Plasmas Torn Apart

Caltech researchers make discovery that hints at origin of phenomena like solar flares

PASADENA, Calif.—January saw the biggest solar storm since 2005, generating some of the most dazzling northern lights in recent memory.

The source of that storm—and others like it—was the sun's magnetic field, described by invisible field lines that protrude from and loop back into the burning ball of gas. Sometimes these field lines break—snapping like a rubber band pulled too tight—and join with other nearby lines, releasing energy that can then launch bursts of plasma known as solar flares. Huge chunks of plasma from the sun's surface can zip toward Earth and damage orbiting satellites or bump them off their paths.

These chunks of plasma, called coronal mass ejections, can also snap Earth's magnetic field lines, causing charged particles to speed toward Earth's magnetic poles; this, in turn, sets off the shimmering light shows we know as the northern and southern lights.

Even though the process of field lines breaking and merging with other lines—called magnetic reconnection—has such significant effects, a detailed picture of what precisely is going on has long eluded scientists, says Paul Bellan, professor of applied physics in the Division of Engineering and Applied Science at the California Institute of Technology (Caltech).

Now, using high-speed cameras to look at jets of plasma in the lab, Bellan and graduate student Auna Moser have discovered a surprising phenomenon that provides clues to just how magnetic reconnection occurs. They describe their results in a paper published in the February 16 issue of the journal Nature.

"Trying to understand nature by using engineering techniques is indeed a hallmark of the Division of Engineering and Applied Science at Caltech," says Ares Rosakis, the Theodore von Kármán Professor of Aeronautics and professor of mechanical engineering and the chair of engineering and applied science.

In the experiments, Moser fired jets of hydrogen, nitrogen, and argon plasmas at speeds of about 10 to 50 kilometers per second across a distance of more than 20 centimeters in a vacuum. Plasma is a gas so hot that atoms are stripped of their electrons. As a throughway for speeding electrons, the jets act like electrical wires. The experiment requires 200 million watts of power to produce jets that are a scorching 20,000 degrees Kelvin and carry a current of 100,000 amps. To study the jets, Moser used cameras that can take a snapshot in less than a microsecond, or one millionth of a second.

As in all electrical currents, the flowing electrons in the plasma jet generate a magnetic field, which then exerts a force on the plasma. These electromagnetic interactions between the magnetic field and the plasma can cause the jet to writhe and form a rapidly expanding corkscrew. This behavior, called a kink instability, has been studied for nearly 60 years, Bellan says.

But when Moser looked closely at this behavior in her experimental plasma jets, she saw something entirely unexpected.

She found that—more often than not—the corkscrew shape that developed in her jets grew exponentially and extremely fast. The jets in the experiment formed 20-centimeter-long coils in just 20 to 25 microseconds. She also noticed tiny ripples that began appearing on the inner edge of the coil just before the jet broke—the moment when there was a magnetic reconnection.

In the beginning, Moser and Bellan say, they did not know what they were seeing—they just knew it was strange. "I thought it was a measurement error," Bellan says. "But it was way too reproducible. We were seeing it day in and day out. At first, I thought we would never figure it out."

But after months of additional experiments, they determined that the kink instability actually spawns a completely different kind of phenomenon, called a Rayleigh-Taylor instability. A Rayleigh-Taylor instability happens when a heavy fluid that sits on top of a light fluid tries to trade places with the light fluid. Ripples form and grow at the interface between the two, allowing the fluids to swap places.

What Moser and Bellan realized is that the kink instability creates conditions that give rise to a Rayleigh-Taylor instability. As the coiled plasma expands—due to the kink instability—it accelerates outward. Just like a passenger being pushed back into the seat of an accelerating car, the accelerated plasma is pushed down on the vacuum behind it. The plasma tries to swap places with the trailing vacuum by forming ripples that then expand—just like when gravity forces a heavy fluid to try to change places with a light fluid underneath. The Rayleigh-Taylor instability—as revealed by the ripples on the trailing side of the accelerating plasma—grows in about a microsecond.

"People have not observed anything like this before," Bellan says.

Although the Rayleigh-Taylor instability has been studied for more than 100 years, no one had considered the possibility that it could be caused by a kink instability, Bellan says. The two types of instabilities are so different that to see them so closely coupled was a shock. "Nobody ever thought there was a connection," he says.

What is notable is that the two instabilities occur at very different scales, the researchers say. While the coil created by the kink instability spans about 20 centimeters, the Rayleigh-Taylor instability is much smaller, making ripples just two centimeters long. Still, those smaller ripples rapidly erode the jet, forcing the electrons to flow faster and faster through a narrowing channel. "You're basically choking it off," Bellan explains. Soon, the jet breaks, causing a magnetic reconnection.

Magnetic reconnection on the sun often involves phenomena that span scales from a million meters to just a few meters. At the larger scales, the physics is relatively simple and straightforward. But at the smaller scales, the physics becomes more subtle and complex—and it is in this regime that magnetic reconnection takes place. Magnetic reconnection is also a key issue in developing thermonuclear fusion as a future energy source using plasmas in the laboratory. One of the key advances in this study, the researchers say, is being able to relate phenomena at large scales, such as the kink instability, to those at small scales, such as the Rayleigh-Taylor instability.

The researchers note that, although kink and Rayleigh-Taylor instabilities may not drive magnetic reconnection in all cases, this mechanism is a plausible explanation for at least some scenarios in nature and the lab.

The title of Moser and Bellan's Nature paper is "Magnetic reconnection from a multiscale instability cascade." This research was funded by the U.S. Department of Energy, the National Science Foundation, and the Air Force Office of Scientific Research.

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Marcus Woo
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Two Caltech Researchers Receive Frontiers of Knowledge Award

For their work in information and communication technologies, and biomedicine, Carver Mead, Moore Professor Emeritus of Engineering and Applied Science, and Alexander Varshavsky, Smits Professor of Cell Biology, have been honored by the BBVA Foundation as recipients of 2011 Frontiers of Knowledge awards. The BBVA Foundation—a social responsibility arm of the multinational Spanish banking group Banco Bilbao Vizcaya Argentaria (BBVA)—presents the 400,000 euro (approximately $520,000) awards to recognize world-class research and "contributions of lasting impact for their originality, theoretical significance, and ability to push back the frontiers of the known world." For more information on the awards, and to read profiles on the winners, click here.

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Katie Neith
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Rosakis Receives Commandeur de l'Ordre des Palmes Academiques

Ares Rosakis, von Kármán Professor of Aeronautics and professor of mechanical engineering, and chair of the Division of Engineering and Applied Science, has been selected to receive the Commandeur de l'Ordre des Palmes Académiques—the Commander grade of the French Republic's Order of Academic Palms. Originally a decoration created by Napoléon in 1808 to honor educators and scholars, it was established as an order in 1955 and now honors recipients from all around the world who have made significant contributions to the development of French culture, science, and education. It is currently awarded by the prime minister of France.

The order comprises three grades—commandeur, officier, and chevalier—and in distinction immediately follows such honors as the Legion of Honor and the Order of the Liberation.

Elected to the Academy of Engineering last year, Rosakis is a fellow of the American Academy of Arts and Sciences, the Society of Experimental Mechanics, and American Society of Mechanical Engineers. His many honors include the Society of Engineering Science's Eringen Medal and the Caltech Graduate Student Council's Excellence in Teaching Award.

He received his BSc from the University of Oxford in 1978 his PhD from Brown University in 1982, the same year he joined Caltech as an assistant professor. He became associate professor in 1988 and professor in 1993. He directed the Graduate Aeronautical Laboratories (GALCIT), now the Graduate Aerospace Laboratories, from 2004—the same year he was named von Kármán Professor—to 2009, when he was appointed division chair.

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Michael Farquhar
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New Faculty Members in EAS Zoom in on the Nanoscale

Three new faculty members in the Division of Engineering and Applied Science (EAS) have big ideas about really small things. Assistant professors Hyuck Choo, Dennis Kochmann, and Austin Minnich focus on quite different challenges, but they all home in on the nanoscale, where they manipulate, model, and measure structures and phenomena at the level of individual atoms.

"Caltech and EAS take pride in lowering the barriers between disciplines to create collaborative environments for researchers such as Hyuck, Dennis, and Austin to work on a variety of topics including understanding and predicting behavior of materials at the nanoscale, which already is an area of strength within EAS," says Ares J. Rosakis, chair of the division, Theodore von Kármán Professor of Aeronautics, and professor of mechanical engineering.

Hyuck Choo, Assistant Professor of Electrical Engineering

Electrical engineer Hyuck Choo is developing a mechanism to focus laser light down to a spot just 25 nanometers (25 billionths of a meter) in diameter. Such a tiny spotlight could help manufacturers of hard disk drives continue to increase the capacity of their disks.

A typical hard drive includes a platter where data is recorded; that platter is made up of many, many individual magnetic grains, called bits. To increase the storage capacity of these disks, developers have to keep squeezing more bits into each square inch of a platter.

But they've hit a wall of sorts. Top-of-the-line hard drive disks currently hold about 750 gigabytes (GB) per square inch. The upper limit, using current methods and materials, is 1 terabyte (1,024 GB) per square inch. However, to reach such a density, the individual bits need to be smaller than 25 nanometers, and at that size, the ferromagnetic materials they're traditionally made from become unstable at room temperature, making them unreliable for data storage.

Switching to a different material, iron-platinum alloy, for the platters, the individual bits can remain stable down to 2.5 nanometers. The only problem is that iron platinum is so stable that it has to be heated in order to record information.

That's where Choo, who is part of the Kavli Nanoscience Institute, comes in. "We are creating a laser spot that is small enough to go down to the 25-nanometer size, so that we can just heat up individual magnetic bits," Choo says. "Our solution could be used to enable the next generation of hard drive disks."

To illuminate such a miniscule spot, Choo's group has developed a device called a "plasmonic waveguide." Made of gold and glass, the three-dimensional guide channels laser light through a tapering geometry to a tip that's just 14 nanometers across. Choo explains that when they shine laser light at an angle along the waveguide, "the energy of the photons gets transferred to the electrons at the interface between the glass and the gold." Once that happens, an electron wave travels down to the tip, compressing the energy into a very small spotlight.

Choo was born in the United States, raised in South Korea, and went to high school in Worcester, Massachusetts. He did his undergrad work at Cornell University and earned his PhD at the University of California, Berkeley. In addition to the light-focusing project, Choo's lab is also working on label-free imaging for glaucoma research.

Dennis Kochmann, Assistant Professor of Aerospace

Aerospace engineer Dennis Kochmann is developing theoretical and computational models of solid materials across multiple time and length scales-from the nano all the way up to the macroscale, which we deal with on an everyday basis.

"The behaviors that you observe at the macroscale usually stem from very many mechanisms from components at lower scales," Kochmann says. "If you zoom in with a microscope on a piece of metal, which looks homogeneous to the naked eye, you will see that it has microstructure-that there are many, many very small things going on across various length scales."

For example, a metal is basically a regular lattice of atoms. But if you zoom in, that lattice is full of defects such as misplaced or completely missing atoms (so-called dislocations or vacancies). Kochmann says these innumerable defects are extremely important because they provide a material with its very distinct properties. "We need to model these lower scales in order to, on the one hand, be able to understand the behavior of materials, but also to predict the behavior of materials," Kochmann says. "If you want to design a material, you need to understand where its properties come from."

In his research, Kochmann focuses on modeling the behavior of crystalline solids bridging scales from the nano all the way up to the macro. At the lowest scale, methods of molecular dynamics help him to investigate the basic mechanisms at play within a crystal. These studies inform higher-scale engineering models and lay the basis for full-physics material models. He hopes that such complete and efficient models will help engineers simulate and predict material behavior with high accuracy.

He's also interested in designing new materials, in particular composite materials with novel properties that can be turned on and off. As a grad student, he was involved in the creation of such a material, which could be made both highly stiff and highly damping, or energy absorbent, at a particular temperature. At Caltech he plans to expand this concept to create new materials with tunable properties.

Kochmann, who was born and raised in Germany, earned his Diplom and doctoral degree in mechanical engineering from Ruhr-University Bochum. He also studied at the University of Wisconsin-Madison as a Fulbright fellow, and earned his MS in engineering mechanics there. He spent the last academic year working as a postdoctoral scholar in the lab of Michael Ortiz, the Dotty and Dick Hayman Professor of Aeronautics and Mechanical Engineering at Caltech.

Austin Minnich, Assistant Professor of Mechanical and Civil Engineering

Mechanical engineer Austin Minnich is interested in thermoelectrics-materials that are capable of converting heat directly into electricity. Thermoelectrics have been around for decades but are receiving increased attention now because they could help with energy conservation efforts. "The simplicity and reliability of thermoelectric devices offer many opportunities to recover wasted energy," Minnich says.

Thermoelectrics require an unusual combination of properties. "You're looking for high electrical conductivity and low thermal conductivity," Minnich explains. Just as electrical conduction is the flow of electrons through a material, thermal conduction refers to the flow of phonons, packets of vibrational energy, through a material. Typically, he says, a material is high in both types of conductivity or low in both. For example, metals are high in both, while glass is low in both.

But over the past decade, researchers have started to find that they can improve existing thermoelectrics by introducing nanostructures-tiny inclusions. If the inclusions are carefully sized, they can disrupt or scatter phonons, while retaining most of the material's electrical conductivity.

Minnich is focused on developing and using different tools to investigate the nanoscale physics at play in such thermoelectric materials. To properly size the nanoinclusions to scatter phonons, Minnich says it would be useful to know a measurement called the mean free path-the distance that phonons travel without being disrupted-for a variety of materials. The problem is, these values haven't been determined for most materials. "This means that we don't really know what size or distribution of sizes of nanostructures are needed to optimally scatter phonons," he says.  

As part of his research program, Minnich has developed a method to measure the details of heat transport at nanometer-length scales using ultrafast lasers. Fourier's law, which governs heat transfer at the macroscopic level, no longer holds true at nanometer scales. Minnich systematically varies the size of the region heated by the laser and observes both how that heat moves through the material and how heat transfer deviates from what would be expected with Fourier's law. From those deviations, he can figure out how different phonon mean free paths contribute to the material's heat conduction.

"We use these techniques with very fine time resolution, looking at thermal conduction over periods of just femtoseconds," he says. "And that allows us to look at details of transport that are normally just lost if you look at longer time scales." So far, he's been able to measure mean free paths for phonons in silicon and sapphire, and he plans to apply the technique to many more semiconductors, including thermoelectric materials.

Minnich grew up on the East Coast and went to high school in New Mexico. He completed his undergraduate education at the University of California, Berkeley, and earned his PhD at the Massachusetts Institute of Technology.

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Kimm Fesenmaier
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Caltech Faculty Receive Gates Foundation Grants for Global Health Initiatives

On December 15, the Gates Foundation and Grand Challenge Canada announced over $31 million in new grants to help advance healthcare in the developing world. James Heath, Gilloon Professor and professor of chemistry, and Axel Scherer, Neches Professor of Electrical Engineering, Applied Physics, and Physics, were among the 12 grant recipients who will be funded by the Bill & Melinda Gates Foundation. Caltech was the only organization to receive more than one award.  

The grants are part of the Point-of-Care Diagnostics (POC Dx) Initiative, which aims to create high-quality, low-cost diagnostic platforms to improve the quality and efficacy of healthcare in the world's poorest countries. POC Dx is the 14th program of the Grand Challenges in Global Health initiative, launched in 2003 to create new healthcare tools across a range of disciplines.   

"New and improved diagnostics to use at the point of care can help health workers around the world save countless lives," said Chris Wilson, director of global health discovery at the Bill & Melinda Gates Foundation, in a press release. "Our hope is that these bold ideas lead to affordable, easy-to-use tools that can rapidly diagnose diseases, trigger timelier treatment and thereby reduce death, disability and transmission of infections in resource-poor communities."

Heath was awarded a grant to develop HIV diagnostic tools that use synthetically created peptides instead of antibodies in diagnostic assays. Their chemical structure would allow them to be transported, stored, and used more easily than antibodies. Scherer will work with collaborators at Dartmouth to develop a prototype technology to detect a wide range of pathogens that is low-cost, low-power, and easy to use.

For more information on the Grand Challenges in Global Health program, visit the program's website

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Katie Neith
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Light as a Feather, Stiffer Than a Board

Caltech Researchers Help Develop World’s Lightest Solid Material

When you pick up the newest material in Julia Greer's office, it takes a second for your mind to adjust. Despite its looks, the little brick of metal weighs next to nothing. 

Greer, assistant professor of materials science and mechanics, is part of a team of researchers from Caltech, HRL Laboratories, LLC, and the University of California, Irvine, who have developed the world's lightest solid material, with a density of just 0.9 milligrams per cubic centimeter, or approximately 100 times lighter than Styrofoam™. Though the material is ultra-low in density, it has incredible strength and absorbs energy well, making it potentially useful for applications ranging from battery electrodes to protective shielding.

"We're entering a new era of materials science where material properties are determined not only by the microscopic makeup of the material but also by the architecture of the constituents," Greer says.

The new material, called a micro-lattice, relies, appropriately, on a lattice architecture: tiny hollow tubes made of nickel-phosphorous are angled to connect at nodes, forming repeating, asterisklike unit cells in three dimensions. Everything between the tubes is open air. In fact, the structure consists of 99.99% open volume. Tobias Schaedler, a research staff scientist at HRL Laboratories, LLC, and lead author on the report described it as "a lattice of interconnected hollow tubes with a wall thickness of 100 nanometers, 1,000 times thinner than a human hair."

The material takes advantage of a hierarchical design: the wall thickness can be measured in nanometers, the diameter of each tube can be measured in microns, each tube is millimeters in length, and the entire micro-lattice material can be measured in centimeters (but might one day be made meters in length). Just as with large-scale structures, such as the Eiffel Tower, where order and hierarchy can lead to more efficient use of materials and improved properties, the same can be achieved by ordering materials on a tiny scale. In addition to its ultra-low density, the micro-lattice's hierarchical architecture allows it to recover almost completely from loads that compress it by as much as 50 percent, making it excellent at absorbing energy.

"The emergence of the unique properties of these ultra-light micro-lattice structures is due, in part, to the different mechanical behavior that emerges in nano-sized solids, which is the focus of my research," Greer says. Her team uses a machine called a SEMentor, which is both an electron microscope and a nanoindenter, to visualize the deformation of nano-sized structures and to concurrently measure mechanical properties, such as how much force it takes to break a material, how much energy it can absorb, and how much it stretches. The extremely small wall thickness-to-diameter ratio in the micro-lattice material makes the individual tubes ductile (i.e. they do not fail catastrophically); at higher aspect ratios, the material simply collapses and cannot recover.

The research appears in the November 18 issue of Science. Additional coauthors on the report, "Ultralight Metallic Lattices," include Caltech postdoctoral scholar Jane Lian, as well as Alan Jacobsen, Adam Sorensen, and Bill Carter from HRL Laboratories, and Anna Torrents and Lorenzo Valdevit from the University of California, Irvine. The research was funded by the Defense Advanced Research Projects Agency. 

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Kimm Fesenmaier
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Auctions, Traffic, Selfishness, and Data Privacy—It All Comes Down to Math

Every time you run a Google search, a split-second automated auction takes place to determine which of many competing companies will get to fill the ad space in your browsing window. The program controlling that auction is designed to fulfill a specific set of goals that probably differ from the goals of the individual companies. Similarly, your motivation during your morning commute is unlikely to be maintaining the overall flow of freeway traffic, and when you give a hospital your personal information, you're probably not trying to improve their data analysis.

These types of problems—where a conflict or tension exists between individual incentives and more global objectives—are Katrina Ligett's bread and butter. They fall into a category known as algorithmic game theory, and Ligett, a new assistant professor of economics and computer science at Caltech, uses ideas from computer science and mathematics to approach them.

"I'm interested in new algorithms, in understanding how difficult it is to solve problems," Ligett says. "Sometimes this comes from a particular application—maybe it's inspired by auctions that Google actually needs to run, for example."

Ligett says the field of algorithmic game theory has emerged in the last decade or so at the interface between computer science and economics. It got its start when computer scientists began to realize that they had something to offer to existing ideas in economics and game theory. For example, much of traditional game theory looks at equilibria, such as the Nash Equilibrium (developed by Nobel Prize–winning mathematician John Forbes Nash, Jr., and made famous on the big screen by A Beautiful Mind). That equilibrium describes a set of decisions that create a scenario in which no "players" want to change their decision, given how everyone else is making their decisions. "There is a huge amount of beautiful work in economics on such equilibria," Ligett says. "But there is relatively little work looking at things like how difficult it is to actually compute an equilibrium, and computer scientists have excellent tools to address such problems."

At first, computer scientists made up most of the field. But today, researchers are coming to algorithmic game theory from both directions—from computer science and from economics. Ligett says she was thrilled to come to Caltech in part because the Institute was looking for researchers to work specifically at this juncture. "There aren't that many places where computer scientists and economists actually talk to each other," Ligett says. "At Caltech, people are really interested in and committed to investigating at this intersection, and that's very appealing. 

It wasn't always clear that Ligett would become a computer scientist. In high school and for a while afterward, she worked in an Army Corps of Engineers research lab in New Hampshire focused on studying the Arctic and Antarctic regions of the world. She got to see real scientists doing research and working in the field. She even got to travel to Barrow, Alaska, to study patterns of seasonal ice melt.

So when Ligett went to college at Brown University, she thought she'd go into a lab science—perhaps chemistry or physics. But that all changed when she took her first computer-science class.  "I would get so wrapped up in problems that I was just really excited every week to work on my homework and projects for the class," she says. "So I thought, maybe I'll do a little bit more of this." Eventually, she switched over to computer science and mathematics, and went on to earn a PhD in computer science at Carnegie Mellon University.

Ligett's main research interests lie in trying to find better ways of thinking about and describing selfish behavior and in modeling alternatives to equilibrium states.  "Some of my work says, 'What makes you think that people are going to end up at an equilibrium?'" she says. "Let's talk about where people might end up or what the whole system might look like if people act in a less prescribed way and just act selfishly. " She analyzes systems that never reach equilibrium and devises alternative models to try to account for situations where, for example, individuals might try to "game the system" and alter outcomes in unexpected ways.

In the area of data privacy, she's looking at the tension between privacy and the usefulness of data, trying to come up with new theorems to describe the relationship. Take, for example, a medical database, where Ligett might examine whether it is possible to mathematically transform the data in such a way that the database would provide researchers with meaningful data while still protecting the privacy of individuals.

Although the problems Ligett deals with involve complex, dynamic situations, most of what she does is good old paper-and-pencil math. "I might write down a mathematical description of people acting a certain way, using equations to establish the types of decisions people make. Once I have a mathematical model, I study the properties and consequences of the model," she says. So those equations on the white board in Ligett's new office? They might represent online auctions, the morning commute, or issues of data privacy. "I'm interested in the fundamental mathematics and in solving problems that are as general as possible," she says. "I hope that can be applied in a bunch of different ways."

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Kimm Fesenmaier
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An Incredible Shrinking Material

Caltech Engineers Reveal How Scandium Trifluoride Contracts with Heat

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

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

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

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

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

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

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

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

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

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Marcus Woo
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Early Years of GALCIT Featured in New Display

The Guggenheim Aeronautical Laboratory at the California Institute of Technology (GALCIT) was a key component in the success of the Southern California aeronautic industry, which took flight in the 1920s. Working Together, Learning to Fly is a historical look at the research and industry collaborations at Caltech during the early years of GALCIT and is currently on display at Parsons-Gates Hall of Administration.

The exhibit, a collaboration of the Caltech Library, the Caltech Archives, GALCIT, and the Division of Engineering and Applied Science, features four cases full of documents, photographs, and images that highlight pioneers in the field, the history of Caltech's legendary wind tunnel, industry partnerships, and the important contributions that GALCIT graduates made to aviation. 

The exhibit is open Monday through Friday from 8:30 a.m. to 4:30 p.m. on the 2nd floor of Parsons-Gates. For more information, visit the exhibit's website.

 

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Caltech Retains Top Spot in Engineering and Technology Rankings

In addition to their recent ranking of Caltech as first among world universities, for the second consecutive year the Times Higher Education has also ranked the Institute first for its engineering and technology programs.

"Once again, Caltech has been recognized for its contributions to academia and society," says Ares Rosakis, chair of the Division of Engineering and Applied Science (EAS) at Caltech. "We know that rankings are often imperfect and a number one position may come and go. In contrast, our sustained impact is in the creation of new schools of thought which are the true indicators of our combined achievements and excellence in both research and education."

This year, Caltech shares the lead position with MIT; Princeton, UC Berkeley, and Stanford University round out the top five. The engineering and technology ranking is one of six subsections of the Times Higher Education World University Rankings 2011-2012. It covers a wide range of subjects—from aerospace engineering to sustainable energy research—making it "one of the most diverse of the subject tables in terms of national representation." The list is compiled data supplied by Thomson Reuters.

For a full list of the world's top 50 engineering and technology schools and all of the performance indicators, go to the Times Higher Education website.

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