Good Eggs: Nanomagnets Offer Food for Thought About Computer Memories

For a study described in a new paper, NIST researchers used electron-beam lithography to make thousands of nickel-iron magnets, each about 200 nanometers (billionths of a meter) in diameter. Each magnet is ordinarily shaped like an ellipse, a slightly flattened circle. Researchers also made some magnets in three different egglike shapes with an increasingly pointy end. It's all part of NIST research on nanoscale magnetic materials, devices and measurement methods to support development of future magnetic data storage systems.

It turns out that even small distortions in magnet shape can lead to significant changes in magnetic properties. Researchers discovered this by probing the magnets with a laser and analyzing what happens to the"spins" of the electrons, a quantum property that's responsible for magnetic orientation. Changes in the spin orientation can propagate through the magnet like waves at different frequencies. The more egg-like the magnet, the more complex the wave patterns and their related frequencies. (Something similar happens when you toss a pebble in an asymmetrically shaped pond.) The shifts are most pronounced at the ends of the magnets.

To confirm localized magnetic effects and"color" the eggs, scientists made simulations of various magnets using NIST's object-oriented micromagnetic framework (OOMMF). Lighter colors indicate stronger frequency signals.

The egg effects explain erratic behavior observed in large arrays of nanomagnets, which may be imperfectly shaped by the lithography process. Such distortions can affect switching in magnetic devices. The egg study results may be useful in developing random-access memories (RAM) based on interactions between electron spins and magnetized surfaces. Spin-RAM is one approach to making future memories that could provide high-speed access to data while reducing processor power needs by storing data permanently in ever-smaller devices. Shaping magnets like eggs breaks up a symmetric frequency pattern found in ellipse structures and thus offers an opportunity to customize and control the switching process.

"For example, intentional patterning of egg-like distortions into spinRAM memory elements may facilitate more reliable switching," says NIST physicist Tom Silva, an author of the new paper.

"Also, this study has provided the Easter Bunny with an entirely new market for product development."

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Creative, Online Learning Tool Helps Students Tackle Real-World Problems

A new computer interface developed at Iowa State University is helping students use what they've learned in the horticulture classroom and apply it to problems they'll face when they are on the job site.

The project, called ThinkSpace, is led by a group of ISU faculty including Ann Marie VanDerZanden, professor of horticulture and associate director of ISU's Center for Excellence in Learning and Teaching.

ThinkSpace has many different features that make it an effective way to teach using ill-structured problems. This type of problem allows students to choose from multiple paths to arrive at a solution.

By contrast, well-structured problems have a straight path to the one, clear solution.

In horticulture, the ThinkSpace platform is being used for upper-level classes and requires students to access what they've learned throughout their time studying horticulture and apply it to real-world problems.

In these classes, VanDerZanden gives students computer-delivered information about residential landscape.

That information includes illustrations of the work site, descriptions of the trees on the property, explanations of the problems the homeowner is experiencing, mock audio interview files with the property owner, and about anything else a horticulture professional would discover when approaching a homeowner with a landscape problem.

Also, just like in real life, some of the information is relevant to the problem, and some information is not.

"It forces students to take this piece of information, and that piece of information, and another piece of information, and then figure out what is wrong -- in this case with a plant," said VanDerZanden.

When the students think they have determined the problem, they enter their responses into the online program.

VanDerZanden can then check the responses.

For those students on the right track, she allows them to continue toward a solution.

For those who may have misdiagnosed the situation, VanDerZanden steers the students toward the right track before allowing them to move forward.

So far, the response from students has been very positive.

"The students like the variety," said VanDerZanden."They like struggling with real-world problems, rather that something that is just made up. On the other hand, they can get frustrated because there is not a clear-cut answer."

The entire process leverages the classroom experience into something the students can use at work.

"I think this really enhances student learning," said VanDerZanden."Students apply material from previous classes to a plausible, real-world situation. For instance students see what happens when a tree was pruned really hard to allow a piece of equipment to get into the customer's yard. As a result, the tree sends out a lot of new succulent shoots, and then there is an aphid infestation in the tree. It helps students start making all of those connections."

The ThinkSpace interface was developed from existing technologies already being used in ISU's College of Veterinary Medicine, College of Engineering and department of English.

VanDerZanden and her group recently received a$446,000 grant from the United States Department of Agriculture Higher Education Challenge Grant program to further develop ThinkSpace so it could more useful to other academic areas and universities.

As part of this research, VanDerZanden is also working with faculty members at University of Pennsylvania, Philadelphia; University of Wisconsin, Madison; and Kansas State University, Manhattan.

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Decoding Human Genes Is Goal of New Open-Source Encyclopedia

In a paper that will be published in the journalPLoS Biologyon 19 April 2011, the project -- called ENCODE (Encyclopedia Of DNA Elements) -- provides an overview of the team's ongoing efforts to interpret the human genome sequence, as well as a guide for using the vast amounts of data and resources produced so far by the project.

Ross Hardison, the T. Ming Chu Professor of Biochemistry and Molecular Biology at Penn State University and one of the principal investigators of the ENCODE Project team, explained that the philosophy behind the project is one of scientific openness, transparency, and collaboration across sub-disciplines. ENCODE comes on the heels of the now-complete Human Genome Project -- a 13-year effort aimed at identifying all the approximately 20,000 to 25,000 genes in human DNA -- which also was based on the belief in open-source data sharing to further scientific discovery and public understanding of science.

The ENCODE Project has accomplished this goal by publishing its database atgenome.ucsc.edu/ENCODE, and by posting tools to facilitate data use atencodeproject.org."ENCODE resources are already being used by scientists for discovery," Hardison said."But what's kind of revolutionary is that they also are being used in classes to train students in all areas of biology. Our classes here at Penn State are using real data on genomic variation and function in classroom problem sets, shortly after the labs have generated them."

Hardison explained that there are about 3-billion base pairs in the human genome, making the cataloging and interpretation of the information a monumental task."We have a very lofty goal: To identify the function of every nucleotide of the human genome," he said."Not only are we discovering the genes that give information to cells and make proteins, but we also want to know what determines that the proteins are made in the right cells, and at the appropriate time. Finding the DNA elements that govern this regulated expression of genes is a major goal of ENCODE." Hardison explained that ENCODE's job is to identify the human genome's functional regions, many of which are quite esoteric."The human DNA sequence often is described as a kind of language, but without a key to interpret it, without a full understanding of the 'grammar,' it might as well be a big jumble of letters." Hardison added that the ENCODE Project supplies data such as where proteins bind to DNA and where parts of DNA are augmented by additional chemical markers. These proteins and chemical additions are keys to understanding how different cells within the human body interpret the language of DNA.

In the soon-to-be-published paper, the team shows how the ENCODE data can be immediately useful in interpreting associations between disease and DNA sequences that can vary from person to person -- single nucleotide polymorphisms (SNPs). For example, scientists know that DNA variants located upstream of a gene called MYC are associated with multiple cancers, but until recently the mechanism behind this association was a mystery. ENCODE data already have been used to confirm that the variants can change binding of certain proteins, leading to enhanced expression of the MYC gene and, therefore, to the development of cancer. ENCODE also has made similar studies possible for thousands of other DNA variants that may be associated with susceptibility to a variety of human diseases.

Another of the principal investigators of the project, Richard Myers, president and director of the HudsonAlpha Institute for Biotechnology, explained that the ENCODE Project is unique because it requires collaboration from multiple people all over the world at the cutting edge of their fields."People are working in a coordinated manner to figure out the function of our human genome," he said."The importance of the project extends beyond basic knowledge of who and what we are as humans, and into an understanding of human health and disease."

Scientists with the ENCODE Project also are applying up to 20 different tests in 108 commonly used cell lines to compile important data. John Stamatoyannopoulos, an assistant professor of genome sciences and medicine at the University of Washington and another principal investigator, explained that the ENCODE Project has been responsible for producing many assays -- molecular-biology procedures for measuring the activity of biochemical agents -- that are now fundamental to biology."Widely used computational tools for processing and interpreting large-scale functional genomic data also have been developed by the project," Stamatoyannopoulos added."The depth, quality, and diversity of the ENCODE data are unprecedented."

Hardison said that the portion of the human genome that actually codes for protein is about 1.1 percent."That's still a lot of data," he said."And to complicate matters even more, most mechanisms for gene expression and regulation lie outside what we call the 'coding' region of DNA." Hardison explained that scientists have a limited number of tools with which to explore the genome, and one that has been used widely is inter-species comparison."For example, we can compare humans and chimpanzees and glean some fascinating information," Hardison said."But very few proteins and other DNA products differ in any fundamental way between humans and chimps. The important difference between us and our close cousins lies in gene expression -- the basic level at which genes give rise to traits such as eye color, height, and susceptibility to a particular disease. ENCODE is helping to map the very proteins involved in gene regulation and gene expression. Our paper not only explains how to find the data, but it also explains how to apply the data to interpret the human genome."

The ENCODE Project is funded, primarily, by the National Human Genome Research Institute of the U. S. National Institutes of Health.

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'3-D Towers' of Information Double Data Storage Areal Density

This greatly enhances the amount of data that can be stored in a magnetic storage device and provides a method to reach beyond a wall of physical limits that the currently used technology is hitting. The team presents their findings in the American Institute of Physics'Journal of Applied Physics.

"Over the past 50 years, with the rise of multimedia devices, the worldwide Internet, and the general growth in demand for greater data storage capacity, the areal density of information in magnetic hard disk drives has exponentially increased by 7 orders of magnitude," says Jerome Moritz, a researcher at SPINTEC, in Grenoble."This areal density is now about 500Gbit/in2, and the technology presently used involves writing the information on a granular magnetic material. This technology is now reaching some physical limits because the grains are becoming so small that their magnetization becomes unstable and the information written on them is gradually lost."

Therefore, new approaches are needed for magnetic data storage densities exceeding 1Tbit/in2.

"Our new approach involves using bit-patterned media, which are made of arrays of physically separated magnetic nanodots, with each nanodot carrying one bit of information. To further extend the storage density, it's possible to increase the number of bits per dots by stacking several magnetic layers to obtain a multilevel magnetic recording device," explains Moritz.

In that context, Moritz and colleagues were able to demonstrate that the best way to achieve a 2-bit-per-dot media involves stacking in-plane and perpendicular-to-plane magnetic media atop each dot. The perpendicularly magnetized layer can be read right above the dot, whereas the in-plane magnetized layer can be read between dots. This enables doubling of the areal density for a given dot size by taking better advantage of the whole patterned media area.

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'Time Machine' Made to Visually Explore Space and Time in Videos: Time-Lapse GigaPans Provide New Way to Access Big Data

Viewers, for instance, can use the system to focus in on the details of a booth within a panorama of a carnival midway, but also reverse time to see how the booth was constructed. Or they can watch a group of plants sprout, grow and flower, shifting perspective to watch some plants move wildly as they grow while others get eaten by caterpillars. Or, they can view a computer simulation of the early universe, watching as gravity works across 600 million light-years to condense matter into filaments and finally into stars that can be seen by zooming in for a close up.

"With GigaPan Time Machine, you can simultaneously explore space and time at extremely high resolutions," said Illah Nourbakhsh, associate professor of robotics and head of the CREATE Lab."Science has always been about narrowing your point of view -- selecting a particular experiment or observation that you think might provide insight. But this system enables what we call exhaustive science, capturing huge amounts of data that can then be explored in amazing ways."

The system is an extension of the GigaPan technology developed by the CREATE Lab and NASA, which can capture a mosaic of hundreds or thousands of digital pictures and stitch those frames into a panorama that be interactively explored via computer. To extend GigaPan into the time dimension, image mosaics are repeatedly captured at set intervals, and then stitched across both space and time to create a video in which each frame can be hundreds of millions, or even billions of pixels.

An enabling technology for time-lapse GigaPans is a feature of the HTML5 language that has been incorporated into such browsers as Google's Chrome and Apple's Safari. HTML5, the latest revision of the HyperText Markup Language (HTML) standard that is at the core of the Internet, makes browsers capable of presenting video content without use of plug-ins such as Adobe Flash or Quicktime.

Using HTML5, CREATE Lab computer scientists Randy Sargent, Chris Bartley and Paul Dille developed algorithms and software architecture that make it possible to shift seamlessly from one video portion to another as viewers zoom in and out of Time Machine imagery. To keep bandwidth manageable, the GigaPan site streams only those video fragments that pertain to the segment and/or time frame being viewed.

"We were crashing the browsers early on," Sargent recalled."We're really pushing the browser technology to the limits."

Guidelines on how individuals can capture time-lapse images using GigaPan cameras are included on the site created for hosting the new imagery's large data files,http://timemachine.gigapan.org. Sargent explained the CREATE Lab is eager to work with people who want to capture Time Machine imagery with GigaPan, or use the visualization technology for other applications.

Once a Time Machine GigaPan has been created, viewers can annotate and save their explorations of it in the form of video"Time Warps."

Though the time-lapse mode is an extension of the original GigaPan concept, scientists already are applying the visualization techniques to other types of Big Data. Carnegie Mellon's Bruce and Astrid McWilliams Center for Cosmology, for instance, has used it to visualize a simulation of the early universe performed at the Pittsburgh Supercomputing Center by Tiziana Di Matteo, associate professor of physics.

"Simulations are a huge bunch of numbers, ugly numbers," Di Matteo said."Visualizing even a portion of a simulation requires a huge amount of computing itself." Visualization of these large data sets is crucial to the science, however."Discoveries often come from just looking at it," she explained.

Rupert Croft, associate professor of physics, said cosmological simulations are so massive that only a segment can be visualized at a time using usual techniques. Yet whatever is happening within that segment is being affected by forces elsewhere in the simulation that cannot be readily accessed. By converting the entire simulation into a time-lapse GigaPan, however, Croft and his Ph.D. student, Yu Feng, were able to create an image that provided both the big picture of what was happening in the early universe and the ability to look in detail at any region of interest.

Using a conventional GigaPan camera, Janet Steven, an assistant professor of biology at Sweet Briar College in Virginia, has created time-lapse imagery of rapid-growing brassicas, known as Wisconsin Fast Plants."This is such an incredible tool for plant biology," she said."It gives you the advantage of observing individual plants, groups of plants and parts of plants, all at once."

Steven, who has received GigaPan training through the Fine Outreach for Science program, said time-lapse photography has long been used in biology, but the GigaPan technology makes it possible to observe a number of plants in detail without having separate cameras for each plant. Even as one plant is studied in detail, it's possible to also see what neighboring plants are doing and how that might affect the subject plant, she added.

Steven said creating time-lapse GigaPans of entire landscapes could be a powerful tool for studying seasonal change in plants and ecosystems, an area of increasing interest for understanding climate change. Time-lapse GigaPan imagery of biological experiments also could be an educational tool, allowing students to make independent observations and develop their own hypotheses.

Google Inc. supported development of GigaPan Time Machine.

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CAPTCHAs With Chaos: Strong Protection for Weak Passwords

Researchers at the Max Planck Institute for the Physics of Complex Systems in Dresden have been inspired by the physics of critical phenomena in their attempts to significantly improve password protection. The researchers split a password into two sections. With the first, easy-to-memorize section they encrypt a CAPTCHA ("completely automated public Turing test to tell computers and humans apart") -- an image that computer programs per se have difficulty in deciphering. The researchers also make it more difficult for computers, whose task it is to automatically crack passwords, to read the passwords without authorization. They use images of a simulated physical system, which they additionally make unrecognizable with a chaotic process. These p-CAPTCHAs enable the Dresden physicists to achieve a high level of password protection, even though the user need only remember a weak password.

Computers sometimes use brute force. Hacking programs use so-called brute-force attacks to try out all possible character combinations to guess passwords. CAPTCHAs are therefore intended as an additional safeguard the input of which originates from a human being and not from a machine. They pose a task for the user which is simple enough for any human, yet very difficult for a program. Users must enter a distorted text which is displayed on the screen, for example. CAPTCHAs are increasingly being bypassed, however. Personal data of members of the"SchülerVZ" social network for school pupils have already been stolen in this way.

Researchers at the Max Planck Institute for the Physics of Complex Systems in Dresden have now developed a new type of password protection that is based on a combination of characters and a CAPTCHA. They also use mathematical methods from the physics of critical phenomena to protect the CAPTCHA from being accessed by computers."We thus make the password protection both more effective and simpler," says Konstantin Kladko, who had the idea for this interdisciplinary approach during his time at the Dresden Max Planck Institute; he is currently a researcher at Axioma Research in Palo Alto/USA.

The Dresden-based researchers initially combine password and CAPTCHA in a completely novel way. The CAPTCHA is no longer generated anew each time in order to distinguish the human user from a computer on a case-by-case basis. Rather, the physicists use the codeword in the image, which can only be deciphered by humans as the real password, which provides access to a social network or an online bank account, for example. The researchers additionally encrypt this password using a combination of characters.

However, that's not all: the CAPTCHA is a snapshot of a dynamic, chaotic Hamiltonian system in two dimensions. For the sake of simplicity, his image can be imagined as a grey-scale pixel matrix, where every pixel represents an oscillator. The oscillators are coupled in a network. Every oscillator oscillates between two states and is affected by the neighbouring oscillators as it does so, thus resulting in the grey scales.

Chaotic development makes password unreadable

The physicists then leave the system to develop chaotically for a period of time. The grey-scale matrix changes the colour of its pixels. The result is an image that no longer contains a recognizable word. The researchers subsequently encrypt this image with the combination of characters and save the result."We therefore talk of a password-protected CAPTCHA or p-CAPTCHA," says Sergej Flach, who teamed up with Tetyana Laptyeva to achieve the decisive research results at the Max Planck Institute for the Physics of Complex Systems. Since the chaotic evolution of the initial image is deterministic, i.e. reversible, the whole procedure can be reversed using the combination of characters, so that the user can again read the password hidden in the CAPTCHA.

"The character combination we use to encrypt the password in the CAPTCHA can be very easy to remember," explains Konstantin Kladko."We thus take account of the fact that most people only want to, or can only, remember simple passwords." The fact that the passwords are correspondingly weak is now no longer important, because the real protection comes from the encrypted password in the CAPTCHA.

On the one hand, the password hidden in the CAPTCHA is too long for computers to be able to guess it using a brute-force attack in a reasonable length of time. On the other, the physicists use a critical system to generate the password image. This system is close to a phase transition: with a phase transition, the system changes from one physical state to another, from the paramagnetic to the ferromagnetic state, for example. Close to the transition, regions repeatedly form which temporarily have already completed the transition."The resulting image is always very grainy. Therefore, a computer cannot distinguish it from the original it is searching for," explains Sergej Flach.

"Although the study has just been submitted to a specialist journal and is only available online in an archive, it has already provoked a large number of responses in the community -- and not only in Hacker News," says Sergej Flach."I was very impressed by the depth of some comments in certain forums -- in Slashdot, for example." The specialists are obviously impressed by the ingenuity of the approach, which means passwords could be very difficult to crack in the future. Moreover, the method is easy and quick to implement in conventional computer systems."An expansion to several p-CAPTCHA levels is obvious," says Sergej Flach. Hoiwever, this requires increased computing power to reverse the chaotic development in a reasonable time:"We therefore want to investigate various Hamiltonian and non-Hamiltonian systems in the future to see whether they provide faster and even more effective protection."

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New Kid on the Plasmonic Block: Researchers Find Plasmonic Resonances in Semiconductor Nanocrystals

"We have demonstrated well-defined localized surface plasmon resonances arising from p-type carriers in vacancy-doped semiconductor quantum dots that should allow for plasmonic sensing and manipulation of solid-state processes in single nanocrystals," says Berkeley Lab director Paul Alivisatos, a nanochemistry authority who led this research."Our doped semiconductor quantum dots also open up the possibility of strongly coupling photonic and electronic properties, with implications for light harvesting, nonlinear optics, and quantum information processing."

Alivisatos is the corresponding author of a paper in the journalNature Materialstitled"Localized surface plasmon resonances arising from free carriers in doped quantum dots." Co-authoring the paper were Joseph Luther and Prashant Jain, along with Trevor Ewers.

The term"plasmonics" describes a phenomenon in which the confinement of light in dimensions smaller than the wavelength of photons in free space make it possible to match the different length-scales associated with photonics and electronics in a single nanoscale device. Scientists believe that through plasmonics it should be possible to design computer chip interconnects that are able to move much larger amounts of data much faster than today's chips. It should also be possible to create microscope lenses that can resolve nanoscale objects with visible light, a new generation of highly efficient light-emitting diodes, and supersensitive chemical and biological detectors. There is even evidence that plasmonic materials can be used to bend light around an object, thereby rendering that object invisible.

The plasmonic phenomenon was discovered in nanostructures at the interfaces between a noble metal, such as gold or silver, and a dielectric, such as air or glass. Directing an electromagnetic field at such an interface generates electronic surface waves that roll through the conduction electrons on a metal, like ripples spreading across the surface of a pond that has been plunked with a stone. Just as the energy in an electromagnetic field is carried in a quantized particle-like unit called a photon, the energy in such an electronic surface wave is carried in a quantized particle-like unit called a plasmon. The key to plasmonic properties is when the oscillation frequency between the plasmons and the incident photons matches, a phenomenon known as localized surface plasmon resonance (LSPR). Conventional scientific wisdom has held that LSPRs require a metal nanostructure , where the conduction electrons are not strongly attached to individual atoms or molecules. This has proved not to be the case as Prashant Jain, a member of the Alivisatos research group and one of the lead authors of the Nature Materials paper, explains.

"Our study represents a paradigm shift from metal nanoplasmonics as we've shown that, in principle, any nanostructure can exhibit LSPRs so long as the interface has an appreciable number of free charge carriers, either electrons or holes," Jain says."By demonstrating LSPRs in doped quantum dots, we've extended the range of candidate materials for plasmonics to include semiconductors, and we've also merged the field of plasmonic nanostructures, which exhibit tunable photonic properties, with the field of quantum dots, which exhibit tunable electronic properties."

Jain and his co-authors made their quantum dots from the semiconductor copper sulfide, a material that is known to support numerous copper-deficient stoichiometries. Initially, the copper sulfide nanocrystals were synthesized using a common hot injection method. While this yielded nanocrystals that were intrinsically self-doped with p-type charge carriers, there was no control over the amount of charge vacancies or carriers.

"We were able to overcome this limitation by using a room-temperature ion exchange method to synthesize the copper sulfide nanocrystals," Jain says."This freezes the nanocrystals into a relatively vacancy-free state, which we can then dope in a controlled manner using common chemical oxidants."

By introducing enough free electrical charge carriers via dopants and vacancies, Jain and his colleagues were able to achieve LSPRs in the near-infrared range of the electromagnetic spectrum. The extension of plasmonics to include semiconductors as well as metals offers a number of significant advantages, as Jain explains.

"Unlike a metal, the concentration of free charge carriers in a semiconductor can be actively controlled by doping, temperature, and/or phase transitions," he says."Therefore, the frequency and intensity of LSPRs in dopable quantum dots can be dynamically tuned. The LSPRs of a metal, on the other hand, once engineered through a choice of nanostructure parameters, such as shape and size, is permanently locked-in."

Jain envisions quantum dots as being integrated into a variety of future film and chip-based photonic devices that can be actively switched or controlled, and also being applied to such optical applications as in vivo imaging. In addition, the strong coupling that is possible between photonic and electronic modes in such doped quantum dots holds exciting potential for applications in solar photovoltaics and artificial photosynthesis

"In photovoltaic and artificial photosynthetic systems, light needs to be absorbed and channeled to generate energetic electrons and holes, which can then be used to make electricity or fuel," Jain says."To be efficient, it is highly desirable that such systems exhibit an enhanced interaction of light with excitons. This is what a doped quantum dot with an LSPR mode could achieve."

The potential for strongly coupled electronic and photonic modes in doped quantum dots arises from the fact that semiconductor quantum dots allow for quantized electronic excitations (excitons), while LSPRs serve to strongly localize or confine light of specific frequencies within the quantum dot. The result is an enhanced exciton-light interaction. Since the LSPR frequency can be controlled by changing the doping level, and excitons can be tuned by quantum confinement, it should be possible to engineer doped quantum dots for harvesting the richest frequencies of light in the solar spectrum.

Quantum dot plasmonics also hold intriguing possibilities for future quantum communication and computation devices.

"The use of single photons, in the form of quantized plasmons, would allow quantum systems to send information at nearly the speed of light, compared with the electron speed and resistance in classical systems," Jain says."Doped quantum dots by providing strongly coupled quantized excitons and LSPRs and within the same nanostructure could serve as a source of single plasmons."

Jain and others in Alivsatos' research group are now investigating the potential of doped quantum dots made from other semiconductors, such as copper selenide and germanium telluride, which also display tunable plasmonic or photonic resonances. Germanium telluride is of particular interest because it has phase change properties that are useful for memory storage devices.

"A long term goal is to generalize plasmonic phenomena to all doped quantum dots, whether heavily self-doped or extrinsically doped with relatively few impurities or vacancies," Jain says.

This research was supported by the DOE Office of Science.

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