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  • richardmitnick 1:55 pm on May 18, 2017 Permalink | Reply
    Tags: , , Princeton   

    From Princeton: “Bright future: Unlocking the potential of light” 

    Princeton University
    Princeton University

    Jan. 9, 2017 [Nothing like being prompt.]
    Bennett McIntosh for the Office of the Dean for Research

    One hundred years ago, Italian chemist Giacomo Ciamician predicted a future society that would run on sunlight.

    1
    Late at night in Princeton’s Frick Chemistry Laboratory, blue LED lights work overtime to catalyze new chemical reactions. The method, developed largely at Princeton, involves using light and a catalyst to join molecules that normally would not react with each other. The technique is already being used in industry to create medicines, solvents, plastics and other products. Shown above is Megan Shaw, a postdoctoral research associate in MacMillan’s group. Photo by Sameer A. Khan/Fotobuddy

    In a paper presented in 1912 to an international meeting of chemists in New York City, he foresaw a future of vibrant desert communities under “a forest of glass tubes and greenhouses of all sizes” where light-driven chemical reactions would produce not just energy but also wondrous medicines and materials.

    Ciamician’s vision has not yet arrived, but a handful of Princeton researchers have succeeded with one part of his legacy: they are harnessing light to perform previously impossible feats of chemistry. In Princeton’s Frick Chemistry Laboratory, blue LED lamps cast light on flask after flask of gently stirring chemicals that are reacting in ways they never have before to create tomorrow’s medicines, solvents, dyes and other industrial chemicals.

    The leader in this emerging field is David MacMillan, who arrived in Princeton’s chemistry department in 2006. He was intrigued by the potential for using light to coax new chemical reactions. Like most chemists, he’d spent years learning the rules that govern the interactions of elements such as carbon, oxygen and hydrogen, and then using those rules to fashion new molecules. Could light help change these rules and catalyze reactions that have resisted previous attempts at manipulation?

    Changing the rules

    The idea for using light as a catalyst had been explored since Ciamician’s time with limited success. Light can excite a molecule to kick loose one or more of its electrons, creating free radicals that are extremely reactive and readily form new bonds with one another. However, most chemists did not think this process could be controlled precisely enough to make a wide variety of precision molecules.

    But that changed in the summer of 2007.

    MacMillan and postdoctoral researcher David Nicewicz were working on a tough problem. The two scientists wanted to create chemical bonds between one group of atoms, called bromocarbonyls, and another group, known as aldehydes. “It was one of those longstanding challenges in the field,” MacMillan said. “It was one of those reactions that was really useful for making new medicines, but nobody knew how to do it.”

    Nicewicz had found a recipe that worked, but it involved using ultraviolet (UV) light. This high-energy form of light causes sunburn by damaging the molecules in the skin, and it also damaged the molecules in the reaction mixture, making the recipe Nicewicz had discovered less useful. MacMillan, who is Princeton’s James S. McDonnell Distinguished University Professor of Chemistry, asked Nicewicz to investigate how to do the transformation without UV light.

    Nicewicz recalled some experiments that he’d seen as a graduate student at the University of North Carolina-Chapel Hill. Researchers led by chemistry professor Malcolm Forbes had split water into oxygen and hydrogen fuel using visible light and a special molecule, a catalyst containing a metal called ruthenium. The approach was known as “photoredox catalysis” because particles of light, or photons, propel the exchange of electrons in a process called oxidation-reduction, or “redox” for short.

    Visible light is lower in energy than ultraviolet light, so Nicewicz and MacMillan reasoned that the approach might work without damaging the molecules. Indeed, when the researchers added a ruthenium catalyst to the reaction mixture and placed the flask under an ordinary household fluorescent lightbulb, the two scientists were astounded to see the reaction work almost perfectly the first time. “More times than not, the reaction you draw on the board never works,” Nicewicz said. Instead, the reaction produced astonishing amounts of linked molecules with high purity. “I knew right away it was a fantastic result,” he said.

    With support from the National Institutes of Health, MacMillan and Nicewicz spent the next year showing that the reaction was useful for many different types of bromocarbonyls and aldehydes, results that the team published in Science in October 2008. Research in the lab quickly expanded beyond this single reaction, and each new reaction hinted at a powerful shift in the rules of organic chemistry. “It just took off like gangbusters,” MacMillan said. “As time goes on you start to realize that there are nine or 10 different things that it can do that you didn’t think of.”

    Old catalysts, new tricks

    At the time that Nicewicz and MacMillan were making their discovery, chemistry professor Tehshik Yoon and his team at the University of Wisconsin-Madison found that combining the ruthenium catalyst with light produced a different chemical reaction. They published their work in 2008 in the Journal of the American Chemical Society the same day MacMillan’s paper appeared in Science. Within a year of MacMillan publishing his paper, Corey Stephenson, a University of Michigan chemistry professor, and his team found yet another photoredox-based reaction.

    With these demonstrations of the versatility of photoredox catalysts, other chemists quickly joined the search for new reactions. About 20 photoredox catalysts were already available for purchase from chemical catalogs due to previous research on water-splitting and energy storage, so researchers could skip the months-long process of building catalysts. However, by designing and tailoring new catalysts, the chemists unlocked the potential to use light to drive numerous new reactions, and today there are more than 400 photoredox catalysts available.

    The secret to these catalysts’ ability to drive specific reactions lies in their design. The catalysts consist of a central atom, often a metal atom such as ruthenium or iridium, surrounded by a halo of other atoms. Light frees an electron from the central atom, and the atoms surrounding the center act as a sort of channel that ushers the freed electrons toward the specific atoms that the chemists want to join.

    One scientist who became intrigued with the power of photoredox catalysts was Abigail Doyle, a Princeton associate professor of chemistry. Doyle, whose work is funded by the National Institutes of Health, uses nickel to help join two molecules. In 2014, she was searching for a way to conduct a reaction that had long eluded other scientists. She wanted to find a catalyst that could make perhaps the most common bond in organic chemistry — between carbon and hydrogen — reactive enough to couple to another molecule. Perhaps a photocatalyst could make a reactive free radical, allowing her to then bring in a nickel catalyst to attach the carbon-carbon bond.

    Unbeknownst to Doyle, the MacMillan lab had recently turned their attention to combining photoredox and nickel catalysts on a similar reaction, coupling molecules at the site of a carboxylate group, a common arrangement of atoms found in biological molecules from vinegar to proteins.

    Given the similarities in their findings, the MacMillan and Doyle labs decided to combine their respective expertise in nickel and photoredox chemistry. Together, the teams found a photocatalyst based on the metal iridium that worked with nickel to carry out both coupling reactions — at the carbon-hydrogen bond and at the carboxylate group. Their collaborative paper, published in Science July 25, 2014, showed the extent of photoredox catalysis’ power to couple molecules with these common features.

    The ability to combine molecules using natural features such as the carbon-hydrogen bond or the carboxylate group makes photoredox chemistry extremely useful. Often, chemists have to significantly modify a natural molecule to make it reactive enough to easily link to another molecule. One popular reaction — which earned a Nobel Prize in 2010 — requires several steps before two molecules can be linked. Skipping all these steps means a far easier and cheaper reaction — and one that is rapidly being applied.

    “It’s one of the fastest-adopted chemistries I’ve seen,” Doyle said. “A couple of months after we published, we were visiting pharmaceutical companies and many of them were using this chemistry.”

    The search for new drugs often involves testing vast libraries of molecules for ones that interact with a biological target, like trying thousands of keys to see which ones open a door. Pharmaceutical companies leapt at the chance to quickly and cheaply make many more kinds of molecules for their libraries.

    Merck & Co., Inc., a pharmaceutical company with research labs in the Princeton area, was one of the first companies to become interested in using the new approach — and in funding MacMillan’s research. The company donated $5 million to start Princeton’s Merck Center for Catalysis in 2006, and recently announced another $5 million in continued research funding.

    In addition to aiding drug discovery, photoredox-catalyzed reactions can produce new or less-expensive fine chemicals for flavorings, perfumes and pesticides, as well as plastic-like polymer materials. And the techniques keep getting cheaper. MacMillan published a paper June 23, 2016, in Science, showing that with the aid of a photoredox catalyst, a widely used reaction to make carbon-nitrogen bonds can be carried out with nickel instead of palladium. Because nickel is thousands of times cheaper than palladium, companies hoping to use the reaction were contacting MacMillan before the paper was even published.

    Spreading the light

    Doyle has continued to explore photoredox chemistry, as have other Princeton faculty members, including two new assistant professors, Robert Knowles and Todd Hyster.

    Hyster combines photoredox catalysis with reactions inspired by biology. Drugs often function by fitting in a protein like a hand fits in a glove. But just as placing a left hand in a right glove results in a poor fit, inserting a left-handed molecule into a protein designed for a right-handed molecule will give poor results. Many catalysts produce both the intended product and its mirror image, but by combining photoredox catalysts with artificial proteins, Hyster is finding reactions that can make that distinction.

    Hyster, who arrived at Princeton in summer 2015, was drawn to Princeton’s chemistry department in part because of the opportunities to share knowledge and experience with other groups researching photoredox catalysis. “The department is quite collegial, so there’s no barrier when talking to colleagues about projects that are broadly similar,” he said.

    Students from different labs chat about their work over lunch, teaching and learning informally — and formally, as the labs encourage collaboration and sharing expertise, said Emily Corcoran, a postdoctoral researcher who works with MacMillan. When Corcoran was trying to determine exactly how one of her reactions worked, she was able to consult with students in Knowles’ lab who had experience using sensitive magnetic measurements to find free radicals in the reaction mixture.

    “If you have a question, you can just walk down the hall and ask,” Corcoran said. “That really pushes all the labs forward at a faster pace.”

    A bright future

    After the graduate students go home at night, the blue LEDs continue to drive new chemical reactions and new discoveries. “This is really just the beginning,” Doyle said.

    Hyster thinks that within a few years, manufacturers may take advantage of photoredox chemistry to produce biological chemicals — such as insulin and the malaria drug artemisinin — to meet human needs. For his part, MacMillan envisions zero-waste chemical plants in the Nevada desert, driven not by fossil fuels but by the sun.

    MacMillan’s vision echoes that of the original photochemist, Ciamician. The Italian’s optimistic vision of a sunlit future is brighter than ever.

    See the full article here .

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    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 1:36 pm on May 18, 2017 Permalink | Reply
    Tags: Computational modeling to the understanding of psychiatric diseases, Princeton, , Rutgers-Princeton Center for Computational Cognitive Neuropsychiatry   

    From Rutgers and Princeton: “New Rutgers-Princeton center uses computational models to understand psychiatric conditions” 

    Rutgers University
    Rutgers University

    Princeton University
    Princeton University

    Feb. 8, 2017 [Just found this, could not pass it up.]
    Catherine Zandonella, Office of the Dean for Research, Princeton

    A new center is bringing together researchers from Princeton and Rutgers universities to apply computational modeling to the understanding of psychiatric diseases. The Rutgers-Princeton Center for Computational Cognitive Neuropsychiatry, which will open its doors this month, aims to improve the diagnosis of mental disorders, better predict their progression and eventually aid in developing treatments.

    The center fosters collaboration between computational neuroscientists, who develop models of brain activity and cognitive processes, and clinical researchers who work directly with patients. The studies conducted at the center will address disorders ranging from depression, anxiety and schizophrenia to obsessive-compulsive disorder and substance abuse.

    The center, located at Rutgers University Behavioral Health Care in Piscataway, features a 1,400-square foot facility with rooms for conducting patient intake and testing. It is supported by matching funds from Princeton and Rutgers.

    “We can learn a lot about how the brain controls behaviors when we create computational models of how life events affect brain circuits, and how these circuits change over time. This center will allow us to bring this knowledge into the patient setting,” said Yael Niv, who co-directs the new center and is an associate professor of psychology and the Princeton Neuroscience Institute.

    “We still lack an understanding of the biological basis of many of the symptoms of psychiatric disorders, and computational approaches can help us start to close that gap,” said Steven Silverstein, co-director of the center with Niv. Silverstein is the director of the Division of Schizophrenia Research at Rutgers University Behavioral Health Care, and a professor of psychiatry at the Rutgers-Robert Wood Johnson Medical School.

    Over the last decade or so, neuroscientists have turned to computational modeling to help them understand how brain activity gives rise to behaviors. For example, researchers can build a computer model that represents how two areas of the brain communicate to generate behavior, and then block that communication to see what happens to the behavior.

    The researchers can then test the model’s predictions by comparing them to the real-life behavior of human volunteers. At present, most of these tests are done with community members or students. The new center will enable researchers to test models of disorders like depression and bipolar disorder in individuals living with those conditions.

    One goal of the research is to better understand the brain’s circuitry and what goes awry in mental disorders — how brain regions are connected, what is the role of each brain area, and how disruptions in brain circuitry can give rise to symptoms.

    “With computational models, you can quickly find out which of your hypotheses about how the brain works are likely to be true, and which are unlikely to be true,” said Silverstein. “This can accelerate scientific progress by maximizing the chances that follow-up experiments with people will lead to useful results, and avoiding long and expensive studies that are unlikely to succeed.”

    Models can also help improve diagnosis, Niv said. “Models allow us to describe behaviors in a precise, quantitative way,” Niv said. “For example, we can quantify the extent to which getting an unexpected reward affects your mood, and how this differs between patients and healthy control. This allows us to start to think about diagnosing psychiatric disorders in a more definitive way, with tools that are more like a blood test rather than a self-report of symptoms.”

    As the models improve, and researchers gain confidence that computational models accurately represent human conditions, it should be possible to use the models to develop new treatments, Silverstein said. “You can ask, what happens to behavior if I add a treatment effect to the model, and this can help us understand what might happen in patients.”

    Rutgers has one of the country’s largest academically affiliated mental health care systems, serving over 12,500 people each year. Princeton is a leading institution in the field of computational neuroscience. Also participating are collaborators at the Max Planck-UCL Centre for Computational Psychiatry and Ageing Research.

    See the full article here .

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    Princeton University Campus

    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    Rutgers, The State University of New Jersey, is a leading national research university and the state’s preeminent, comprehensive public institution of higher education. Rutgers is dedicated to teaching that meets the highest standards of excellence; to conducting research that breaks new ground; and to providing services, solutions, and clinical care that help individuals and the local, national, and global communities where they live.

    Founded in 1766, Rutgers teaches across the full educational spectrum: preschool to precollege; undergraduate to graduate; postdoctoral fellowships to residencies; and continuing education for professional and personal advancement.

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    As a ’67 graduate of University college, second in my class, I am proud to be a member of

    Alpha Sigma Lamda, National Honor Society of non-tradional students.

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  • richardmitnick 3:07 pm on May 14, 2017 Permalink | Reply
    Tags: Marisa Salazar, Princeton,   

    From Princeton: Women in STEM – Marisa Salazar 

    Princeton University
    Princeton University

    5.14.17

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    Marisa Salazar, Class of 2017, chemistry major and co-winner of the Pyne Prize, awarded to the senior who has most clearly manifested excellent scholarship, strength of character and effective leadership. Photo by Briana Christophers, Class of 2017#Princetagram

    “Something about my background that is very important to me is that I am a second-generation immigrant.

    My grandparents and my dad moved to New Mexico when my dad was a child, and my grandparents worked hard at several jobs in order to provide my father access to opportunities and education.

    My father ended up getting a bachelor’s degree in engineering and a master’s in business.

    He and my mother prioritized and encouraged a love of education and learning throughout my life.

    I grew up in southern New Mexico, a state that is beautiful but consistently underperforms in education. In general, those who didn’t drop out in high school would go to the community college or to the local state university in my hometown.

    I was homeschooled, and in that environment I had a quality education where my love of learning was nurtured.

    Even so, because so few people in the community left the area to go to college, the thought never entered my mind.

    I always expected I would go to the state university where my parents and my siblings had gone because of precedent and because it provided generous scholarships for residents. Considering finances, it did not seem worth moving to attend another university if I would have to take on debt to cover the costs.

    However, when I started high school — I went part-time to a public school for a few years — I began to realize that I had more options than I had thought. Teachers at the public school encouraged me to consider alternatives.

    In many ways, when applying to these schools my guidance counselor was the internet, because I did not know anyone who had ended up going to an Ivy League (or even to the Northeast), and the entire process was new to me.

    I ended up applying to Princeton largely due to its quality education and its financial aid program.” — Marisa Salazar, Class of 2017, chemistry major and co-winner of the Pyne Prize, awarded to the senior who has most clearly manifested excellent scholarship, strength of character and effective leadership. Photo by Briana Christophers, Class of 2017#Princetagram

    See the full article in Facebook here.

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    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 9:23 pm on May 5, 2017 Permalink | Reply
    Tags: , , , Continuous monitoring of the entire sky now possible, , , Princeton   

    From Gordon and Betty Moore Foundation via Princeton: “Continuous monitoring of the entire sky now possible” 

    Princeton University
    Princeton University

    1

    Gordon and Betty Moore Foundation

    2
    An extremely bright supernova, designated ASASSN-15lh Source: Stellarium

    Astronomy plays a central role in advancing our understanding of the universe, but human lives are very short compared to astronomical timescales. Looking for changes in the universe demands powerful technology to catch rare events. Much of astronomy is conducted by focusing large and sensitive telescopes on narrow regions of sky. This approach is very successful when you know where to look, but transient events are constantly occurring elsewhere in areas of the sky that are not observed. Until now.

    With funding from the Moore Foundation, The Ohio State University’s existing All-Sky Automated Search for Supernovae project will be expanded – doubling in size so that the entire night sky can be observed once a night, every night. Currently the All-Sky project, with initial funding from the National Science Foundation, observes the night’s sky on every clear night through two telescopes located in Chile and Hawaii. These telescopes help discover exploding stars and violent eruptions from black holes in the centers of galaxies. These transient events get reported rapidly to the astronomical community for further study. The All-Sky network has made important scientific contributions in its two years of operation. It has discovered many bright local supernovae, accounting for about 60 percent of the world’s discoveries over the two-year period. And, in the summer of 2015, the network found the most energetic supernova ever detected.

    Added capacity for the project will produce the first complete and unbiased all-sky census of local supernovae, leading to an improved understanding of which stars produce each of the known types of supernova explosions. It also has the capacity to find unknown types that have eluded less capable searches. Expanding the telescopes from two sites to four not only doubles the data acquisition rate to cover the whole nighttime sky every day, it also helps reduce weather-related gaps in the data. The two new telescopes will be placed in Texas and South Africa: along with the sites in Chile and Hawaii, this system will provide a strong hedge against being clouded out and missing an important event.

    In addition to doubling the size of the project, funding from the foundation will improve the system’s ability to share discoveries with the broader astronomy community. The typical path to astronomical discovery has a small number of researchers analyzing data from “their” telescope time and eventually publishing papers without others ever seeing the data. The All-Sky Survey will take a more collaborative path to discovery anchored by a public alert system and a publicly accessible database. The public alert system uses automated data analysis to rapidly identify phenomena of interest and alerts the astronomy community. Read more from The Ohio State University here.

    Sharing data across the entire astronomy community is an approach the foundation has been taking in this field. Recently, the foundation provided funding to make data from Pan-STARRS – the world’s largest digital sky survey to date – publicly available.

    Pan-STARRS1 located on Haleakala, Maui, HI, USA

    The images from the Pan-STARRS survey also serve as a reference for All-Sky Survey scientists and researchers, who now have a “before” image for almost every patch of the sky and can consult the Pan-STARRS image to see if something detected in the All-Sky Survey data is actually new or not.

    Complementing the abilities of the All-Sky Survey is another foundation-funded project at Princeton dubbed HATPI – “HAT” from the Hungarian Automated Telescope constructed and operated by the principal investigator Gaspar Bakos and covering a solid angle of PI steradians on the sky at all times – about the angle covered by an umbrella held over your head.

    Where the All-Sky Survey uses a smaller array of larger telescopes to make daily observations and is aimed at the discovery of events with a timescale of a few days, HATPI has a 30-second cadence through a hedgehog-like array of 63 telescopes on a single mount.

    3
    HATPI

    HATPI will create very high precision measurements to detect the shadow of a planet crossing the (unseen) disk of nearby stars. It is also great for finding earth-threatening asteroids, all kinds of variable stars, exploding stars and possibly even the optical emission from a gravitational wave source. The telescope will be sited at Las Campanas Observatory in northern Chile.

    Carnegie Las Campanas Observaory in the southern Atacama Desert of Chile in the Atacama Region approximately 100 kilometres (62 mi) northeast of the city of La Serena

    It will cover the whole sky every clear night and over the course of a year, it will record three-quarters of the entire sky. The Princeton team also aims to process the data in real time and make the results public without delay.

    Our portfolio of work in astronomy is aimed at creating new opportunities for the astronomical community by enabling the development of new technologies and by making the data accessible to astronomers and enthusiasts alike. Human life is short compared to astronomical timescales, but systems of telescopes, detectors and software can reveal changes in the cosmos even when they are very rare, giving us insight into the full range of nature’s events.

    See the full article here .

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    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 3:30 pm on April 20, 2017 Permalink | Reply
    Tags: Casimir force, Mysterious force harnessed in a silicon chip, Princeton   

    From Princeton: “Mysterious force harnessed in a silicon chip” 

    Princeton University
    Princeton University

    April 20, 2017
    Catherine Zandonella

    Getting something from nothing sounds like a good deal, so for years scientists have been trying to exploit the tiny amount of energy found in nearly empty space. It’s a source of energy so obscure it was once derided as a source of “perpetual motion.” Now, a research team including Princeton scientists has found a way to harness this energy using a silicon-chip device, potentially enabling applications.

    This energy, predicted seven decades ago by the Dutch scientist Hendrik Casimir, arises from quantum effects and can be seen experimentally by placing two opposing plates very close to each other in a vacuum. At close range, the plates repel each other, which could be useful to certain technologies. Until recently, however, harnessing this “Casimir force” to do anything useful seemed impossible.

    A new silicon chip built by researchers at Hong Kong University of Science and Technology and Princeton University is a step toward harnessing the Casimir force. Using a clever assembly of micron-sized shapes etched into the plates, the researchers demonstrated that the plates repel as they are brought close together. Constructing this device entirely out of a single silicon chip could open the way to using the Casimir force for practical applications such as keeping tiny machine parts from sticking to each other. The work was published in the February issue of the journal Nature Photonics.

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    Researchers created a silicon device that enabled them to observe the Casimir force. (Image credit: Nature Photonics)

    “This is among the first experimental verifications of the Casimir effect on a silicon chip,” said Alejandro Rodriguez, an assistant professor of electrical engineering at Princeton University, who provided theoretical calculations for the device, which was built by a team led by Ho Bun Chan at Hong Kong University of Science and Technology. “And it also allows you to make measurements of forces in very nontrivial structures like these that cause repulsion. It is a double-whammy.”

    The silicon structure looks like two plates lined with teeth that face each other across a tiny gap which is only about 100 nanometers wide. (A human hair is 60,000-80,000 nanometers wide.) As the two plates are pushed closer together, the Casimir force comes into play and pushes them apart.

    This repulsive effect happens without any input of energy and to all appearances, in a vacuum. These characteristics led this energy to be called “zero-point energy.” They also fueled earlier claims that the Casimir force could not exist because its existence would imply some sort of perpetual motion, which would be impossible according to the laws of physics.

    The force, which has since been experimentally confirmed to exist, arises from the normal quantum fluctuations of the few atoms that persist in the chasm despite the evacuation of all the air.

    The team demonstrated that it is possible to build a device in silicon to control the Casimir force.

    “Our paper shows that it is possible to control the Casimir force using structures of complex, tailor-made shapes,” said Ho Bun Chan, senior author on the paper and a scientist at the Hong Kong University of Science and Technology. His team drew on earlier work by Rodriguez published in 2008 that proposed shapes that would be expected to yield a Casimir force that could both attract and repel. “This paper is the experimental realization using a structure inspired by Rodriguez’s design,” Chan said.

    Rodriguez and his team at Princeton developed techniques that allowed the researchers to compute interactions between two parallel plates as they approach each other. With these tools, they were then able to explore what would happen if more complex geometries were used. This led to some of the first predictions of a repulsive Casimir force in 2008.

    The Rodriguez group used nanophotonic techniques, which involved measuring how light would interact with the structures, to get at the complex equations of how the force arises from the interaction of two plates.

    The silicon device included a small mechanical spring that the researchers used to measure the force between the two plates, and to verify that the quantum force can be repulsive. The roughly T-shaped silicon teeth are what allow the repulsive force to form. The repulsion comes from how different parts of the surface interact with the opposite surface.

    “We tried to think about what kind of shapes Chan’s group would have to fabricate to lead to a significant repulsive force, so we did some background studies and calculations to make sure they would see enough non-monotonicity as to be measurable,” Rodriguez said.

    Going forward, the researchers plan to explore other configurations that may give rise to even larger repulsive forces and more well-defined repulsion at larger separations.

    Funding for the study came from the Research Grants Council of Hong Kong and the National Science Foundation (grant no. DMR-1454836).

    The paper, Measurement of non-monotonic Casimir forces between silicon nanostructures, by L. Tang, M. Wang, C. Y. Ng, M. Nikolic, C. T. Chan, A. W. Rodriguez and H. B. Chan was published in the journal Nature Photonics online Jan. 9, 2017 and in the February 2017 issue. Nature Photonics 97–101(2017) doi:10.1038/nphoton.2016.254.

    See the full article here .

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    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 4:55 pm on April 5, 2017 Permalink | Reply
    Tags: , Artificial topological matter opens new research directions, Princeton   

    From Princeton: “Artificial topological matter opens new research directions” 

    Princeton University
    Princeton University

    April 5, 2017
    Catherine Zandonella, Office of the Dean for Research

    An international team of researchers have created a new structure that allows the tuning of topological properties in such a way as to turn on or off these unique behaviors. The structure could open up possibilities for new explorations into the properties of topological states of matter.

    “This is an exciting new direction in topological matter research,” said M. Zahid Hasan, professor of physics at Princeton University and an investigator at Lawrence Berkeley National Laboratory in California who led the study, which was published March 24th in the journal Science Advances. “We are engineering new topological states that do not occur naturally, opening up numerous exotic possibilities for controlling the behaviors of these materials.”

    The new structure consists of alternating layers of topological and normal, or trivial, insulators, an architecture that allows the researchers to turn on or off the flow of current through the structure. The ability to control the current suggests possibilities for circuits based on topological behaviors, but perhaps more importantly presents a new artificial crystal lattice structure for studying quantum behaviors.

    Theories behind the topological properties of matter were the subject of the 2016 Nobel Prize in physics awarded to Princeton University’s F. Duncan Haldane and two other scientists. One class of matter is topological insulators, which are insulators on the inside but allow current to flow without resistance on the surfaces.

    In the new structure, interfaces between the layers create a one-dimensional lattice in which topological states can exist. The one-dimensional nature of the lattice can be thought of as if one were to cut into the material and remove a very thin slice, and then look at the thin edge of the slice. This one-dimensional lattice resembles a chain of artificial atoms. This behavior is emergent because it arises only when many layers are stacked together.

    1
    The researchers made different samples where they could control how the electrons tunnel from interface to interface through alternating layers of trivial and topological insulators, forming an emergent, tunable one-dimensional quantum lattice. The top panel (A, B, C, and D) shows a structure where the trivial layer is relatively thin, enabling electron-like particles to tunnel through the layers (topological phase). The bottom panel (G, H, I, and J) shows a structure where the trivial insulator is relatively thick and blocks tunneling (trivial phase). (Image courtesy of Science/AAAS)

    By changing the composition of the layers, the researchers can control the hopping of electron-like particles, called Dirac fermions, through the material. For example, by making the trivial-insulator layer relatively thick – still only about four nanometers – the Dirac fermions cannot travel through it, making the entire structure effectively a trivial insulator. However, if the trivial-insulator layer is thin – about one nanometer – the Dirac fermions can tunnel from one topological layer to the next.

    To fashion the two materials, the Princeton team worked with researchers at Rutgers University led by Seongshik Oh, associate professor of physics, who in collaboration with Hasan and others showed in 2012 [Physical Review Letters]that adding indium to a topological insulator, bismuth selenide, caused it to become a trivial insulator. Prior to that, bismuth selenide (Bi2Se3) was theoretically and experimentally identified as a topological insulator by Hasan’s team, a finding which was published in the journal Nature in 2009.

    “We had shown that, depending on how much indium you add, the resulting material had this nice tunable property from trivial to topological insulator,” Oh said, referring to the work published in Physical Review Letters in 2012.

    Graduate students Ilya Belopolski of Princeton and Nikesh Koirala of Rutgers combined two state-of-the-art techniques with new instrumentation development and worked together on layering these two materials, bismuth selenide and indium bismuth selenide, to design the optimal structure. One of the challenges was getting the lattice structures of the two materials to match up so that the Dirac fermions can hop from one layer to the next. Belopolski and Suyang Xu worked with colleagues at Princeton University, Lawrence Berkeley National Laboratory and multiple institutions to use high resolution angle-resolved photoemission spectroscopy to optimize the behavior of the Dirac fermions based on a growth to measurement feedback loop.

    Although no topologically similar states exist naturally, the researchers note that analogous behavior can be found in a chain of polyacetylene, which is a known model of one-dimensional topological behavior as described by the 1979 Su-Schrieffer-Heeger’s theoretical model of an organic polymer.

    The research presents a foray into making artificial topological materials, Hasan said. “In nature, whatever a material is, topological insulator or not, you are stuck with that,” Hasan said. “Here we are tuning the system in a way that we can decide in which phase it should exist; we can design the topological behavior.”

    The ability to control the travel of light-like Dirac fermions could eventually lead future researchers to harness the resistance-less flow of current seen in topological materials. “These types of topologically tunable heterostructures are a step toward applications, making devices where topological effects can be utilized,” Hasan said.

    The Hasan group plans to further explore ways to tune the thickness and explore the topological states in connection to the quantum Hall effect, superconductivity, magnetism, and Majorana and Weyl fermion states of matter.

    In addition to work done at Princeton and Rutgers, the research featured contributions from the following institutions: South University of Science and Technology of China; Swiss Light Source, Paul Scherrer Institute; National University of Singapore; University of Central Florida; Universität Würzburg; Diamond Light Source, Didcot, U.K.; and Synchrotron SOLEIL, Saint-Aubin, France.

    Work at Princeton University and synchrotron-based ARPES measurements led by Princeton researchers were supported by the U.S. Department of Energy under Basic Energy AQ29 Sciences grant no. DE-FG-02-05ER46200 (to M.Z.H.). I.B. was supported by an NSF Graduate Research Fellowship. N.K., M.B., and S.O. were supported by the Emergent Phenomena in Quantum Systems Initiative of the Gordon and Betty Moore Foundation under grant no. GBMF4418 and by the NSF under grant no. NSF-EFMA-1542798. H.L. acknowledges support from the Singapore National Research Foundation under award no. NRF-NRFF2013-03. M.N. was supported by start-up funds from the University of Central Florida. The work acknowledges support of Diamond Light Source, Didcot, U.K., for time on beamline I05 under proposal SI11742-1. Some measurements were carried out at the ADRESS beamline (24) of the Swiss Light Source, Paul Scherrer Institute, Switzerland. This study was in part supported by grant AQ30 no. 11504159 of the National Natural Science Foundation of China (NSFC), grant no. 2016A030313650 of NSFC Guangdong, and project no. JCY20150630145302240 of the Shenzhen Science and Technology Innovations Committee.

    The paper, A novel artificial condensed matter lattice and a new platform for one-dimensional topological phases, by Ilya Belopolski, Su-Yang Xu, Nikesh Koirala, Chang Liu, Guang Bian, Vladimir Strocov, Guoqing Chang, Madhab Neupane, Nasser Alidoust, Daniel Sanchez, Hao Zheng, Matthew Brahlek, Victor Rogalev, Timur Kim, Nicholas C. Plumb, Chaoyu Chen, François Bertran, Patrick Le Fèvre, Amina Taleb-Ibrahimi, Maria-Carmen Asensio, Ming Shi, Hsin Lin, Moritz Hoesch, Seongshik Oh and M. Zahid Hasan, was published in the journal Science Advances on March 24, 2017. (Belopolski et al., Sci. Adv. 2017;3: e1501692 24 March 2017)

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    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

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  • richardmitnick 12:51 pm on April 4, 2017 Permalink | Reply
    Tags: Princeton, , Study reveals the multitasking secrets of an RNA-binding protein   

    From Princeton: “Study reveals the multitasking secrets of an RNA-binding protein” 

    Princeton University
    Princeton University

    April 4, 2017
    Staff, Department of Molecular Biology

    1
    Two views of one of Glo’s RNA-binding domains highlight the amino acids required for binding G-tract RNA (left) and U-A stem structures (right). Courtesy of Cell Reports.

    Researchers from Princeton University and the National Institute of Environmental Health Sciences have discovered how a fruit fly protein binds and regulates two different types of RNA target sequence. The study, which will be published April 4 in the journal Cell Reports, may help explain how various RNA-binding proteins, many of which are implicated in cancer and neurodegenerative disease, perform so many different functions in the cell.

    There are hundreds of RNA-binding proteins in the human genome that together regulate the processing, turnover and localization of the many thousands of RNA molecules expressed in cells. These proteins also control the translation of RNA into proteins. RNA-binding proteins are crucial for maintaining normal cellular function, and defects in this family of proteins can lead to disease. For example, RNA-binding proteins are overexpressed in many human cancers, and mutations in some of these proteins have been linked to neurological and neurodegenerative disorders such as amyotrophic lateral sclerosis. “Understanding the fundamental properties of this class of proteins is very relevant,” said Elizabeth Gavis, the Damon B. Pfeiffer Professor in the Life Sciences and a professor of molecular biology.

    Gavis and colleagues are particularly interested in a protein called Glorund (Glo), a type of RNA-binding protein that performs several functions in fruit fly development. This protein was originally identified due to its ability to repress the translation of an RNA molecule called nanos to protein in fly eggs. By binding to a stem structure formed by uracil and adenine nucleotides in the nanos RNA, Glo prevents the production of Nanos protein at the front of the embryo, a step that enables the fly’s head to form properly.

    Like many other RNA-binding proteins, however, Glo is multifunctional. It regulates several other steps in fly development, apparently by binding to RNAs other than nanos. The mammalian counterparts of Glo, known as heterogeneous nuclear ribonucleoprotein (hnRNP) F/H proteins, bind to RNAs containing stretches of guanine nucleotides known as G-tracts, and, rather than repressing translation, mammalian hnRNP F/H proteins regulate processes such as RNA splicing, in which RNAs are rearranged to produce alternative versions of the proteins they encode.

    To understand how Glo might bind to diverse RNAs and regulate them in different ways, Gavis and graduate student Joel Tamayo collaborated with Traci Tanaka Hall and Takamasa Teramoto from the National Institute of Environmental Health Sciences to generate X-ray crystallographic structures of Glo’s three RNA-binding domains. As expected, the three domains were almost identical to the corresponding domains of mammalian hnRNP F/H proteins. They retained, for example, the amino acid residues that bind to G-tract RNA, and the researchers confirmed that, like their mammalian counterparts, each RNA-binding domain of Glo can bind to this type of RNA sequence.

    However, the researchers also saw something new. “When we looked at the structures, we realized that there were also some basic amino acids that projected from a different part of the RNA-binding domains that could be involved in contacting RNA,” Gavis explained.

    The researchers found that these basic amino acids mediate binding to uracil-adenine (U-A) stem structures like the one found in nanos RNA. Each of Glo’s RNA-binding domains therefore contains two distinct binding surfaces that interact with different types of RNA target sequence. “While there have been examples previously of RNA-binding proteins that carry more than one binding domain, each with a different specificity, this represents the first example of a single domain harboring two different specificities,” said Howard Lipshitz, a professor of molecular genetics at the University of Toronto who was not involved in the study.

    To investigate which of Glo’s two RNA-binding modes was required for its different functions in flies, Gavis and colleagues generated insects carrying mutant versions of the RNA-binding protein. Glo’s ability to repress nanos translation during egg development required both of the protein’s RNA-binding modes. The researchers discovered that, as well as binding the U-A stem in the nanos RNA, Glo also recognized a nearby G-tract sequence. But Glo’s ability to regulate other RNAs at different developmental stages only depended on the protein’s capacity to bind G-tracts.

    “We think that the binding mode may correlate with Glo’s activity towards a particular RNA,” said Gavis. “If it binds to a G-tract, Glo might promote RNA splicing. If it simultaneously binds to both a G-tract and a U-A stem, Glo acts as a translational repressor.”

    The RNA-binding domains of mammalian hnRNP F/H proteins probably have a similar ability to bind two different types of RNA, allowing them to regulate diverse target RNAs within the cell. “This paper represents an exciting advance in a field that has become increasingly important with the discovery that defects in RNA-binding proteins contribute to human diseases such as metabolic disorders, cancer and neurodegeneration,” Lipshitz said. “Since these proteins are evolutionarily conserved from fruit flies to humans, experiments of this type tell us a lot about how their human versions normally work or can go wrong.”

    The research was supported in part by a National Science Foundation Graduate Research Fellowship (DGE 1148900), a Japan Society for the Promotion of Science fellowship, the National Institutes of Health (R01 GM061107) and the Intramural Research Program of the National Institute of Environmental Health Sciences. The Advanced Photon Source used for this study is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract W-31-109-Eng-38.

    The study, “The Drosophila hnRNP F/H Homolog Glorund Uses Two Distinct RNA-binding Modes to Diversify Target Recognition,” by Joel Tamayo, Takamasa Teramoto, Seema Chatterjee, Traci Tanaka Hall, and Elizabeth Gavis, was published in the journal Cell Reports on April 4, 2017. http://dx.doi.org/10.1016/j.celrep.2017.03.022

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    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 10:31 am on March 29, 2017 Permalink | Reply
    Tags: A Seismic Mapping Milestone, , , , , Princeton,   

    From ORNL: “A Seismic Mapping Milestone” 

    i1

    Oak Ridge National Laboratory

    March 28, 2017

    Jonathan Hines
    hinesjd@ornl.gov
    865.574.6944

    1
    This visualization is the first global tomographic model constructed based on adjoint tomography, an iterative full-waveform inversion technique. The model is a result of data from 253 earthquakes and 15 conjugate gradient iterations with transverse isotropy confined to the upper mantle. Credit: David Pugmire, ORNL

    When an earthquake strikes, the release of energy creates seismic waves that often wreak havoc for life at the surface. Those same waves, however, present an opportunity for scientists to peer into the subsurface by measuring vibrations passing through the Earth.

    Using advanced modeling and simulation, seismic data generated by earthquakes, and one of the world’s fastest supercomputers, a team led by Jeroen Tromp of Princeton University is creating a detailed 3-D picture of Earth’s interior. Currently, the team is focused on imaging the entire globe from the surface to the core–mantle boundary, a depth of 1,800 miles.

    These high-fidelity simulations add context to ongoing debates related to Earth’s geologic history and dynamics, bringing prominent features like tectonic plates, magma plumes, and hotspots into view. In September 2016, the team published a paper in Geophysical Journal International on its first-generation global model. Created using data from 253 earthquakes captured by seismograms scattered around the world, the team’s model is notable for its global scope and high scalability.

    “This is the first global seismic model where no approximations—other than the chosen numerical method—were used to simulate how seismic waves travel through the Earth and how they sense heterogeneities,” said Ebru Bozdag, a coprincipal investigator of the project and an assistant professor of geophysics at the University of Nice Sophia Antipolis. “That’s a milestone for the seismology community. For the first time, we showed people the value and feasibility of running these kinds of tools for global seismic imaging.”

    The project’s genesis can be traced to a seismic imaging theory first proposed in the 1980s. To fill in gaps within seismic data maps, the theory posited a method called adjoint tomography, an iterative full-waveform inversion technique. This technique leverages more information than competing methods, using forward waves that travel from the quake’s origin to the seismic receiver and adjoint waves, which are mathematically derived waves that travel from the receiver to the quake.

    The problem with testing this theory? “You need really big computers to do this,” Bozdag said, “because both forward and adjoint wave simulations are performed in 3-D numerically.”

    In 2012, just such a machine arrived in the form of the Titan supercomputer, a 27-petaflop Cray XK7 managed by the US Department of Energy’s (DOE’s) Oak Ridge Leadership Computing Facility (OLCF), a DOE Office of Science User Facility located at Oak Ridge National Laboratory.


    ORNL Cray XK7 Titan Supercomputer

    After trying out its method on smaller machines, Tromp’s team gained access to Titan in 2013. Working with OLCF staff, the team continues to push the limits of computational seismology to deeper depths.

    Stitching Together Seismic Slices

    As quake-induced seismic waves travel, seismograms can detect variations in their speed. These changes provide clues about the composition, density, and temperature of the medium the wave is passing through. For example, waves move slower when passing through hot magma, such as mantle plumes and hotspots, than they do when passing through colder subduction zones, locations where one tectonic plate slides beneath another.

    Each seismogram represents a narrow slice of the planet’s interior. By stitching many seismograms together, researchers can produce a 3-D global image, capturing everything from magma plumes feeding the Ring of Fire, to Yellowstone’s hotspots, to subducted plates under New Zealand.

    This process, called seismic tomography, works in a manner similar to imaging techniques employed in medicine, where 2-D x-ray images taken from many perspectives are combined to create 3-D images of areas inside the body.

    In the past, seismic tomography techniques have been limited in the amount of seismic data they can use. Traditional methods forced researchers to make approximations in their wave simulations and restrict observational data to major seismic phases only. Adjoint tomography based on 3-D numerical simulations employed by Tromp’s team isn’t constrained in this way. “We can use the entire data—anything and everything,” Bozdag said.

    Digging Deeper

    To improve its global model further, Tromp’s team is experimenting with model parameters on Titan. For example, the team’s second-generation model will introduce anisotropic inversions, which are calculations that better capture the differing orientations and movement of rock in the mantle. This new information should give scientists a clearer picture of mantle flow, composition, and crust–mantle interactions.

    Additionally, team members Dimitri Komatitsch of Aix-Marseille University in France and Daniel Peter of King Abdullah University in Saudi Arabia are leading efforts to simulate higher-frequency seismic waves. This would allow the team to model finer details in the Earth’s mantle and even begin mapping the Earth’s core.

    To make this leap, Tromp’s team is preparing for Summit, the OLCF’s next-generation supercomputer.


    ORNL IBM Summit supercomputer depiction

    Set to arrive in 2018, Summit will provide at least five times the computing power of Titan. As part of the OLCF’s Center for Accelerated Application Readiness, Tromp’s team is working with OLCF staff to take advantage of Summit’s computing power upon arrival.

    “With Summit, we will be able to image the entire globe from crust all the way down to Earth’s center, including the core,” Bozdag said. “Our methods are expensive—we need a supercomputer to carry them out—but our results show that these expenses are justified, even necessary.”

    See the full article here .

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    ORNL is managed by UT-Battelle for the Department of Energy’s Office of Science. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.

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  • richardmitnick 9:44 am on February 9, 2017 Permalink | Reply
    Tags: , , Princeton, Wave of the future: Terahertz chips a new way of seeing through matter   

    From Princeton: “Wave of the future: Terahertz chips a new way of seeing through matter” 

    Princeton University
    Princeton University

    February 8, 2017
    Tien Nguyen

    1
    Princeton University researchers have drastically shrunk the equipment for producing terahertz — important electromagnetic pulses lasting one millionth of a millionth of a second — from a tabletop setup with lasers and mirrors to a pair of microchips small enough to fit on a fingertip (above). The simpler, cheaper generation of terahertz has potential for advances in medical imaging, communications and drug development. (Photos by Frank Wojciechowski for the Office of Engineering Communications)

    Electromagnetic pulses lasting one millionth of a millionth of a second may hold the key to advances in medical imaging, communications and drug development. But the pulses, called terahertz waves, have long required elaborate and expensive equipment to use.

    Now, researchers at Princeton University have drastically shrunk much of that equipment: moving from a tabletop setup with lasers and mirrors to a pair of microchips small enough to fit on a fingertip.

    In two articles recently published in the IEEE Journal of Solid State Circuits, the researchers describe one microchip that can generate terahertz waves, and a second chip that can capture and read intricate details of these waves.

    “The system is realized in the same silicon chip technology that powers all modern electronic devices from smartphones to tablets, and therefore costs only a few dollars to make on a large scale” said lead researcher Kaushik Sengupta, a Princeton assistant professor of electrical engineering.

    Terahertz waves are part of the electromagnetic spectrum — the broad class of waves that includes radio, X-rays and visible light — and sit between the microwave and infrared light wavebands. The waves have some unique characteristics that make them interesting to science. For one, they pass through most non-conducting material, so they could be used to peer through clothing or boxes for security purposes, and because they have less energy than X-rays, they don’t damage human tissue or DNA.

    Terahertz waves also interact in distinct ways with different chemicals, so they can be used to characterize specific substances. Known as spectroscopy, the ability to use light waves to analyze material is one of the most promising — and the most challenging — applications of terahertz technology, Sengupta said.

    To do it, scientists shine a broad range of terahertz waves on a target then observe how the waves change after interacting with it. The human eye performs a similar type of spectroscopy with visible light — we see a leaf as green because light in the green light frequency bounces off the chlorophyll-laden leaf.

    The challenge has been that generating a broad range of terahertz waves and interpreting their interaction with a target requires a complex array of equipment such as bulky terahertz generators or ultrafast lasers. The equipment’s size and expense make the technology impractical for most applications.

    Researchers have been working for years to simplify these systems. In September, Sengupta’s team reported a way to reduce the size of the terahertz generator and the apparatus that interprets the returning waves to a millimeter-sized chip. The solution lies in re-imaging how an antenna functions. When terahertz waves interact with a metal structure inside the chip, they create a complex distribution of electromagnetic fields that are unique to the incident signal. Typically, these subtle fields are ignored, but the researchers realized that they could read the patterns as a sort of signature to identify the waves. The entire process can be accomplished with tiny devices inside the microchip that read terahertz waves.

    “Instead of directly reading the waves, we are interpreting the patterns created by the waves,” Sengupta said. “It is somewhat like looking for a pattern of raindrops by the ripples they make in a pond.”

    2
    In two recently published articles, researchers Kaushik Sengupta (left), an assistant professor of electrical engineering, and Xue Wu (right), a Princeton graduate student in computer science, describe one microchip that can generate terahertz waves, and a second chip that can capture and read intricate details of these waves. Terahertz waves sit between the microwave and infrared light wavebands on the electromagnetic spectrum and have unique characteristics, such as the ability to pass through most non-conducting material such as clothing or boxes without damaging human tissue or DNA.

    Daniel Mittleman, a professor of engineering at Brown University, said the development was “a very innovative piece of work, and it potentially has a lot of impact.” Mittleman, who is the vice chair of the International Society for Infrared Millimeter and Terahertz Waves, said scientists still have work to do before the terahertz band can begin to be used in everyday devices, but the developments are promising.

    “It is a very big puzzle with many pieces, and this is just one, but it is a very important one,” said Mittleman, who is familiar with the work but had no role in it.

    On the terahertz-generation end, much of the challenge is creating a wide range of wavelengths within the terahertz band, particularly in a microchip. The researchers realized they could overcome the problem by generating multiple wavelengths on the chip. They then used precise timing to combine these wavelengths and create very sharp terahertz pulses.

    In an article published Dec. 14 in the IEEE Journal of Solid State Circuits, the researchers explained how they created a chip to generate the terahertz waves. The next step, the researchers said, is to extend the work farther along the terahertz band. “Right now we are working with the lower part of the terahertz band,” said Xue Wu, a Princeton doctoral student in electrical engineering and an author on both papers.

    “What can you do with a billion transistors operating at terahertz frequencies?” Sengupta asked. “Only by re-imagining these complex electromagnetic interactions from fundamental principles can we invent game-changing new technology.”

    The paper On-chip THz spectroscope exploiting electromagnetic scattering with multi-port antenna was published Sept. 2, and the paper Dynamic waveform shaping with picosecond time widths was published Dec. 14, both by IEEE Journal of Solid State Circuits. The research was supported in part by the National Science Foundation’s Division of Electrical, Communications and Cyber Systems (grant nos. ECCS-1408490 and ECCS-1509560).

    See the full article here .

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    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 12:39 pm on January 30, 2017 Permalink | Reply
    Tags: , , , , Princeton, Symmetry in patterns, symmorphic and nonsymmorphic patterns of atoms, Theorists propose new class of topological metals with exotic electronic properties, tungsten telluride (WTe2)   

    From Princeton: “Theorists propose new class of topological metals with exotic electronic properties (Physics Review X)” 

    Princeton University
    Princeton University

    January 30, 2017
    Tien Nguyen, Department of Chemistry

    1
    A new theory explains the behavior of a class of metals with exotic electronic properties. Credit: Muechler et al., Physics Review X

    Researchers at Princeton, Yale, and the University of Zurich have proposed a theory-based approach to characterize a class of metals that possess exotic electronic properties that could help scientists find other, similarly-endowed materials.

    Published in the journal Physical Review X, the study described a new class of metals based on their symmetry and a mathematical classification known as a topological number, which is predictive of special electronic properties. Topological materials have drawn intense research interest since the early 2000s culminating in last year’s Nobel Prize in Physics awarded to three physicists, including F. Duncan Haldane, Princeton’s Eugene Higgins Professor of Physics, for theoretical discoveries in this area.

    “Topological classification is a very general way of looking at the properties of materials,” said Lukas Muechler, a Princeton graduate student in the laboratory of Roberto Car, Princeton’s Ralph W. *31 Dornte Professor in Chemistry and lead author on the article.

    A popular way of explaining this abstract mathematical classification involves breakfast items. In topological classification, donuts and coffee cups are equivalent because they both have one hole and can be smoothly deformed into one another. Meanwhile donuts cannot deform into muffins which makes them inequivalent. The number of holes is an example of a topological invariant that is equal for the donut and coffee cup, but distinguishes between the donut and the muffin.

    “The idea is that you don’t really care about the details. As long as two materials have the same topological invariants, we can say they are topologically equivalent,” he said.

    Muechler and his colleagues’ interest in the topological classification of this new class of metals was sparked by a peculiar discovery in the neighboring laboratory of Robert Cava, Princeton’s Russell Wellman Moore Professor of Chemistry. While searching for superconductivity in a crystal called tungsten telluride (WTe2), the Cava lab instead found that the material could continually increase its resistance in response to ever stronger magnetic fields – a property that might be used to build a sensor of magnetic fields.

    The origin of this property was, however, mysterious. “This material has very interesting properties, but there had been no theory around it,” Muechler said.

    The researchers first considered the arrangement of the atoms in the WTe2 crystal. Patterns in the arrangement of atoms are known as symmetries, and they fall into two fundamentally different classes – symmorphic and nonsymmorphic – which lead to profound differences in electronic properties, such as the transport of current in an electromagnetic field.

    2
    a) Symmorphic symmetry b) Nonsymmorphic symmetry Credit: Lukas Muechler

    While WTe2 is composed of many layers of atoms stacked upon each other, Car’s team found that a single layer of atoms has a particular nonsymmorphic symmetry, where the atomic arrangement is unchanged overall if it is first rotated and then translated by a fraction of the lattice period (see figure).

    Having established the symmetry, the researchers mathematically characterized all possible electronic states having this symmetry, and classified those states that can be smoothly deformed into each other as topologically equivalent, just as a donut can be deformed into a cup. From this classification, they found WTe2 belongs to a new class of metals which they coined nonsymmorphic topological metals. These metals are characterized by a different electron number than the nonsymmorphic metals that have previously been studied.

    In nonsymmorphic topological metals, the current-carrying electrons behave like relativistic particles, in other words, as particles traveling at nearly the speed of light. This property is not as susceptible to impurities and defects as ordinary metals, making them attractive candidates for electronic devices.

    The abstract topological classification also led the researchers to suggest some explanations for some of the outstanding electronic properties of bulk WTe2, most importantly its perfect compensation, meaning that it has an equal number of holes and electrons. Through theoretical simulations, the researchers found that this property could be achieved in the three-dimensional crystalline stacking of the WTe2 monolayers, which was a surprising result, Muechler said.

    “Usually in theory research there isn’t much that’s unexpected, but this just popped out,” he said. “This abstract classification directly led us to explaining this property. In this sense, it’s a very elegant way of looking at this compound and now you can actually understand or design new compounds with similar properties.”

    Recent photoemission experiments have also shown that the electrons in WTe2 absorb right-handed photons differently than they would left-handed photons. The theory formulated by the researchers showed that these photoemission experiments on WTe2 can be understood based on the topological properties of this new class of metals.

    In future studies, the theorists want to test whether these topological properties are also present in atomically-thin layers of these metals, which could be exfoliated from a larger crystal to make electronic devices. “The study of this phenomena has big implications for the electronics industry, but it’s still in its infant years,” Muechler said.

    This work was supported by the U.S. Department of Energy (DE-FG02-05ER46201), the Yale Postdoctoral Prize Fellowship, the National Science Foundation (NSF CAREER DMR-095242 and NSF-MRSEC DMR-0819860), the Office of Naval Research (ONR-N00014-11-1- 0635), the U.S. Department of Defense (MURI-130-6082), the David and Lucile Packard Foundation, the W. M. Keck Foundation, and the Eric and Wendy Schmidt Transformative Technology Fund.

    See the full article here .

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