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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