at the Institute of Theoretical Physics at CEA in France. "[González] and her team proposed a very interesting and new idea of constructing the network of road usage, which allows us to understand where individuals on a given road are coming from, and enables us to propose new strategies for mitigating congestion. This approach will certainly open new avenues of research in the very active field of mobility in urban systems."

The study was funded by grants from the New England University Transportation Center, the NEC Corporation Fund, the Solomon Buchsbaum Research Fund and the National Natural Science Foundation of China. Wang received funding from the Shenghua Scholar Program of Central South University.

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Pu Wang, Timothy Hunter, Alexandre M. Bayen, Katja Schechtner, Marta C. González. Understanding Road Usage Patterns in Urban Areas. Scientific Reports, 2012; 2 DOI: 10.1038/srep01001

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Meteorite triggered scientific 'Gold Rush'

Dec. 20, 2012 — A meteorite that exploded as a fireball over California's Sierra foothills this past spring was among the fastest, rarest meteorites known to have hit Earth, and it traveled a highly eccentric orbital route to get here.

An international team of scientists presents these and other findings in a study published Dec. 21, in the journal Science. The 70-member team included nine researchers from UC Davis, along with scientists from the SETI Institute, NASA and other institutions.

The researchers found that the meteorite that fell over Northern California on April 22 was the rarest type known to have hit Earth -- a carbonaceous chondrite. It is composed of cosmic dust and presolar materials that helped form the planets of the solar system.

The scientists learned that the meteorite formed about 4.5 billion years ago. It was knocked off its parent body, which may have been an asteroid or a Jupiter-family comet, roughly 50,000 years ago. That began its journey to Sutter's Mill, the gold discovery site that sparked the California Gold Rush.

As it flew toward Earth, it traveled an eccentric course through the solar system, flying from an orbit close to Jupiter toward the sun, passing by Mercury and Venus, and then flying out to hit Earth.

The high-speed, minivan-sized meteorite entered the atmosphere at about 64,000 miles per hour. The study said it was the fastest, "most energetic" reported meteorite that's fallen since 2008, when an asteroid fell over Sudan.

"If this were a much bigger object, it could have been a disaster," said co-author and UC Davis geology professor Qing-zhu Yin. "This is a happy story in this case. "

Before entering Earth's atmosphere, the meteorite is estimated to have weighed roughly 100,000 pounds. Most of that mass burned away when the meteorite exploded. Scientists and private collectors have recovered about 2 pounds remaining.

UC Davis is 60 miles west of the El Dorado county towns of Coloma and Lotus, where pieces of the meteorite were found on residents' driveways and in local forests and parks.

When the meteorite fell, Yin, whose lab contains some of the country's most specialized equipment to measure the age and composition of meteorites, searched for and collected pieces of the fallen meteorite with students and volunteers. He also led a 35-member subgroup of international researchers to study and share information about the meteorite's mineralogy, internal textures, chemical and isotopic compositions and magnetic properties.

Meteorites like Sutter's Mill are thought to have delivered oceans of water to Earth early in its history. Using neutron-computed tomography, UC Davis researchers helped identify where hydrogen, and therefore water-rich fragments, resides in the meteorite without breaking it open.

For the first time, the Doppler weather radar network helped track the falling carbonaceous chondrite meteorite pieces, aiding scientists in the quick recovery of them, the study reports. Yin expects that the weather radar data in the public domain could greatly enhance and benefit future meteorite recoveries on land.

"For me, the fun of this scientific gold rush is really just beginning," said Yin. "This first report based on the initial findings provides a platform to propel us into more detailed research. Scientists are still finding new and exciting things in Murchison, a similar type of meteorite to Sutter's Mill, which fell in Victoria, Australia, in 1969, the same year Apollo astronauts Neil Armstrong and Buzz Aldrin returned the first lunar samples to the Earth. We will learn a lot more with Sutter's Mill."

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New kind of magnetism discovered: Experiments demonstrate ‘quantum spin liquid'

Dec. 20, 2012 — Following up on earlier theoretical predictions, MIT researchers have now demonstrated experimentally the existence of a fundamentally new kind of magnetic behavior, adding to the two previously known states of magnetism.

Ferromagnetism -- the simple magnetism of a bar magnet or compass needle -- has been known for centuries. In a second type of magnetism, antiferromagnetism, the magnetic fields of the ions within a metal or alloy cancel each other out. In both cases, the materials become magnetic only when cooled below a certain critical temperature. The prediction and discovery of antiferromagnetism -- the basis for the read heads in today's computer hard disks -- won Nobel Prizes in physics for Louis Neel in 1970 and for MIT professor emeritus Clifford Shull in 1994.

"We're showing that there is a third fundamental state for magnetism," says MIT professor of physics Young Lee. The experimental work showing the existence of this new state, called a quantum spin liquid (QSL), is reported this week in the journal Nature, with Lee as the senior author and Tianheng Han, who earned his PhD in physics at MIT earlier this year, as lead author.

The QSL is a solid crystal, but its magnetic state is described as liquid: Unlike the other two kinds of magnetism, the magnetic orientations of the individual particles within it fluctuate constantly, resembling the constant motion of molecules within a true liquid.

Finding the evidence

There is no static order to the magnetic orientations, known as magnetic moments, within the material, Lee explains. "But there is a strong interaction between them, and due to quantum effects, they don't lock in place," he says.

Although it is extremely difficult to measure, or prove the existence, of this exotic state, Lee says, "this is one of the strongest experimental data sets out there that [does] this. What used to just be in theorists' models is a real physical system."

Philip Anderson, a leading theorist, first proposed the concept in 1987, saying that this state could be relevant to high-temperature superconductors, Lee says. "Ever since then, physicists have wanted to make such a state," he adds. "It's only in the past few years that we've made progress."

The material itself is a crystal of a mineral called herbertsmithite. Lee and his colleagues first succeeded in making a large, pure crystal of this material last year -- a process that took 10 months -- and have since been studying its properties in detail.

"This was a multidisciplinary collaboration, with physicists and chemists," Lee explains. "You need both … to synthesize the material and study it with advanced physics techniques. Theorists were also crucial to this."

Through its experiments, the team made a significant discovery, Lee says: They found a state with fractionalized excitations, which had been predicted by some theorists but was a highly controversial idea. While most matter has discrete quantum states whose changes are expressed as whole numbers, this QSL material exhibits fractional quantum states. In fact, the researchers found that these excited states, called spinons, form a continuum. This observation, they say in their Nature paper, is "a remarkable first."

Scattering neutrons

To measure this state, the team used a technique called neutron scattering, which is Lee's specialty. To actually carry out the measurements, they used a neutron spectrometer at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md.

The results, Lee says, are "really strong evidence of this fractionalization" of the spin states. "That's a fundamental theoretical prediction for spin liquids that we are seeing in a clear and detailed way for the first time."

It may take a long time to translate this "very fundamental research" into practical applications, Lee says. The work could possibly lead to advances in data storage or communications, he says -- perhaps using an exotic quantum phenomenon called long-range entanglement, in which two widely separated particles can instantaneously influence each other's states. The findings could also bear on research into high-temperature superconductors, and could ultimately lead to new developments in that field, he says.

"We have to get a more comprehensive understanding of the big picture," Lee says. "There is no theory that describes everything that we're seeing."

Subir Sachdev, a professor of physics at Harvard University who was not connected with this work, says that these findings, which have been anticipated for decades, "are very significant and open a new chapter in the study of quantum entanglement in many-body systems." The detection of such states, he says, was an "exceptionally difficult task. Young Lee and his group brilliantly overcame these challenges in their beautiful experiment."

In addition to Lee and Han, the work was carried out by J.S. Helton of NIST, research scientist Shaoyan Chu of MIT's Center for Materials Science and Engineering, MIT chemistry professor Daniel Nocera, Jose Rodriguez-Rivera of NIST and the University of Maryland, and Colin Broholm of Johns Hopkins University. The work was supported by the U.S. Department of Energy and the National Science Foundation.

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Tian-Heng Han, Joel S. Helton, Shaoyan Chu, Daniel G. Nocera, Jose A. Rodriguez-Rivera, Collin Broholm, Young S. Lee. Fractionalized excitations in the spin-liquid state of a kagome-lattice antiferromagnet. Nature, 2012; 492 (7429): 406 DOI: 10.1038/nature11659

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Saturn mission: Cassini instrument learns new tricks

Dec. 20, 2012 — For seven years, a mini-fridge-sized instrument aboard NASA's Cassini spacecraft reliably investigated weather patterns swirling around Saturn; the hydrocarbon composition of the surface of Saturn's moon Titan; the aerosol layers of Titan's haze; and dirt mixing with ice in Saturn's rings. But this year the instrument -- the visual and infrared mapping spectrometer (VIMS) -- has been testing out some new telescopic muscles.

This Friday, Dec. 21, the spectrometer will be tracking the path of Venus across the face of the sun from its perch in the Saturn system. Earthlings saw such a transit earlier this year, from June 5 to 6. But the observation in December will be the first time a spacecraft has tracked a transit of a planet in our solar system from beyond Earth orbit.

Cassini will collect data on the molecules in Venus's atmosphere as sunlight shines through it. But learning about Venus actually isn't the point of the observation. Scientists actually want to use the occasion to test the VIMS instrument's capacity for observing planets outside our solar system.

"Interest in infrared investigations of extrasolar planets has exploded in the years since Cassini launched, so we had no idea at the time that we'd ask VIMS to learn this new kind of trick," said Phil Nicholson, the VIMS team member based at Cornell University, Ithaca, N.Y., who is overseeing the transit observations. "But VIMS has worked so well at Saturn so far that we can start thinking about other things it can do."

VIMS will be able to complement exoplanet studies by space telescopes such as NASA's Hubble and Spitzer space telescopes. VIMS scientists are particularly interested in investigating atmospheric data -- such as signatures of methane -- from far-off star systems in near-infrared wavelengths.

The pointing has to be very accurate to get one of those extrasolar planets in VIMS's viewfinder, but the instrument has had lots of practice pointing at other stars. Earlier this year, VIMS obtained its first successful observation of a transit by the exoplanet HD 189733b. Scientists want to improve these observations by reducing the amount of noise in the signal.

In April, VIMS demonstrated another kind of flexibility by turning its eyes to the warm fissures slashing cross the surface of Saturn's moon Enceladus. VIMS is particularly good at taking thermal data in temperatures around minus 100 degrees Fahrenheit (200 kelvins). So while it is good at tracking hotspots and turbulent clouds on Saturn, VIMS is generally unable to detect thermal emission from Titan, the icy satellites or the rings, since their temperatures are much colder than that.

But the fissures on Enceladus, which scientists have called tiger stripes, are just hot enough for VIMS to detect heat coming from them.

"For the first time, we were able to see that the jets coming from the surface of Enceladus originated in very small, very hot spots," said Bonnie Buratti, a VIMS scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "This new observation is good evidence for liquid water underneath the surface."

VIMS is one of 12 instruments on Cassini, which launched in 1997 and began orbiting Saturn in 2004. "We built Cassini to be hardy, and we're pleased that the spacecraft has been weathering the extreme conditions of the Saturn system remarkably well," said Robert Mitchell, Cassini program manager at JPL. "It isn't too tired to try something new."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The visual and infrared mapping spectrometer team is based at the University of Arizona, Tucson.

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Clean air: New paints break down nitrogen oxides

Dec. 20, 2012 — Surfaces with photo-catalytic characteristics clean the air off nitrogen oxides and other health-endangering substances. Using a new test procedure, Fraunhofer researchers can find out how the coatings behave during a long-term test.

The Seventies: Smog alert in the Ruhr area, acid rain, dying spruce trees in the Bavarian Forest. In those days, the solution was filter systems for the smokestacks in the Ruhr area. Today, people in the urban areas are suffering from high levels of pollution that is being caused by, among other things, automotive traffic. Particularly undesired: the nitrogen oxides (NOX). In the meantime, the European Union tightened the limit values even further; in many communities they are being exceeded. Michael Hüben of the Fraunhofer Institute for Molecular Biology and Applied Ecology IME in Schmallenberg, Germany, knows that "on stretches with heavy traffic there is a particular need for action."

During the next two years, the Fraunhofer researchers want to examine in the project "Effectiveness of photo-catalytic removal of nitrogen oxide on coated building test panels" how photo-catalytic surfaces contribute to the removal of NOx and how the coatings prove themselves during long-term operation. On behalf of the German Federal Ministry of Transport, the German Federal Highway Research Institute is sponsoring and supporting the project. The process will be introduced at the 2013 BAU trade fair at the joint booth of the Fraunhofer Building Innovation Alliance.

"Coatings that are photo-catalytically active can help to reduce nitrogen oxides," explains Dr. Michael Hüben, "There are already a number of products available for the photo-catalytic coating of surfaces, but the measurement method standardized according to ISO 22197-1 cannot be applied to all problems. At the IME, we have now developed a special measurement cell which we are using in our project." At the A 4 interstate at Bergisch Gladbach, we will shortly be setting out weathering noise barrier samples that were coated with reactive material. Prepared test samples will be measured at predetermined intervals in the measuring cell. Hüben explains the set-up of the test: "The surface of the test sample must be photo-catalytically active, meaning it removes NOx when exposed to light."

The surfaces contain titanium dioxide catalysts, a material that is affordable and available in large quantities. Then, exposed to daylight, titanium dioxide catalyzes the nitrogen oxide into nitrate. "The photo-catalytic activities of the samples are determined using a flow-through process," says the scientist. During the next two years, the experts will determine regularly how much nitrous oxide is being removed. In this manner, they will obtain a solid basis for the long term effects of the coatings. Only then will we be sure that the coatings really do help and that larger surfaces, such as entire housing tracts, can be economically furnished with coats that are photo-catalytically effective. This would make it possible to reduce the particle pollution in urban areas.

"Another area of application for the measurement process are interior rooms. Here, too, there are products commercially available that promise to improve air quality in interior rooms," explains Mr. Hüben's colleague, Frank Neumann from the Fraunhofer Institute for Surface Engineering and Thin Films IST.

Researchers will introduce the test at the booth of the Fraunhofer Building Innovation Alliance at the Trade Fair BAU that will take place from 14 -- 19 January, 2013, in Munich, Germany.

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Paths of photons are random -- but coordinated

Dec. 20, 2012 — Researchers at the Niels Bohr Institute have demonstrated that photons (light particles) emitted from light sources embedded in a complex and disordered structure are able to mutually coordinate their paths through the medium. This is a consequence of the photons' wave properties, which give rise to the interaction between different possible routes.

The results are published in the scientific journal Physical Review Letters.

The real world is complex and messy. The research field of photonics, which explores and exploits light, is no exception, and in, for example, biological systems the statistical disorder is unavoidable.

Drunken people and photons

"We work with nanophotonic structures in order to control the emission and propagation of photons. We have discovered in the meantime, that inevitable inaccuracies in the structures lead to random scattering. As a consequence, the transport of photons follow a random path -- like a drunken man staggering through the city's labyrinthine streets after an evening in the pub," explains David García, postdoc in Quantum Photonics at the Niels Bohr Institute at the University of Copenhagen.

If we continue with this analogy, then it is not certain that just because one drunken man comes home safely, then a whole crowd of drunken people spreading out from the pub will also find their way through the city's winding streets. There is no relationship between the different random travellers.

But there is when you are talking about photons. They can 'sense' each other and coordinate their travel through a material, according to new research.

"We have inserted a very small light source in a nanophotonic structure, which contains disorder in the form of a random collection of light diffusing holes. The light source is a so-called quantum dot, which is a specially designed nanoscopic light source that can emit photons. The photons are scattered in all directions and are thrown back and forth. But photons are not just light particles, they are also waves, and waves interact with each other. This creates a link between the photons and we can now demonstrate in our experiments that the photons' path through the material is not independent from the other photons," explains David García.

Spectroscopy of complex materials

By analysing the path of the photons through the medium valuable insight is potentially gained about microscopic complex structures.

"The method could be a new way to measure the spatial properties of complex disordered materials, like biological tissue, and since the light sources are very small, you will be able to place them without destroying the material and you have the potential for very high spatial resolution," explains David García.

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The above story is reprinted from materials provided by University of Copenhagen, via EurekAlert!, a service of AAAS.

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Journal Reference:

Pedro García, Søren Stobbe, Immo Söllner, Peter Lodahl. Nonuniversal Intensity Correlations in a Two-Dimensional Anderson-Localizing Random Medium. Physical Review Letters, 2012; 109 (25) DOI: 10.1103/PhysRevLett.109.253902

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Clays on Mars: More plentiful than expected

Dec. 20, 2012 — A new study co-authored by the Georgia Institute of Technology indicates that clay minerals, rocks that usually form when water is present for long periods of time, cover a larger portion of Mars than previously thought. In fact, Assistant Professor James Wray and the research team say clays were in some of the rocks studied by Opportunity when it landed at Eagle crater in 2004. The rover only detected acidic sulfates and has since driven about 22 miles to Endeavour Crater, an area of the planet Wray pinpointed for clays in 2009.

The study is published online in the current edition of Geophysical Research Letters.

The project, which was led by Eldar Noe Dobrea of the Planetary Science Institute, identified the clay minerals using a spectroscopic analysis from the Mars Reconnaissance Orbiter. The research shows that clays also exist in the Meridiani plains that Opportunity rolled over as it trekked toward its current position.

"It's not a surprise that Opportunity didn't find clays while exploring," said Wray, a faculty member in the School of Earth and Atmospheric Sciences. "We didn't know they existed on Mars until after the rover arrived. Opportunity doesn't have the same tools that have proven so effective for detecting clays from orbit."

The clay signatures near Eagle crater are very weak, especially compared to those along the rim and inside Endeavour crater. Wray believes clays could have been more plentiful in the past, but Mars' volcanic, acidic history has probably eliminated some of them.

"It was also surprising to find clays in geologically younger terrain than the sulfates," said Dobrea. Current theories of Martian geological history suggest that clays, a product of aqueous alteration, actually formed early on when the planet's waters were more alkaline. As the water acidified due to volcanism, the dominant alteration mineralogy became sulfates. "This forces us to rethink our current hypotheses of the history of water on Mars," he added.

Even though Opportunity has reached an area believed to contain rich clay deposits, the odds are still stacked against it. Opportunity was supposed to survive for only three months. It's still going strong nine years later, but the rover's two mineralogical instruments don't work anymore. Instead, Opportunity must take pictures of rocks with its panoramic camera and analyze targets with a spectrometer to try and determine the composition of rock layers.

"So far, we've only been able to identify areas of clay deposits from orbit," said Wray. "If Opportunity can find a sample and give us a closer look, we should be able to determine how the rock was formed, such as in a deep lake, shallow pond or volcanic system."

As for the other rover on the other side of Mars, Curiosity's instruments are better equipped to search for signs of past or current conditions for habitable life, thanks in part to Opportunity. Wray is a member of Curiosity's science team.

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Journal Reference:

E. Z. Noe Dobrea, J. J. Wray, F. J. Calef, T. J. Parker, S. L. Murchie. Hydrated minerals on Endeavour Crater's rim and interior, and surrounding plains: New insights from CRISM data. Geophysical Research Letters, 2012; 39 (23) DOI: 10.1029/2012GL053180

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