Physics experiment with ultrafast laser pulses produces a previously unseen phase of matter.
|An artist's impression of a light-induced charge density wave (CDW). The wavy mesh represents distortions of the material’s lattice structure caused by the formation of CDWs. Glowing spheres represent photons. In the center, the original CDW is suppressed by a brief pulse of laser light, while a new CDW appears at right angles to the first. Image, Alfred Zong.|
Adding energy to any material, such as by heating it, almost always makes its structure less orderly. Ice, for example, with its crystalline structure, melts to become liquid water, with no order at all.
But in new experiments by physicists at MIT and elsewhere, the opposite happens: When a pattern called a charge density wave in a certain material is hit with a fast laser pulse, a whole new charge density wave is created — a highly ordered state, instead of the expected disorder. The surprising finding could help to reveal unseen properties in materials of all kinds.
The discovery is being reported today in the journal Nature Physics, in a paper by MIT professors Nuh Gedik and Pablo Jarillo-Herrero, postdoc Anshul Kogar, graduate student Alfred Zong, and 17 others at MIT, Harvard University, SLAC National Accelerator Laboratory, Stanford University, and Argonne National Laboratory.
The experiments made use of a material called lanthanum tritelluride, which naturally forms itself into a layered structure. In this material, a wavelike pattern of electrons in high- and low-density regions forms spontaneously but is confined to a single direction within the material. But when hit with an ultrafast burst of laser light — less than a picosecond long, or under one trillionth of a second — that pattern, called a charge density wave or CDW, is obliterated, and a new CDW, at right angles to the original, pops into existence.
This new, perpendicular CDW is something that has never been observed before in this material. It exists for only a flash, disappearing within a few more picoseconds. As it disappears, the original one comes back into view, suggesting that its presence had been somehow suppressed by the new one.
Gedik explains that in ordinary materials, the density of electrons within the material is constant throughout their volume, but in certain materials, when they are cooled below some specific temperature, the electrons organize themselves into a CDW with alternating regions of high and low electron density. In lanthanum tritelluride, or LaTe3, the CDW is along one fixed direction within the material. In the other two dimensions, the electron density remains constant, as in ordinary materials.
The perpendicular version of the CDW that appears after the burst of laser light has never before been observed in this material, Gedik says. It “just briefly flashes, and then it’s gone,” Kogar says, to be replaced by the original CDW pattern which immediately pops back into view.
Gedik points out that “this is quite unusual. In most cases, when you add energy to a material, you reduce order.”
“It’s as if these two [kinds of CDW] are competing — when one shows up, the other goes away,” Kogar says. “I think the really important concept here is phase competition.”
The idea that two possible states of matter might be in competition and that the dominant mode is suppressing one or more alternative modes is fairly common in quantum materials, the researchers say. This suggests that there may be latent states lurking unseen in many kinds of matter that could be unveiled if a way can be found to suppress the dominant state. That is what seems to be happening in the case of these competing CDW states, which are considered to be analogous to crystal structures because of the predictable, orderly patterns of their subatomic constituents.
Normally, all stable materials are found in their minimum energy states — that is, of all possible configurations of their atoms and molecules, the material settles into the state that requires the least energy to maintain itself. But for a given chemical structure, there may be other possible configurations the material could potentially have, except that they are suppressed by the dominant, lowest-energy state.
“By knocking out that dominant state with light, maybe those other states can be realized,” Gedik says. And because the new states appear and disappear so quickly, “you can turn them on and off,” which may prove useful for some information processing applications.
The possibility that suppressing other phases might reveal entirely new material properties opens up many new areas of research, Kogar says. “The goal is to find phases of material that can only exist out of equilibrium,” he says — in other words, states that would never be attainable without a method, such as this system of fast laser pulses, for suppressing the dominant phase.
Gedik adds that “normally, to change the phase of a material you try chemical changes, or pressure, or magnetic fields. In this work, we are using light to make these changes.”
The new findings may help to better understand the role of phase competition in other systems. This in turn can help to answer questions like why superconductivity occurs in some materials at relatively high temperatures, and may help in the quest to discover even higher-temperature superconductors. Gedik says, “What if all you need to do is shine light on a material, and this new state comes into being?”
The work was supported by the U.S. Department of Energy, SLAC National Accelerator Laboratory, the Skoltech-MIT NGP Program, the Center for Excitonics, and the Gordon and Betty Moore Foundation.
– David L. Chandler | MIT News Office
November 11, 2019
Study of minerals widely used in industrial processes could lead to discovery of new materials for catalysis and filtering.
Zeolites are a class of natural or manufactured minerals with a sponge-like structure, riddled with tiny pores that make them useful as catalysts or ultrafine filters. But of the millions of zeolite compositions that are theoretically possible, so far only about 248 have ever been discovered or made. Now, research from MIT helps explain why only this small subset has been found, and could help scientists find or produce more zeolites with desired properties.
The new findings are reported in the journal Nature Materials, in a paper by MIT graduate students Daniel Schwalbe-Koda and Zach Jensen, and professors Elsa Olivetti and Rafael Gomez-Bombarelli.
|Traditional structure-based representations of the many forms of zeolites, some of which are illustrated here, provide little guidance as to how they can convert to other forms, but a new graph-based system does a much better job. Illustration courtesy of the researchers.|
Previous attempts to figure out why only this small group of possible zeolite compositions has been identified, and to explain why certain types of zeolites can be transformed into specific other types, have failed to come up with a theory that matches the observed data. Now, the MIT team has developed a mathematical approach to describing the different molecular structures. The approach is based on graph theory, which can predict which pairs of zeolite types can be transformed from one to the other.
This could be an important step toward finding ways of making zeolites tailored for specific purposes. It could also lead to new pathways for production, since it predicts certain transformations that have not been previously observed. And, it suggests the possibility of producing zeolites that have never been seen before, since some of the predicted pairings would lead to transformations into new types of zeolite structures.
Zeolites are widely used today in applications as varied as catalyzing the “cracking” of petroleum in refineries and absorbing odors as components in cat litterbox filler. Even more applications may become possible if researchers can create new types of zeolites, for example with pore sizes suited to specific types of filtration.
All kinds of zeolites are silicate minerals, similar in chemical composition to quartz. In fact, over geological timescales, they will all eventually turn into quartz — a much denser form of the mineral — explains Gomez-Bombarelli, who is the Toyota Assistant Professor in Materials Processing. But in the meantime, they are in a “metastable” form, which can sometimes be transformed into a different metastable form by applying heat or pressure or both. Some of these transformations are well-known and already used to produce desired zeolite varieties from more readily available natural forms.
Currently, many zeolites are produced by using chemical compounds known as OSDAs (organic structure-directing agents), which provide a kind of template for their crystallization. But Gomez-Bombarelli says that if instead they can be produced through the transformation of another, readily available form of zeolite, “that’s really exciting. If we don’t need to use OSDAs, then it’s much cheaper [to produce the material]. The organic material is pricey. Anything we can make to avoid the organics gets us closer to industrial-scale production.
Traditional chemical modeling of the structure of different zeolite compounds, researchers have found, provides no real clue to finding the pairs of zeolites that can readily transform from one to the other.
Compounds that appear structurally similar sometimes are not subject to such transformations, and other pairs that are quite dissimilar turn out to easily interchange. To guide their research, the team used an artificial intelligence system previously developed by the Olivetti group to “read” more than 70,000 research papers on zeolites and select those that specifically identify interzeolite transformations. They then studied those pairs in detail to try to identify common characteristics.
What they found was that a topological description based on graph theory, rather than traditional structural modeling, clearly identified the relevant pairings. These graph-based descriptions, based on the number and locations of chemical bonds in the solids rather than their actual physical arrangement, showed that all the known pairings had nearly identical graphs. No such identical graphs were found among pairs that were not subject to transformation.
The finding revealed a few previously unknown pairings, some of which turned out to match with preliminary laboratory observations that had not previously been identified as such, thus helping to validate the new model. The system also was successful at predicting which forms of zeolites can intergrow — forming combinations of two types that are interleaved like the fingers on two clasped hands. Such combinations are also commercially useful, for example for sequential catalysis steps using different zeolite materials.
Ripe for further research
The new findings might also help explain why many of the theoretically possible zeolite formations don’t seem to actually exist. Since some forms readily transform into others, it may be that some of them transform so quickly that they are never observed on their own. Screening using the graph-based approach may reveal some of these unknown pairings and show why those short-lived forms are not seen.
Some zeolites, according to the graph model, “have no hypothetical partners with the same graph, so it doesn’t make sense to try to transform them, but some have thousands of partners” and thus are ripe for further research, Gomez-Bombarelli says.
In principle, the new findings could lead to the development of a variety of new catalysts, tuned to the exact chemical reactions they are intended to promote. Gomez-Bombarelli says that almost any desired reaction could hypothetically find an appropriate zeolite material to promote it.
“Experimentalists are very excited to find a language to describe their transformations that is predictive,” he says.
This work is “a major advancement in the understanding of interzeolite transformations, which has become an increasingly important topic owing to the potential for using these processes to improve the efficiency and economics of commercial zeolite production,” says Jeffrey Rimer, an associate professor of chemical and biomolecular engineering at the University of Houston, who was not involved in this research.
Manuel Moliner, a tenured scientist at the Technical University of Valencia, in Spain, who also was not connected to this research, says: “Understanding the pairs involved in particular interzeolite transformations, considering not only known zeolites but also hundreds of hypothetical zeolites that have not ever been synthesized, opens extraordinary practical opportunities to rationalize and direct the synthesis of target zeolites with potential interest as industrial catalysts.”
This research was supported, in part, by the National Science Foundation and the Office of Naval Research.
– David L. Chandler | MIT News Office
October 7, 2019
MIT Materials Research Laboratory 2019 Poster Session winners are Mechanical Engineering graduate student Erin Looney, Media Arts and Sciences graduate student Bianca Datta, and Materials Science and Engineering Postdoctoral Associate Michael Chon.
The Poster Session was held immediately after the Materials Day Symposium on Oct. 9, 2019. Winners, who were selected by non-MIT affiliated attendees, each receive a $500 award.
Media Arts and Sciences graduate student
POSTER: “Simulation-based optimization towards fabrication of bio-inspired nanostructures exhibiting structural coloration.”
Datta is using simulation techniques and rapid prototyping to design surfaces that display color like butterfly wings.
Advisor: Christine Ortiz, Morris Cohen Professor of Materials Science and Engineering
POSTER: “High capacity CMOS-compatible thin film batteries on flexible substrates”
Chon is developing all solid-state flexible microbatteries that combine a germanium anode, a ruthenium dioxide cathode and lithium phosphorous oxynitride (LiPON) solid electrolyte. The thin film batteries can be stacked and folded or incorporated directly into integrated circuits.
Advisor: Carl V. Thompson, Stavros Salapatas Professor of Materials Science and Engineering, and Director, Materials Research Laboratory
Erin E. Looney
Mechanical Engineering graduate student
POSTER: “Machine learning-based classification of environmental conditions for PV module testing and design”
Looney simulates solar cell material operation under real world conditions by combining temperature, solar spectra and humidity data to estimate performance with 95 percent accuracy. She showed that a statistical method called a k-means algorithm can produce these results with 1,000 times fewer data inputs.
Advisor: Tonio Buonassisi, Professor of Mechanical Engineering
– Materials Research Laboratory
Updated October 28, 2019
New method from MIT’s research enterprise in Singapore paves the way for improved optoelectronic and 5G devices.
|A LEES researcher reviews a 200 mm Silicon III-V wafer. Photo: SMART|
The Singapore-MIT Alliance for Research and Technology (SMART), MIT’s research enterprise in Singapore, has announced the successful development of a commercially viable way to manufacture integrated silicon III-V chips with high-performance III-V devices inserted into their design.
In most devices today, silicon-based CMOS chips are used for computing, but they are not efficient for illumination and communications, resulting in low efficiency and heat generation. This is why current 5G mobile devices on the market get very hot upon use and can shut down after a short time.
This is where III-V semiconductors are valuable. III-V chips are made with compounds including elements in the third and fifth columns of the periodic table, such as gallium nitride (GaN) and indium gallium arsenide (InGaAs). Due to their unique properties, they are exceptionally well-suited for optoelectronics (such as LEDs) and communications (such as 5G wireless), boosting efficiency substantially.
“By integrating III-V into silicon, we can build upon existing manufacturing capabilities and low-cost volume production techniques of silicon and include the unique optical and electronic functionality of III-V technology,” says Eugene Fitzgerald, CEO and director of SMART and the Merton C. Flemings-SMA Professor of Materials Science and Engineering at MIT. “The new chips will be at the heart of future product innovation and power the next generation of communications devices, wearables, and displays.”
Kenneth Lee, senior scientific director of the SMART Low Energy Electronic Systems (LEES) research program, adds: “Integrating III-V semiconductor devices with silicon in a commercially viable way is one of the most difficult challenges faced by the semiconductor industry, even though such integrated circuits have been desired for decades. Current methods are expensive and inefficient, which is delaying the availability of the chips the industry needs. With our new process, we can leverage existing capabilities to manufacture these new integrated silicon III-V chips cost-effectively and accelerate the development and adoption of new technologies that will power economies.”
The new technology developed by SMART builds two layers of silicon and III-V devices on separate substrates and integrates them vertically together within a micron, which is 1/50th the diameter of a human hair. The process can use existing 200 micrometer manufacturing tools, which will allow semiconductor manufacturers in Singapore and around the world to make new use of their current equipment. Today, the cost of investing in a new manufacturing technology is in the range of tens of billions of dollars; the new integrated circuit platform is highly cost-effective, and will result in much lower-cost novel circuits and electronic systems.
SMART is focusing on creating new chips for pixelated illumination/display and 5G markets, which has a combined potential market of over $100 billion. Other markets that SMART’s new integrated silicon III-V chips will disrupt include wearable mini-displays, virtual reality applications, and other imaging technologies.
The patent portfolio has been exclusively licensed by New Silicon Corporation (NSC), a Singapore-based spinoff from SMART. NSC is the first fabless silicon integrated circuit company with proprietary materials, processes, devices, and design for monolithic integrated silicon III-V circuits.
SMART’s new integrated Silicon III-V chips will be available next year and expected in products by 2021.
SMART’s LEES Interdisciplinary Research Group is creating new integrated circuit technologies that result in increased functionality, lower power consumption, and higher performance for electronic systems. These integrated circuits of the future will impact applications in wireless communications, power electronics, LED lighting, and displays. LEES has a vertically-integrated research team possessing expertise in materials, devices, and circuits, comprising multiple individuals with professional experience within the semiconductor industry. This ensures that the research is targeted to meet the needs of the semiconductor industry both within Singapore and globally.
– Singapore-MIT Alliance for Research and Technology | MIT News
October 3, 2019
The American Ceramic Society honors Prof. Michael Cima with The W. David Kingery Award at the Society’s Annual Honor and Awards banquet, September 30, 2019, in Portland, Ore., during the ACerS Annual Meeting held at the Materials Science and Technology Conference.
The Materials Research Laboratory (MRL) will celebrate renovations to the Electron Microscopy (EM) Shared Experimental Facility in Building 13 on Monday, Oct. 7, 2019, from 3:00 to 5:00 p.m. The event is open only to the MIT community.
|A specially designed transmission electron microscope in MIT Materials Research Laboratory’s newly renovated Electron Microscopy (EM) Shared Facility in Building 13. Photo, Denis Paiste, Materials Research Laboratory.|
The EM suite, which is part of the National Science Foundation-funded Materials Research Science and Engineering Center (MRSEC) within MRL, is now home to an ultra-high vacuum evaporator system and specially designed transmission electron microscope, which IBM donated to MIT. The equipment will be used by Frances M. Ross, the Ellen Swallow Richards Professor in Materials Science and Engineering, who joined the Department of Materials Science and Engineering (DMSE) faculty last year, moving from the Nanoscale Materials Analysis department at the IBM Thomas J. Watson Research Center.
Greene Construction completed the EM suite renovations, which included new flooring and lighting, a new entrance, repainting, and an updated meeting area with video presentation capability for meetings or teaching.
Research shows that, contrary to accepted rule of thumb, a 10- or 15-year lifetime can be good enough.
|A new study shows that replacing new solar panels after just 10 or 15 years, using the existing mountings and control systems, can make economic sense, contrary to industry expectations that a 25-year lifetime is necessary.|
A new study shows that, contrary to widespread belief within the solar power industry, new kinds of solar cells and panels don’t necessarily have to last for 25 to 30 years in order to be economically viable in today’s market.
Rather, solar panels with initial lifetimes of as little as 10 years can sometimes make economic sense, even for grid-scale installations — thus potentially opening the door to promising new solar photovoltaic technologies that have been considered insufficiently durable for widespread use.
The new findings are described in a paper in the journal Joule, by Joel Jean, a former MIT postdoc and CEO of startup company Swift Solar; Vladimir Bulović, professor of electrical engineering and computer science and director of MIT.nano; and Michael Woodhouse of the National Renewable Energy Laboratory (NREL) in Colorado.
“When you talk to people in the solar field, they say any new solar panel has to last 25 years,” Jean says. “If someone comes up with a new technology with a 10-year lifetime, no one is going to look at it. That’s considered common knowledge in the field, and it’s kind of crippling.”
Jean adds that “that’s a huge barrier, because you can’t prove a 25-year lifetime in a year or two, or even 10.” That presumption, he says, has left many promising new technologies stuck on the sidelines, as conventional crystalline silicon technologies overwhelmingly dominate the commercial solar marketplace. But, the researchers found, that does not need to be the case.
“We have to remember that ultimately what people care about is not the cost of the panel; it’s the levelized cost of electricity,” he says. In other words, it’s the actual cost per kilowatt-hour delivered over the system’s useful lifetime, including the cost of the panels, inverters, racking, wiring, land, installation labor, permitting, grid interconnection, and other system components, along with ongoing maintenance costs.
Part of the reason that the economics of the solar industry look different today than in the past is that the cost of the panels (also known as modules) has plummeted so far that now, the “balance of system” costs — that is, everything except the panels themselves — exceeds that of the panels. That means that, as long as newer solar panels are electrically and physically compatible with the racking and electrical systems, it can make economic sense to replace the panels with newer, better ones as they become available, while reusing the rest of the system.
“Most of the technology is in the panel, but most of the cost is in the system,” Jean says. “Instead of having a system where you install it and then replace everything after 30 years, what if you replace the panels earlier and leave everything else the same? One of the reasons that might work economically is if you’re replacing them with more efficient panels,” which is likely to be the case as a wide variety of more efficient and lower-cost technologies are being explored around the world.
He says that what the team found in their analysis is that “with some caveats about financing, you can, in theory, get to a competitive cost, because your new panels are getting better, with a lifetime as short as 15 or even 10 years.”
Although the costs of solar cells have come down year by year, Bulović says, “the expectation that one had to demonstrate a 25-year lifetime for any new solar panel technology has stayed as a tautology. In this study we show that as the solar panels get less expensive and more efficient, the cost balance significantly changes.”
He says that one aim of the new paper is to alert the researchers that their new solar inventions can be cost-effective even if relatively short lived, and hence may be adopted and deployed more rapidly than expected. At the same time, he says, investors should know that they stand to make bigger profits by opting for efficient solar technologies that may not have been proven to last as long, knowing that periodically the panels can be replaced by newer, more efficient ones.
“Historical trends show that solar panel technology keeps getting more efficient year after year, and these improvements are bound to continue for years to come,” says Bulović. Perovskite-based solar cells, for example, when first developed less than a decade ago, had efficiencies of only a few percent. But recently their record performance exceeded 25 percent efficiency, compared to 27 percent for the record silicon cell and about 20 percent for today’s standard silicon modules, according to Bulović. Importantly, in novel device designs, a perovskite solar cell can be stacked on top of another perovskite, silicon, or thin-film cell, to raise the maximum achievable efficiency limit to over 40 percent, which is well above the 30 percent fundamental limit of today’s silicon solar technologies. But perovskites have issues with longevity of operation and have not yet been shown to be able to come close to meeting the 25-year standard.
Bulović hopes the study will “shift the paradigm of what has been accepted as a global truth.” Up to now, he says, “many promising technologies never even got a start, because the bar is set too high” on the need for durability.
For their analysis, the team looked at three different kinds of solar installations: a typical 6-kilowatt residential system, a 200-kilowatt commercial system, and a large 100-megawatt utility-scale system with solar tracking. They used NREL benchmark parameters for U.S. solar systems and a variety of assumptions about future progress in solar technology development, financing, and the disposal of the initial panels after replacement, including recycling of the used modules. The models were validated using four independent tools for calculating the levelized cost of electricity (LCOE), a standard metric for comparing the economic viability of different sources of electricity.
In all three installation types, they found, depending on the particulars of local conditions, replacement with new modules after 10 to 15 years could in many cases provide economic advantages while maintaining the many environmental and emissions-reduction benefits of solar power. The basic requirement for cost-competitiveness is that any new solar technology that is to be installed in the U.S should start with a module efficiency of at least 20 percent, a cost of no more than 30 cents per watt, and a lifetime of at least 10 years, with the potential to improve on all three.
Jean points out that the solar technologies that are considered standard today, mostly silicon-based but also thin-film variants such as cadmium telluride, “were not very stable in the early years. The reason they last 25 to 30 years today is that they have been developed for many decades.” The new analysis may now open the door for some of the promising newer technologies to be deployed at sufficient scale to build up similar levels of experience and improvement over time and to make an impact on climate change earlier than they could without module replacement, he says.
The study was supported by the Tata-MIT GridEdge Solar research program.
– David L. Chandler | MIT News Office
September 19, 2019
Brian Anthony, co-leader of SENSE.nano, discusses sensing for augmented and virtual reality and for advanced manufacturing.
Sensors are everywhere today, from our homes and vehicles to medical devices, smart phones, and other useful tech. More and more, sensors help detect our interactions with the environment around us — and shape our understanding of the world.
SENSE.nano is an MIT.nano Center of Excellence, with a focus on sensors, sensing systems, and sensing technologies. The 2019 SENSE.nano Symposium, taking place on Sept. 30 at MIT, will dive deep into the impact of sensors on two topics: sensing for augmented and virtual reality (AR/VR) and sensing for advanced manufacturing.
MIT Principal Research Scientist Brian W. Anthony is the associate director of MIT.nano and faculty director of the Industry Immersion Program in Mechanical Engineering. He weighs in on why sensing is ubiquitous and how advancements in sensing technologies are linked to the challenges and opportunities of big data.
Q: What do you see as the next frontier for sensing as it relates to augmented and virtual reality?
A: Sensors are an enabling technology for AR/VR. When you slip on a VR headset and enter an immersive environment, sensors map your movements and gestures to create a convincing virtual experience.
But sensors have a role beyond the headset. When we're interacting with the real world we're constrained by our own senses — seeing, hearing, touching, and feeling. But imagine sensors providing data within AR/VR to enhance your understanding of the physical environment, such as allowing you to see air currents, thermal gradients, or the electricity flowing through wires superimposed on top of the real physical structure. That's not something you could do any place else other than a virtual environment.
Another example: MIT.nano is a massive generator of data. Could AR/VR provide a more intuitive and powerful way to study information coming from the metrology instruments in the basement, or the fabrication tools in the clean room? Could it allow you to look at data on a massive scale, instead of always having to look under a microscope or on a flat screen that's the size of your laptop? Sensors are also critical for haptics, which are interactions related to the sensation of touch. As I apply pressure to a device or pick up an object — real or virtual — can I receive physical feedback that conveys that state of interaction to me?
You can’t be an engineer or a scientist without being involved with sensing instrumentation in some way. Recognizing the widespread presence of sensing on campus, SENSE.nano and MIT.nano — with MIT.nano’s new Immersion Lab providing the tools and facility — are trying to bring together researchers on both the hardware and software sides to explore the future of these technologies.
Q: Why is SENSE.nano focusing on sensing for advanced manufacturing?
A: In this era of big data, we sometimes forget that data comes from someplace: sensors and instruments. As soon as the data industry as a whole has solved the big data challenges we have now with the data that's coming from current sensors — wearable physiological monitors, or from factories, or from your automobiles — it is going to be starved for new sensors with improved functionality.
Coupled with that, there are a large number of manufacturing technologies — in the U.S. and worldwide — that are either coming to maturity or receiving a lot of investment. For example, researchers are looking at novel ways to make integrated photonics devices combining electronics and optics for on-chip sensors; exploring novel fiber manufacturing approaches to embed sensors into your clothing or composites; and developing flexible materials that mold to the body or to the shape of an automobile as the substrate for integrated circuits or as a sensor. These various manufacturing technologies enable us to think of new, innovative ways to create sensors that are lower in cost and more readily immersed into our environment.
Q: You’ve said that a factory is not just a place that produces products, but also a machine that produces information. What does that mean?
A: Today’s manufacturers have to approach a factory not just as a physical place, but also as a data center. Seeing physical operation and data as interconnected can improve quality, drive down costs, and increase the rate of production. And sensors and sensing systems are the tools to collect this data and improve the manufacturing process.
Communications technologies now make it easy to transmit data from a machine to a central location. For example, we can apply sensing techniques to individual machines and then collect data across an entire factory so that information on how to debug one computer-controlled machine can be used to improve another in the same facility. Or, suppose I'm the producer of those machines and I've deployed them to any number of manufacturers. If I can get a little bit of information from each of my customers to optimize the machine’s operating performance, I can turn around and share improvements with all the companies who purchase my equipment. When information is shared amongst manufacturers, it helps all of them drive down their costs and improve quality.
– MIT.nano | MIT News Office
September 20, 2019
MIT researchers answer puzzling question why magnetism in certain materials is different in atomically thin layers and their bulk forms.
|MIT Physics graduate student Dahlia R. Klein, left, and postdoc David MacNeill showed that the magnetic order and stacking order are very strongly linked in two-dimensional (2D) magnets such as chromium chloride and chromium iodide, giving engineers a tool to vary the material’s magnetic properties. Photo, Denis Paiste, Materials Research Laboratory.|
Researchers led by MIT Physics Professor Pablo Jarillo-Herrero last year showed that rotating layers of hexagonally structured graphene at a particular “magic angle” could change the material’s electronic properties from an insulating state to a superconducting state. Now researchers in the same group and their collaborators have demonstrated that in a different ultra-thin material that also features a honeycomb-shaped atomic structure – chromium trichloride (CrCl3) – they can alter the material’s magnetic properties by shifting the stacking order of layers.
The researchers peeled away two-dimensional (2D) layers of chromium trichloride using tape in the same way researchers peel away graphene from graphite. Then they studied the 2D chromium trichloride’s magnetic properties using electron tunneling. They found that the magnetism is different in 2D and 3D crystals due to different stacking arrangements between atoms in adjacent layers.
At high temperatures, each chromium atom in chromium trichloride has a magnetic moment that fluctuates like a tiny compass needle. Experiments show that as the temperature drops below 14 Kelvin (-434.47 F), deep in the cryogenic temperature range, these magnetic moments freeze into an ordered pattern, pointing in opposite directions in alternating layers (antiferromagnetism). The magnetic direction of all the layers of chromium trichloride can be aligned by applying a magnetic field. But the researchers found that in its 2D form, this alignment needs a magnetic force 10 times stronger than in the 3D crystal. The results were recently published online in Nature Physics.
“What we’re seeing is that it’s 10 times harder to align the layers in the thin limit compared to the bulk, which we measure using electron tunneling in a magnetic field,” says MIT Physics graduate student Dahlia R. Klein, one of the paper’s lead authors. Klein is an NSF Graduate Research Fellow. Physicists call the energy required to align the magnetic direction of opposing layers the interlayer exchange interaction. “Another way to think of it is that the interlayer exchange interaction is how much the adjacent layers want to be anti-aligned,” fellow lead author and MIT postdoc David MacNeill suggests.
The researchers attribute this change in energy to the slightly different physical arrangement of the atoms in 2D chromium chloride. “The chromium atoms form a honeycomb structure in each layer, so it’s basically stacking the honeycombs in different ways,” Klein says. “The big thing is we’re proving that the magnetic and stacking orders are very strongly linked in these materials.”
"Our work highlights how the magnetic properties of 2D magnets can differ very substantially from their 3D counterparts,” says senior author Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics. “This means that we have now a new generation of highly tunable magnetic materials, with important implications for both new fundamental physics experiments and potential applications in spintronics and quantum information technologies."
Layers are very weakly coupled in these materials, known as van der Waals magnets, which is what makes it easy to remove a layer from the 3D crystal with adhesive tape. “Just like with graphene, the bonds within the layers are very strong but there are only very weak interactions between adjacent layers, so you can isolate few-layer samples using tape,” Klein says.
MacNeill and Klein grew the chromium chloride samples, built and tested nanoelectronic devices, and analyzed their results. The researchers also found that as chromium trichloride is cooled from room temperature to cryogenic temperatures, 3D crystals of the material undergo a structural transition that the 2D crystals do not. This structural difference accounts for the higher energy required to align the magnetism in the 2D crystals.
|Bulk single crystal of chromium trichloride, a layered two-dimensional (2D) van der Waals antiferromagnet. Photo, David MacNeill, MIT.|
The researchers measured the stacking order of 2D layers through the use of Raman spectroscopy and developed a mathematical model to explain the energy involved in changing the magnetic direction. Co-author and Harvard University postdoc Daniel T. Larson says he analyzed a plot of Raman data that showed variations in peak location with the rotation of the chromium trichloride sample, determining that the variation was caused by the stacking pattern of the layers. “Capitalizing on this connection, Dahlia and David have been able to use Raman spectroscopy to learn details about the crystal structure of their devices that would be very difficult to measure otherwise,” Larson explains. “I think this technique will be a very useful addition to the toolbox for studying ultra-thin structures and devices.” Materials Science and Engineering graduate student Qian Song carried out the Raman spectroscopy experiments in the lab of MIT Assistant Professor of Physics Riccardo Comin. Both also are co-authors of the paper.
“This research really highlights the importance of stacking order on understanding how these van der Waals magnets behave in the thin limit,” Klein says.
MacNeill adds, “The question of why the 2D crystals have different magnetic properties had been puzzling us for a long time. We were very excited to finally understand why this is happening, and it’s because of the structural transition.”
This work builds on two years of prior research into 2D magnets in which Jarillo-Herrero’s group collaborated with researchers at the University of Washington led by Professor Xiaodong Xu, who holds joint appointments in the departments of Materials Science & Engineering, Physics, and Electrical & Computer Engineering, and others. Their work, which was published in a Nature letter in June 2017, showed for the first time that a different material with a similar crystal structure – chromium triiodide (CrI3) – also behaved differently in the 2D form than in the bulk, with few-layer samples showing antiferromagnetism unlike the ferromagnetic 3D crystals.
Jarillo-Herrero’s group went on to show in a May 2018 Science paper that chromium triiodide exhibited a sharp change in electrical resistance in response to an applied magnetic field at low temperature. This work demonstrated that electron tunneling is a useful probe for studying magnetism of 2D crystals. Klein and MacNeill were also the first authors of this paper.
University of Washington Professor Xiaodong Xu says of the latest findings, “The work presents a very clever approach, namely the combined tunneling measurements with polarization resolved Raman spectroscopy. The former is sensitive to the interlayer antiferromagnetism, while the latter is a sensitive probe of crystal symmetry. This approach gives a new method to allow others in the community to uncover the magnetic properties of layered magnets.”
“This work is in concert with several other recently published works,” Xu says. “Together, these works uncover the unique opportunity provided by layered van der Waals magnets, namely engineering magnetic order via controlling stacking order. It is useful for arbitrary creation of new magnetic states, as well as for potential application in reconfigurable magnetic devices.”
Other authors contributing to this work include Efthimious Kaxiras, the John Hasbrouck Van Vleck Professor of Pure and Applied Physics at Harvard University; Harvard graduate student Shiang Fang; Iowa State University Distinguished Professor (Condensed Matter Physics) Paul C. Canfield; Iowa State graduate student Mingyu Xu; and Raquel A. Ribeiro, of Iowa State University and the Federal University of ABC, Santo André, Brazil. This work was supported in part by the Center for Integrated Quantum Materials; the DOE Office of Science, Basic Energy Sciences; the Gordon and Betty Moore Foundation’s EPiQS Initiative; and the Alfred P. Sloan Foundation.
Annual MIT Materials Day Symposium highlights latest innovations on Oct. 9, 2019.
Machine learning tools are both helping to design new materials and devices and to help those devices run at their best.
Optical spectrometers, for example, are devices that record Iight intensity as a function of wavelength and identify chemicals based on their response to light. MIT Associate Professor of Materials Science and Engineering Juejun (JJ) Hu, last year developed a new chip-based spectrometer that employs an algorithm which improves resolution 100 percent compared to the textbook limits, called Rayleigh limits.
“We developed an algorithm that allows us to extract the information with much better signal-to-noise ratio,” Hu explains. “We have validated the algorithm for many different kinds of spectrum.”
Unlike the conventional shape of glass lenses which are often curved, his new optical devices feature an array of specially designed optical antennas that add a phase delay to the incoming light, which enables many different functions. Hu currently is working with UMass researchers to perfect an algorithm that can screen potential designs for these devices. The algorithm can evaluate the workability of irregular shapes that go beyond conventional shapes likes circles and rectangles.
“The algorithm allows us to train it with existing data,” Hu says. “It can recognize the underlying connections between complex geometries and the electromagnetic response.” The algorithm can find hidden relations much faster than conventional full-scale simulation methods. The algorithm can also screen out potential combinations of materials and functions that just won’t work. “If you use conventional methods, you have to waste lots of time to exhaust all the possible design space and then come to this conclusion, but now our algorithm can tell you really quickly,” he says.
Hu will present his research at the MIT Materials Research Laboratory’s annual Materials Day Symposium on Wednesday, Oct. 9, in Kresge Auditorium. The Symposium runs from 8 a.m. to 3:30 p.m. and is immediately followed by a Poster Session in La Sala de Puerto Rico on the second floor of Stratton Student Center. Register here.
Atlantic Richfield Associate Professor of Energy Studies Elsa A. Olivetti will discuss her work on an artificial-intelligence system that scours through scientific papers to deduce materials science “recipes.” Her team is currently working on experimental verification, particularly focused on catalysts materials.
“We are constantly refining and improving our system from improving overall accuracy to expanding to other parts of the paper, such as results, to other kinds of documents, such as patents,” Olivetti says.
AI can also help to improve sustainability. “If we can know better how to make new materials, we might be able to inform how to make them in a lower resource consuming way,” Olivetti says.
Keynote speaker Dr. Brian Storey, Toyota Research Institute’s Director of Accelerated Materials Design & Discovery, will discuss several collaborative projects focusing on research and development of materials for battery and fuel cell electric vehicles.
Other Materials Day speakers are: Professor Carl V. Thompson, Director, Materials Research Laboratory; Professor Klavs F. Jensen, Departments of Chemical Engineering and Materials Science & Engineering; Professor Asu Ozdaglar, Department Head, Electrical Engineering & Computer Science; Professor Ju Li, Departments of Nuclear Science & Engineering and Materials Science & Engineering; and Assistant Professor Rafael Gomez-Bombarelli, Department of Materials Science & Engineering.
MIT graduate students and postdocs will give two-minute talks on their research during a “Poster Previews” session before the lunch break. The Poster Session runs 3:35 to 5:45 p.m. with an awards presentation at 5:30 p.m.