SCIENCE, DISCOVERY & MAGNETISM
Twisted Twisted Physics Physics
GRAPHENE GETS GROOVY IN HIGH FIELDS
ROCK PAPER MAGNETS
Grab some scissors and create a mini magnet lab
Are harmful chemicals finding their way to your faucet? SPRING 2019
Japan Blows Doors off World Record Who said physics is boring? These scientists had a blast developing a technique for studying electrons.
Tracking Fracking This scientist wants to know if harmful chemicals are finding their way to your faucet.
These Particles Just Wonâ€™t Commit Electrons in this cool high-field experiment flip-flopped between liquid and solid.
Double Whammy What happens when a kid with ADHD sustains a concussion? Researchers use high-field magnets to find out.
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Scientists probing 2D materials are discovering intriguing behaviors that could revolutionize our 3D world.
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Japan Blows Doors off World Record By Jen A. Miller
Scientists have a blast developing a technique for studying electrons in ultra-high magnetic fields.
hojiro Takeyama was braced for an explosion. A blast with the force of five sticks of dynamite was about to rock his laboratory at the Institute for Solid State Physics at the University of Tokyo. But he wasn’t frightened: He was excited. After all, he had been carefully planning the explosion for more than five years. The point wasn’t to make a big bang — although he would. Rather, that big bang was just a tool to generate an extremely high magnetic field using a technique Takeyama had been developing for years, called electromagnetic flux-compression. On that day in April 2018, he and his team were hunkered down, their experimental device contained within an iron “anti-explosion house” nearby. They were far enough away that they didn’t need earplugs, and could see what happened via a monitor.
house, “but the machine was designed in such a careful way that I knew this could be achieved.” Such high magnetic fields have been made before, but only with TNT detonated outside and resulting in an uncontrolled explosion. Takeyama’s goal was to create a field that was both ultra-high and manageable, so that it could be used in experiments to study materials in extreme environments. Instead of using explosives, the Takeyama group used a set of nested coils. The first coil created a static magnetic field of about 3.2 teslas (in the range of what an MRI machine generates). In the middle, they added a coil attached to capacitors storing five megajoules of energy (think of a minivan moving at 100 miles per hour). Inside that coil was a lightweight copper ring. When the capacitors released their charge, it created, thanks to electromagnetic induction, a sudden, strong magnetic field that counteracted
When the system did, in fact, explode, the team was safe — and they did hear “a big sound,” Takeyama said. They were also overjoyed. They had hoped to create a magnetic field of 700 teslas (the unit of magnetic field strength). Instead, they reached 1200 teslas, about 400 times stronger than a typical MRI machine, and a new world record for a controlled magnetic field. “I was surprised,” said Takeyama of the explosion, which dislodged the doors of the anti-explosion 4
Illustration of the magnet infrastucture.
Shojiro Takeyama (right) and engineer Hironobu Sawabe in the “anti-explosion house” before the big blast.
that static magnetic field. The resulting forces caused the copper ring to implode, which in turn compressed the magnetic field inside the ring, causing it to surge to a whopping 1200 teslas. When it couldn’t compress anymore, it exploded out. In a matter of 40 microseconds, it was all over.
said. “The novelty of professor Takeyama’s design is that one pulsed magnetic field is used to compress the other, as opposed to chemical explosives. This enables the apparatus to be operated in a laboratory as opposed to outside at a dedicated firing site.”
“In general, producing higher magnetic fields comes at the expense of shorter duration,” said Ross McDonald, deputy director of the Pulsed Field Facility, a branch of the National High Magnetic Field Laboratory located in Los Alamos, New Mexico. The facility has developed and maintained a set of pulsed magnets, including a 100-tesla instrument that creates the highest nondestructive field in the world.
Because the magnetic field created by Takeyama’s device is far briefer and more compact than that of the 100-tesla magnet, it also requires less energy — only a few megajoules compared to several hundred.
Reaching fields up to 45 teslas, continuous-field research electromagnets can run indefinitely, McDonald said, as long as you keep running current through them and the heat produced is dissipated. The higher-field pulsed magnets can only operate for seconds, or even milliseconds, at a time. Still, that’s enough time to learn a lot about the material placed inside that field for the experiment. Takeyama’s magnet is a kind of pulsed magnet. Although it self-destructs, scientists should still be able to get valuable data out of an experiment during its brief duration. “Above 100 teslas, there is no current apparatus strong enough to generate even a short-duration field pulse without being destroyed,” McDonald
Takeyama said these ultra-high fields could reveal never-before-seen behaviors in electrons, which could have implications for both material science and fusion power generation. “We can expect to see new physics of electrons in solids,” he said. Many future breakthroughs will require the kind of tools Takeyama is developing. “Society’s ability to take advantage of new materials, in particular for new electronics applications, requires deep, fundamental understanding of how the electrons in a given material behave,” said McDonald. “High-field research provided this information for the semiconductors in today’s electronics decades prior to their common application. Our ability to design using materials with new functionalities ultimately requires even higher fields to gain comparable understanding.”
TRACKING FRACKING Got two minutes? That’s all it takes to get the drift of how scientists use high fields to see if harmful chemicals are coming out of your faucet.
usan Richardson, an environmental chemist at the University of South Carolina, is the first to use high-field magnets to see if dangerous chemicals form when water associated with hydraulic fracturing is later processed for drinking water. Read her two-minute interview with non-scientist Maggie Brandenburg below. Then go to fieldsmagazine.org to watch it and an equally short (but nerdier) conversation with a fellow scientist. MB: So what are you doing here at the National MagLab? SR: This week we’re running some of our drinking water samples. We’re looking specifically for new chemicals, unknown chemicals, that are formed when we have hydraulic fracturing impacts on our drinking water. How do you determine those unknown chemicals?
There’s a very special instrument here called a mass spectrometer, and it’s a special kind of a mass spectrometer that gives us very high resolution and that allows us to know exactly how many carbons, hydrogens, oxygens — whatever — are in the molecules. And we can use that data to identify these unknown chemicals.
When you put something into the mass spectrometer what does that process look like?
So we have our drinking water extracts, and we have this syringe that we load. And we slowly inject little bits of that extract into the instrument, and it goes into the big magnet and gives us the data we need.
That’s really cool. So is there any other research happening like this? Or are you guys the first to do it?
We’re the first to do this kind of work, actually. There was a scientist earlier that got some more basic information on these “disinfection byproducts” that are formed when we chlorinate that water. But we are looking at a lot of unknown chemicals. Nobody had ever looked at these unknown chemicals before. We’re hoping this special instrument is going to give us a lot more information than we’ve had from other mass spectrometers. In fact we know: We’ve already had really good data the last day and a half.
Where is hydraulic fracturing happening in the country?
All over. You’ve probably heard of the Marcellus Shale; it’s under New York, Ohio, Pennsylvania and other states. We’re actually collecting the hydraulic fracturing wastewater from the Barnett Shale in Texas. But there are places all over the country where they are doing hydraulic fracturing now.
So it’s really important work then.
I think so. What happens is that the “produce water” that comes back out of the fracking wells can go into our rivers. And if you’re in a city that’s downstream, it can impact your drinking water. So we’re trying to figure out what those impacts are.
SLOW TRAIN TO SCIENCE
HOP ON HERE! 1
BY KRISTEN COYNE Physicists love when matter changes phases. It’s understandable: Even the most mundane of phase changes — water turning solid at 0 degrees Celsius and gaseous at 100 degrees Celsius — are fascinating when you think about them. Nature has loads of other phasechanging tricks up its sleeve. They can be driven by temperature, as is the case with H20, or other parameters, such as high magnetic fields. In an experiment published last year involving scientists from the RIKEN Center for Emergent Matter Science, the University of Tokyo in Japan and the High Field Magnet Laboratory (HFML) in the Netherlands, scientists created a material featuring a special two-dimensional gas layer, then subjected it to both high fields and extremely low temperatures. They wanted to see what combinations of field and temperature would prompt that gas to change to a liquid and then a solid. Anytime physicists can provoke a phase transition, they learn a little more about how the world works while gaining knowledge that could one day translate into an advance in electronics, energy or other applications. What they discovered surprised them. Board this “Slow Train to Science” and check out the stops along the way to learn more. Science Advisor: Uli Zeitler
Composite fermion liquid to Wigner solid transition in the lowest Landau level of zinc oxide, D. Maryenko et al., Nature Communications 9, 4356 (2018).
WHAT’S A HETEROSTRUCTURE?
GROW THE MATERIAL Before you can study cool behavior, you have to create an environment where you’re likely to find it, like a heterostructure.
COOL IT DOWN Using a cryostat, scientists lower the temperature around the material so they can slow down the atoms and observe the subtler quantum behaviors inside. It’s like looking for a shivering person at a disco: You have to stop the music so the other people quit dancing before you can detect that much more subtle movement. The chillier temps also make the electron gas an even better metal, with the electrons moving more freely.
It’s what happens when you layer one atomically thin, two-dimensional material over another.
CRYOSTAT? TRANSLATION, PLEASE! It’s a fancy fridge. Instead of Freon, scientists use liquid nitrogen and liquid helium to create extremely cold temperatures. The cryostat extends into the magnet so the material can be really cold while being exposed to high fields.
WHAT’S KINETIC ENERGY? TURN ON THE MAGNET The scientists will gradually increase the field strength over the course of the experiment and observe what happens. The negatively charged electrons in the gas respond to the magnetic field by spinning around it. The higher the field, the faster this cyclotron motion, and the more kinetic energy used by the electrons. So begins a tug-of-war between kinetic and potential energy in the system.
ELECTRONS LIKE EASY STREET There’s no free lunch for electrons. They have to spend energy either moving around, or sitting still on a crystal lattice and overcoming the repulsive force of their neighbors. They default to whichever option requires less energy. Putting electrons in a magnet increases their kinetic energy. If the field climbs high enough, the kinetic energy required to maintain their gas phase becomes too high, and they hunker down and fall under the influence of potential energy instead. That’s when they transition to a liquid or solid.
Energy related to motion.
ELECTRONS GET WEIRD But before this electron gas transitions to another phase, another kind of change occurs. This happens just as the magnetic field reaches a value of about 14 teslas: The electrons turn into composite fermions.
WHAT’S A TESLA? A unit of magnetic field strength. A typical hospital MRI magnet is 2 or 3 teslas.
WHAT’S A COMPOSITE FERMION? As electrons interact with a magnetic field, they can sometimes steal, or pin down, a part of it. In a sense, they become a kind of hybrid particle: a composite fermion.
WHY THE MAGNESIUM? WHAT DID THE SCIENTISTS CREATE? Atom by atom, they built a thin layer of zinc oxide (ZnO) topped by a thin layer of magnesium zinc oxide (MgZnO). Both materials are insulators.
The presence of Mg at the interface between ZnO and MgZnO creates an electric field that traps electrons, resulting in a gas made of freely moving electrons between the two layers. That in-between gas layer, or heterojunction, is what interests the scientists.
SO … THE COLD TURNS THE GAS TO A LIQUID? Not in this case — this is different than sticking water in the freezer. The purpose of the cold is to “sedate” the atoms. Instead of temperature, scientists will turn up the magnetic field to trigger a phase transition in this electron system.
WHAT’S POTENTIAL ENERGY?
REMIND ME ABOUT GASES. They’re a phase of matter (like steam) in which the molecules move around a lot. Liquids have less kinetic energy than gases, and solids have even less kinetic energy than liquids.
THINGS GET INTERESTING The magnetic field keeps rising until, at about 16 teslas, two changes abruptly occur: The resistance of the electrons plummets to almost zero, and the composite fermion gas condenses briefly into a special kind of liquid called a Laughlin liquid.
The electron gas at the junction of these two insulators is actually a metal!
TERMINUS THE BIG QUESTION While the scientists could accurately measure the system’s resistance and deduce its states of matter, they can’t say for sure whether the Wigner solids they observed were made up of electrons or composite fermions. This is an important question that further experiments and theory work could help answer, said HFML physicist Uli Zeitler, who was part of the research team. Either way, he said, the results are exciting. “The interactions between these electrons are much stronger in this material than in other materials,” he said. “This is data that help us to understand interactions between electrons and even to use them [one day] for something useful.”
Generally, potential energy is the possibility of motion. If you put a ball on top of a hill, it has potential energy because it could roll down. In this case, the potential energy is the repulsion between the electrons, all of which have the same negative charge. If they had more space, they would move further apart.
WAIT — FREE ELECTRONS IN AN INSULATOR?
9 FLIP-FLOPPING PHASES This strange cycle repeats itself a few more time as the system vacillates between an insulating solid and highly conductive liquid.
LIQUID TO SOLID The liquid’s life is brief. As the magnetic field continues to climb, the liquid transitions into a type of solid called a Wigner solid, and the resistance of the system shoots back up. The colder the system, the higher its resistance, i.e., the better an insulator it becomes.
SOLID TO LIQUID But as the magnetic field reaches about 17.5 teslas, it’s déjà vu: The resistance takes another nosedive, and the solid reverts to a liquid.
WHAT IS A WIGNER SOLID?
A measure of how well electricity (electrons) travel through a material. Metals have low resistance, insulators have high resistance.
Named after Hungarian-American scientist Eugene Wigner, who first predicted it in 1934, it’s a solid-like state that can occur in electrons in a 2D material at a high magnetic field.
WHAT IS A LAUGHLIN LIQUID? Named after Nobel Prize–winning physicist Robert Laughlin, it’s a quantum fluid that can form at high fields and low temperatures.
DOUBLE WHAMMY By Zachary Boehm
What happens when a kid with ADHD sustains a concussion? Using high-field magnets, researchers are working to find out.
hen a child is diagnosed with attention deficit hyperactivity disorder (ADHD), doctors will often seek to salve the anxieties of parents by dispensing a piece of straightforward advice: “Get your child involved in sports.” It’s a good outlet, they suggest, with its inbuilt socialization, confidence building and directed physical exertion. A perfectly constructive way to channel that excess energy. But sports aren’t an ADHD panacea, and they come with their own constellation of potential dangers, including head injuries. According to the Centers for Disease Control, between 1.6 and 3.8 million sports- and recreation-related concussions occur every year in the United States alone, and those numbers will continue to swell as concussion awareness grows. “So what happens when you have a person with ADHD, and they sustain a concussion, or even repetitive concussions?” asks Cathy Levenson, a professor of Biochemical Sciences and Neuroscience at Florida State University’s College of Medicine in Tallahassee, Fla. “How does that affect their ADHD? Do the symptoms of concussion look different in someone with ADHD? Is the treatment of someone with ADHD different after they’ve sustained a concussion?” These are critical questions for the estimated 6.1 million children in the U.S. diagnosed with ADHD — so critical, in fact, that Harvard University’s Spaulding Rehabilitation Hospital, which works often with people who have sustained traumatic brain injuries (TBIs) through its state-of-the-art Brain Injury Rehabilitation Program, is funding research from Levenson and her colleagues on the neurobiological interaction of ADHD and concussions.
Levenson and other research groups have observed minute tissue changes as a result of concussions that linger for as long as six months after head injuries, as well as months-long indications of depressive and risk-taking behavior. There have also been studies on novel treatment strategies for concussion-related depression. But to better understand the effects of concussion on a brain with ADHD, Levenson needed to create those same conditions in mice. For past TBI research, she and her team had developed a mouse model of repetitive concussion, but that was only one half of the equation. She needed a high-fidelity model of both repetitive concussion and ADHD in a single brain. In other words, she needed to build a brain that was specifically tailored to her study.
An MRI tractography image of a mouse brain that has both ADHD and concussion. The colors denote different directions in which water is moving through bundles of neurons.
Enter Dr. Pradeep Bhide, director of the Center for Brain Repair at FSU’s College of Medicine. Long before Levenson’s current concussion project, his research group had developed its own ADHD mouse model to study the psychiatric effects of nicotine. When bred with Levenson’s TBI mouse model, Bhide’s ADHD model proved to be the perfect complement.
Photo: Stephen Bilenky
Cathy Levenson runs experiments with National MagLab physicist Victor Schepkin (left) and Florida State University undergraduate Nicholas Kynast on the MagLab’s 21.1-tesla MRI magnet.
“My group’s mouse model shows behavioral, neuroanatomical and neurochemical changes that are consistent with ADHD, and treatment with stimulant drugs such as methylphenidate (Ritalin) produces benefits similar to those seen in ADHD patients receiving the same treatment,” Bhide says. “Our mouse model provides an excellent experimental tool to examine potential interactions between ADHD and TBIs in terms of cognitive function and cellular and molecular changes in the brain.”
metabolic markers, blood flow deficits, white matter integrity and other important indicators of brain injury. Along with the researchers’ uniquely representative experimental model, these tools are key to unlocking the questions posed by Levenson and her collaborators.
Now, armed with their new mouse model, Levenson and her team are poised to disentangle the molecular responses particular to people with both brain injuries and ADHD.
“Without the facilities at the MagLab, this work would not be possible,” Levenson says. “It’s the ultra-high field strengths that enable us to make these kind of measurements. Our hope is that if we can learn the system at these high fields, then we may learn how to better apply them clinically.”
But even with a well-suited model, stripping back the convoluted layers of these interactions is no easy feat. Clinical-grade magnetic resonance imaging (MRI) technologies, which doctors use to snap detailed pictures of the brain, aren’t strong enough to produce the kind of highresolution images needed to credibly explore these infinitesimal effects. A complicated mystery requires more powerful tools. The good news: Those tools exist, and they’re only a short drive away from Levenson’s College of Medicine office. At the National High Magnetic Field Laboratory’s headquarters, Levenson is using the world’s strongest MRI machine to run tissue scans with extraordinary levels of sensitivity, revealing
“We might not be able to prevent the concussion, but we can have a better idea of what to do about it.”
And that, Levenson says, is the goal: to make discoveries that clinicians can leverage to improve the lives of concussion patients with ADHD. “Ultimately, it’s about treatment, management and having the best outcomes possible,” she says. “We might not be able to prevent the concussion, but we can have a better idea of what to do about it. With this repetitive concussion model combined with the ADHD model and the power of the magnets at the MagLab, we’re hoping we can pinpoint some areas of the brain that we can target.”
Twisted Twisted Physics Physics Scientists probing an exotic, two-dimensional realm are discovering astonishing behaviors that could revolutionize our 3D world. Story by Bennett McIntosh Illustrations by Caroline McNiel
Fifteen years ago,
a team led by physicists Andre Geim and Konstantin Novoselov published a groundbreaking paper in the journal Science about a one-atom-thin variation of the element carbon. Called graphene, its discovery created a huge stir in the scientific community, in part due to its many incredible properties: extremely thin and light, yet incredibly strong; highly electrically and thermally conductive; elastic and impermeable to gas and liquid. Although spell-checkers still reject the word, graphene has already spawned thousands of patents. While scientists continue to probe graphene’s wonders, they are also exploring other two-dimensional materials. In fact, physicists have been inventively stacking these materials to create “fillings” between them that constitute entirely new systems with intriguing behaviors. As our cover story by science writer Bennett McIntosh (see next page) describes, scientists are making important discoveries by subjecting these 2D sandwiches to high magnetic fields, putting them under pressure or twisting
the layers in just the right way, a nascent field dubbed twistronics. (See recipes for some of these “sandwiches” on page 18.) These heterojunctions, as the interfaces are called, are to condensed matter physicists what new planets are to astronomers: An uncharted environment where nature might be hiding some of its deepest secrets. Stacking his hands one atop the other, National High Magnetic Field Laboratory Director Greg Boebinger described how each of these tiny assemblies is a unique world where never-before-seen physics could arise. “There is this freedom to decide to take an insulating layer, then put a metal layer on top, another insulating layer, and then a magnetic layer and, finally, perhaps a superconductor,” he said. “In a sense, we can design and build tiny, new universes that never existed before.” — K.C To find out more about what these wee worlds are teaching us, please read on.
I can guarantee you we will have something useful in five years’ time, but I don’t know what it is...
thin sheet of carbon sits A inn impossibly the bowels of the National High
Magnetic Field Laboratory’s Superconducting Magnet #2 at over 20,000 atmospheres — a far higher pressure than exists even at the ocean’s nadir. The magnet’s field can reach as high as 18 teslas, but for now it is held much lower, allowing a subtle but profound shift in the dance of electrons through the pinhead-sized sheet. As the temperature in the experimental chamber drops toward absolute zero, individual electrons synchronize their quantum fluctuations, joining in a delicate choreography suffusing the sheet. This erases the material’s electronic resistance, and electrons begin to flow through it easily, frictionless, in a manner scientists have seen only in a handful of other materials. And with that, the sheet, made up of two layers of the 1-atom-thick form of carbon known as graphene, has transformed from metallic to superconducting. This experiment, performed last summer at the National MagLab’s Tallahassee headquarters, is one of many revealing the exciting, surprising properties of atomically thin materials. Some, including graphene, are single layers of hexagonally bonded atoms; other arrangements may be 2 or even 3 atoms thick. This sheer diversity of structures makes atomically thin materials exciting, says Matthew Yankowitz, the Columbia University postdoctoral researcher who performed the graphene experiment. He thinks of the temperature, magnetic field and even the order in which different sheets are stacked as
experimental “knobs” that allow scientists to dial up new electronic properties, exploring the frontiers of materials engineering. “By turning the knobs,” Yankowitz says, “you can introduce new physics that simply doesn’t exist in the parent materials.”
Stack Stack It! It!
The thinness of materials like graphene gives another advantage to the scientists who are tuning their properties and studying them. With so few atoms in the picture, it takes only a few electrons to dramatically alter the properties of a material — say, from metal to superconductor. By contrast, physicists trying to induce different properties in threedimensional materials with far more atoms often must introduce electrons into the system, usually by adding elements with “extra” or “missing” electrons, a process called doping.
“The beauty of graphene,” says Yankowitz, “is you can just change the voltage, and tune from a metal to an insulator to a superconductor or any number of other configurations.” Studying how these configurations emerge can help physicists understand what goes on within harder-to-probe three-dimensional materials. For instance, the properties of high-temperature superconductors, an area of
intense physics research, appear to depend on electrons confined to two-dimensional layers within their three-dimensional structure. “The whole field of high-temperature superconductors is based on two-dimensional copper-oxygen layers in which the electrons travel, and we still don’t understand why they’re such good superconductors,” says Greg Boebinger, director of the National MagLab. High-field magnets are particularly valuable tools for understanding two-dimensional behavior like superconductivity. In all materials, magnetic fields deflect the movement of electrons. But electrons confined to a 2D plane exhibit more interesting deflections. “Applying magnetic fields in two dimensions means that suddenly electrons have to start dancing with the magnetic flux lines in a complex way,” says Boebinger. Scientists first began producing atomically thin materials with the Nobel Prize–winning isolation of a single graphene layer in 2004, which soon led to a veritable zoo of other materials. Stacking and manipulating these materials provided new and rapidly expanding possibilities for exploring fundamental mysteries in physics and materials science — and new technologies.
Twist Twist It! It!
In the years since graphene’s discovery, there have been tantalizing hints that its behavior might shed insight onto the enigma of high-temperature superconductors, or even be a superconductor itself. Because superconductivity, like other quantum effects, depends on fragile correlations between electrons, most superconductors only work at a few degrees above absolute zero. Even “high-temperature” superconductors must be cooled to -196 degrees Celsius by liquid nitrogen to work. But in 2007, a team including Zeitler and Boebinger showed that powerful magnetic fields could induce another quantum phenomenon, the quantum Hall effect, in room-temperature graphene. Firm evidence of these fragile states remained elusive until early 2018. But that March, a team led by MIT physicist Pablo Jarillo-Herrero added a new twist — literally — to the search for quantum effects in graphene. “Not only did we find these correlation effects, but we found them in spectacular ways,” says
One measure of the possibilities of 2D materials is the number of patents the field has generated worldwide. For graphene alone, that number soared from just a few hundred in 2010 to more than 6,000 by 2018. “This is an extremely powerful way to make devices with new functionalities,” says Uli Zeitler, a physicist at the High Field Magnet Laboratory and Radboud University in the Netherlands. In addition to providing fundamental physics insights, Zeitler says, these new materials will enter the consumer market in exciting and unpredictable ways. “I can guarantee you we will have something useful in five years’ time,” he says, “but I don’t know what it is.”
Jarillo-Herrero. The team revealed that by stacking two sheets of graphene — one rotated 1.1 degrees with respect to the other — and cooling the assembly to a fraction of a degree above absolute zero, they could create an insulator. If a slight charge to the graphene was added, it became a superconductor! The discovery was lauded as the breakthrough of the year by Physics World and marked the beginning of a new subfield dubbed “twistronics.” Jarillo-Herrero’s successful search for what he calls the “magic angle” in graphene is part of a wider trend in research on stacked atomically thin materials, making use of the large-scale “moiré patterns” generated when small-scale patterns overlap (think about the striping that appears when you photograph a screen with a digital camera). Five years previously, three teams, one of them including Jarillo-Herrero and Yankowitz, had shown that the moiré pattern produced by stacking graphene between sheets of the similarly hexagonal boron nitride produced a beautiful fractal energy pattern called Hofstadter’s butterfly, named for the scientist who predicted it nearly 40 years earlier. In Jarillo-Herrero’s 2018 experiment, stacking graphene sheets at a 1.1-degree angle results in a hexagonal moiré pattern that repeats approximately every 13 nanometers, a distance nearly 100 times farther apart than the pattern for single-layer graphene. This large-scale repeating structure puts a different set of constraints on the electrons’ wave functions, enabling the new behavior the team observed. “The electrons are walking through a different landscape, changing their properties,” explains Zeitler, who was not involved in this research.
Press Press It! It!
How, precisely, the quantum interactions between layers of graphene create a superconductor is still a mystery — but it’s one that Yankowitz is eager to help solve. Physicists were quick to notice some similarities between the particular flavor of superconductivity in graphene and that in high-temperature superconductors, but without knowing the precise mechanism in either case, the question of how high the critical temperature could go — whether it could, perhaps, even reach room temperature — remains uncertain. To better explore inter-layer quantum interactions, Yankowitz resolved to add a new experimental dial — tuning the strength of these interactions by literally pressing the layers closer together. Enter David Graf, a physicist at the National MagLab. For years, Graf had worked with other MagLab users to place their samples, usually three-dimensional crystals, under enormous pressure within the experimental chambers of the magnets. But nobody had developed a comparable protocol for the 1-atom-thick layers Yankowitz wanted to work with, which are not only microscopically small, but thin enough that a simple static charge can make them explode. “Matt had this crazy amount of enthusiasm,” recalls Graf, “so I thought, Why don’t we give this a try?”
There’s an enormous number of materials that we can apply together in an infinite number of combinations. Working together, the two showed one layer of graphene could be safely pressed to nearly 25,000 times atmospheric pressure between layers of boron nitride, then demonstrated that the same could be done in two layers with a moiré-generating twist, changing the strength of the quantum correlations between layers. In fact, in two layers, as the pressure increased, so too did the “magic angle” between the two graphene layers that yields superconductivity. The results, published in the journal Science in January, are a step closer to exploring the full spectrum of graphene’s superconducting states. Yankowitz hopes this will help generate new theories to predict and design the structure of superconductors that work at ever-higher temperatures. “This pressure work provides a road map for driving up that critical temperature as high as it can possibly be,” says Yankowitz. Cory Dean, Yankowitz’s advisor at Columbia and lead author of one of the other papers on Hofstadter’s butterfly, says this is only the beginning of what the combination of remarkable physics and high-powered magnet facilities can do. “The fact that we were able to do this [experiment] on this system is really a testament to the confluence of expertise that exists at the National MagLab,” he says.
Graphene is just the beginning. Physicists are now experimenting with atomically thin sheets of materials from highly reactive black phosphorus and the semiconductor indium selenide. Dean and Yankowitz are exploring the properties of a newer class of atomically thin materials, transition metal dichalcogenides (TMDCs), which bring their own sets of experimental challenges (see the double-decker recipe on page 19) and exciting properties, including the ability to convert light to electricity. While high magnetic fields typically destroy superconductivity, Uli Zeitler, the Nijmegen physicist, was on a research team that recently found that one of these TMDCs, molybdenum disulfide, can maintain its superconductivity even under extremely powerful magnetic fields in excess of 35 teslas. It’s a vast quantum playground promising endless exploration and revolutionary discoveries. Says Dean, “There’s an enormous number of materials that we can apply together in an infinite number of combinations.”
WHAT’S A MOIRÉ? What a question! And — mamma mia! — do we have an answer for you! Check out “That’s a Moiré!” for an animated explanation with an unusual musical twist! Visit nationalmaglab.org/moire.
HOW TO WHIP UP NEW PHYSICS Like good chefs, physicists creatively experiment with their ingredients. Chefs attempting a fresh take on the humble sandwich, for example, will test different layers — meats, cheeses, condiments — pressed between slices of wheat or rye. Physicists exploring two-dimensional materials create their own quantum concoctions called van der Waals heterostructures, layers of 1-atom-thin materials, weakly bonded, that are arranged in a very particular way. They put these sandwiches inside a high magnetic field, where the electrons start doing the darndest things. Below are some of the “recipes” scientists have used to make exciting physics discoveries with heterostructures. Whether you don a lab coat or a white toque, we hope these recipes whet your appetite for science.
Hofstadter Hoagie Recipe Note: Three groups of creative chefs simultaneously developed this zesty recipe, each adding its own special techniques and flavoring. Here’s the basic recipe:
Preparation: • Precool experimental chamber to -273 °C. • Ramp up magnetic field to 35 teslas.
Ingredients: • 1 graphene flake • 1 layer boron nitride
Directions: • Gently place the graphene flake atop the boron nitride to create a lovely moiré pattern. • Carefully place this combination inside the pre cooled, high-field experimental space. • When your data reveals a fractional quantum Hall state, your sandwich is done and your data is ready to share with Science and Nature! • Turn off the magnet to find that the graphene has become a semiconductor with a band gap — potentially useful as a transistor.
Yield: A beautiful fractal energy pattern and validation of a decades-old physics prediction. DETAILED INSTRUCTIONS:
The Columbia University variation: Hofstadter’s butterfly and the fractal quantum Hall effect in moiré superlattices, Nature, Vol. 497 (30 May 2013). The Chefs: C.R. Dean, L. Wang, P. Maher, C. Forsythe, F. Ghahari, Y. Gao, J. Katoch, M. Ishigami, P. Moon, M. Koshino, T. Taniguchi, K. Watanabe, K. L. Shepard, J. Hone & P. Kim. The University of Manchester variation: Cloning of Dirac fermions in graphene superlattices, Nature, Vol. 497 (30 May 2013). The Chefs: L.A. Ponomarenko, R.V. Gorbachev, G.L. Yu, D.C. Elias, R. Jalil, A.A. Patel, A. Mishchenko, A.S. Mayorov, C.R. Woods, J.R. Wallbank, M. Mucha-Kruczynski, B.A. Piot, M. Potemski, I.V. Grigorieva, K.S. Novoselov, F. Guinea, V. I. Fal’ko & A.K. Geim. The MIT variation: Massive Dirac Fermions and Hofstadter Butterfly in a van der Waals Heterostructure, Science, Vol. 340, Issue 6139 (21 Jun 2013). The Chefs: B. Hunt, J.D. SanchezYamagishi, A.F. Young, M. Yankowitz, B.J. LeRoy, L. Watanabe, T. Taniguchi, P. Moon, M. Koshino, P. JarilloHerrero, R.C. Ashoori.
Black Phosphorus Panini Preparation:
• Precool experimental chamber to -267 °C. • Ramp up magnetic field to 35 teslas.
A beautiful demonstration of 2D behavior in the elemental semiconductor known as black phosphorus.
Ingredients: • 1 fresh black phosphorus crystal • 1 teaspoon acrylic
Directions: • Gently peel black phosphorus in a nitrogen atmosphere. • Sparingly apply a thin acrylic coating. Be sure to cover the entire sample. • Carefully place your lightly glazed sample inside the pre-cooled, high-field experimental space. • As soon as your phosphorus shows Shubnikov-de Haas oscillations, you are ready to share your data with Nature Communications.
CHEFS: V. Tayari, N. Hemsworth, I. Fakih, A. Favron, E. Gaufrès, G.Gervais, R. Martel & T. Szkopek DETAILED INSTRUCTIONS: Two-dimensional magnetotransport in a black phosphorus naked quantum well, Nature Communications, Vol. 6, Article number 7702 (7 July 2015).
Tungsten Diselenide Double-Decker Recipe Note: The tungsten diselenide (WSe2 ) layer is a sandwich within a sandwich, a tungsten layer between selenium ions. It rejects most electrodes, turning the interface with them into an insulator. So rather than run electric currents through it to measure the material’s properties, chefs use strong electric fields.
Preparation: • Pre-cool experimental chamber to -273 °C. • Ramp up magnetic field to 35 teslas.
Ingredients: • Tungsten (W) and selenium (Se) pellets, in 1:2 molar ratio • Crystals of graphite and hexagonal boron nitride (BN)
Directions: • Heat W and Se together at 1000 °C for two days. • Place resulting WSe2 crystals in a vacuum and anneal at 450 °C to melt off excess Se. • Use Scotch tape to remove single layers from WSe2, BN and graphite.
• Stack in the following order: graphite, BN, WSe2, graphite, BN. • Place the sandwich inside the pre-cooled magnet. • Use a single-electron transistor to control the electric field across the WSe2. • The way electric fields pass through the sample should give a stunning picture of energy levels — enough to publish in Nature Materials.
Yield: A comprehensive picture of the electronic structure of a new 2D material: a transition metal dicalcogenide. CHEFS: M.V. Gustafsson, M. Yankowitz, C. Forsythe, D. Rhodes, K. Watanabe, T. Taniguchi, J. Hone, X. Zhu & C.R. Dean DETAILED INSTRUCTIONS: Ambipolar Landau levels and strong bandselective carrier interactions in monolayer WSe2, Nature Materials, Vol. 17 (26 March 2018).
Good Mentors MAKE GOOD SCIENTISTS
Researchers talk about their favorite mentors and why they are so inspiring.
“It would be Jimmy Eng. He’s an electrical engineer and programmer at the University of Washington who invented the first database search algorithm for proteomics. He was willing to teach me how to write computer code in C++. He sat over my shoulder, even though I wasn’t a programmer or trained in that way. He was willing to do whatever it took to get me to learn that. He’s also very, very smart but has a lot of humility to go with it.”
RESEARCH SCIENTIST, NATIONAL HIGH MAGNETIC FIELD LABORATORY
“Probably the most important mentor I had was Brian Pettitt, a theoretical chemist and my undergraduate advisor at the University of Winnipeg. He absolutely insisted that I needed an interdisciplinary approach to my education and forced me to take a wide range of courses in physics, mathematics and chemistry. When I asked him once about which area was the most important, he said that they all were equally important, and he was right! He taught me that science does not have boundaries, and I should follow my passions.”
CHEMIST, UNIVERSITY OF WINNIPEG
JAMES BROOKS (FAR LEFT) WITH MENTEES, INCLUDING ELIZABETH GREEN (BACK ROW, WITH GLASSES)
“Without a doubt, my Ph.D. advisor, Dr. James Brooks. He taught me to think outside the box and to believe in the impossible. He had this contagious excitement for discovery and, although he passed away a few years ago, his excitement still reverberates throughout the scientific community.”
Elizabeth L. Green
PHYSICIST, DRESDEN HIGH MAGNETIC FIELD LABORATORY
“Probably my graduate school advisor, John Singleton. He was extremely motivating, and he worked really hard. He was just a really, really inspirational guy to do experiments with. … I guess everybody else I’d worked with before was extremely focused on one particular thing. But he was interested in a bunch of different things. And that was eye-opening for me. That was what was really very different about him.”
PHYSICIST, NATIONAL HIGH MAGNETIC FIELD LABORATORY
“The best mentors of mine are Prof. Tatsuo Okano and Prof. Katsuyuki Fukutani of the University of Tokyo, who were my Ph.D. supervisors. They were so patient that they let me find the research theme myself and allowed me a high degree of freedom. Rather than guiding directly, they supported me and my research from one step away, like parenting. Such a way of supervising is more like mentoring to me, and I liked it.”
RESEARCH ASSOCIATE, INSTITUTE FOR SOLID STATE PHYSICS, UNIVERSITY OF TOKYO
“My most influential mentor is Adalbert Mayer-Heinricy, professor of physics and biophysics at the University of Bremen, Germany. In 1989 I was a fifth-year diploma student in physics at the University of Leipzig, in what was then East Germany. When the Berlin Wall was opened, the first thing I did was search the physics department at the University of Bremen for an intriguing topic and inspiring supervisor for my diploma thesis. Most professors turned me down on the grounds that my East German curriculum and certificates might not be in full alignment with the guidelines of the university. Professor Mayer-Heinricy was not scared by the guidelines, and said that he would find a way or make one. With this courage and encouragement he welcomed me as a diploma student in his group; work I did then triggered my interest in magnetic resonance imaging (MRI). I have enjoyed an exciting and productive career since. Professor Mayer-Heinricy planted the seed for my career in magnetic resonance. He still closely follows my research and also loves dancing tango at the age of 84.”
“My most influential mentor as a young scientist was Marshall Dixon of Butler University in Indiana. He believed that smart young people, women or men, could learn quantum mechanics while still in high school, and the training I got from him, and the confidence he had in me, extended their influence throughout my college years and beyond. I am still in contact with him, and he is still teaching quantum mechanics to high school students, now at Cathedral High School in Indianapolis.”
Barbara A. Jones
RESEARCH STAFF MEMBER, IBM ALMADEN RESEARCH CENTER
HEAD, BERLIN ULTRAHIGH FIELD FACILITY, MAX DELBRÜCK CENTER FOR MOLECULAR MEDICINE
HANNAH AND TARIQ SHAFAAT
“My dad, an engineer, was a huge inspiration and mentor to me as a young scientist. He always pushed me and my brother (who also became a chemist) to really understand how things worked. He coached our Science Olympiad teams (to victory, I might add!) and encouraged us to create and modify our own tools and toys when commercial ones didn’t meet all of our needs.”
“My most influential mentor as a young scientist was (and still is) Joe Thompson because of his scientific integrity, hard work and ability to guide his mentees into becoming independent scientists. Enabling young researchers to think critically by themselves is challenging, and Joe excels at it.”
Hannah S. Shafaat
PHYSICAL CHEMIST, THE OHIO STATE UNIVERSITY
JOE THOMPSON (FAR RIGHT, NEXT TO MENTEE PRISCILA ROSA) WAS NAMED AN OUTSTANDING MENTOR BY LOS ALAMOS NATIONAL LAB IN 2016.
PHYSICIST, LOS ALAMOS NATIONAL LABORATORY
MAKESHIFT MAGNETS Turn your trash into treasure by creating your own high-field magnet models.
By Kristin Roberts
magnets can require millions of dollars and many years to design and build. But with a few repurposed resources, you can craft your very own world-record magnets and understand how scientists use them to conduct experiments that lead to groundbreaking discoveries. R esearch You’ll need scissors, tape or glue, recycled toilet tissue or paper towel roll and a straw. You’ll also need (as any scientist does) curiosity and imagination! First, you’ll be an engineer as you build two world-unique magnets housed at the National MagLab and some probes to go with them. 1. Start snipping! Cut along all the blue and green dashed lines to create all the pieces and parts. 2. Build your magnets. High-field magnets are cylindrical with a small opening down the center (called the bore) where the experiment takes place. Create your cylinders by first gluing the rectangular cutouts to your recycled toilet tissue or paper towel rolls. Then glue the tops and bottoms into place. Voilà : Magnets! Specifically, you’re building the 900 MHz, 21.1-tesla NMR–MRI magnet and the 45-tesla hybrid magnet. 3. Build your probes. These are holders you’ll need to put your sample (the thing you’re studying) inside the magnet. It also measures what’s happening to the sample inside the magnetic field. Glue or tape the cut-out probes to upcycled straws.
Now, step into the shoes of a scientist and try your hand at an experiment of your own! 1. Select a sample. High-field magnets can illuminate everything from new materials to oil samples to the human body itself! 2. Pick the right magnet for the job. Want to understand migraines in rodent brains? Use the strongest MRI scanner in the world — the 900 MHz. Trying to see how electrons behave in special materials? Use the world’s strongest magnet — the 45-tesla hybrid magnet. 3. Match your sample with the right probe. We color-coordinated them for you to make it easy! 4. Run your experiment. Insert your prepared probe into the magnet. Try to get the sample right in the center of the magnet where the field is the strongest. Then (using that imagination) envision your amazing discoveries! (For inspiration, explore some of the MagLab’s science highlights at nationalmaglab.org/scihigh) 5. Repeat with different samples and probes. 6. Shout: “Eureka! I love science!”
900 MHZ NMR-MRI MAGNET
TIP: ATTACH TOP AND BOTTOM SIDES TO ROLL FIRST
TIP: USE THE DASHED LINE TO GUIDE YOUR CUT
45 T PROBE
WORLDâ€™S STRONGEST MAGNET
21.1 T MAGNET
45 T SAMPLES
45 T MAGNET
PASTE SAMPLE HERE
TOP GL TAB UE
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Semiconductor Superconductor Graphene 21.1 T PROBES
21.1 T SAMPLES
MAKE BORE HERE
45 T MAGNET
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TIP: FOLD IN HALF TO CUT OUT CENTER.
21.1 T MAGNET
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MAKE BORE HERE
PASTE SAMPLE HERE
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Illustrations by Caroline McNiel
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Non-profit Organization U.S. Postage PAI D Tallahassee, FL Permit No. 55
National MagLab Florida State University 600 W. College Ave. Tallahassee, FL 32306
What can this colorful image, created by a high-field MRI machine, teach us about traumatic brain injury? Turn to page 10 to find out.
Headquartered in Tallahassee, Florida, the National High Magnetic Field Laboratory is home to some of the worldâ€™s strongest and most unique magnets, and belongs to a network of high-field magnet labs around the world offering scientists cutting-edge instruments for their discoveries.
@NationalMagLab Florida State University
University of Florida
Los Alamos National Laboratory
The National MagLab is supported by the National Science Foundation (DMR â€“ 1644779) and the state of Florida.
Learn why research into 2D materials could revolutionize our 3D world, how strong magnets reveal the impacts of fracking, and much more.
Published on Feb 21, 2019
Learn why research into 2D materials could revolutionize our 3D world, how strong magnets reveal the impacts of fracking, and much more.