The discovery of graphene – a material made of a single layer of carbon atoms – 15 years ago was the first step in what has become an ongoing revolution. Using such two-dimensional layers of carbon or other compounds, materials can now be precisely engineered for particular properties in ways that had not previously been possible. Around two years ago, scientists at the Massachusetts Institute of Technology (MIT) added yet another twist – literally. They created a material made of two layers of graphene in which the top layer was slightly askew – twisted at a “magic angle” of a tad over one degree. A single layer of graphene generally behaves as a semimetal, but the magic twist turns the two graphene layers into a superconductor, in which electrons can carry electric current with no loss of energy. This superconductor somewhat resembles a completely different group of materials – so-called high-temperature superconductors – that have subject of intense research for decades but are still not fully understood.
Researchers at the Weizmann Institute of Science recently teamed up with the magic-angle group at MIT to uncover the physics of this interesting twist. Along the way, they identified a new kind of disorder – a discovery that could advance the emerging field of “twistronics.”
PhD student Aviram Uri and Dr. Sameer Grover, who led the research in the group of Prof. Eli Zeldov of the Weizmann’s Condensed Matter Physics Department, together with Yuan Cao and colleagues from the group of Prof. Pablo Jarillo-Herrero at MIT, measured the flow of electrons in magic-angle graphene using the scanning SQUID-on-tip microscope developed in Zeldov’s lab. An ultra-sensitive magnetometer with nanoscale resolution, the SQUID-on-tip is perfect for this purpose, explains Aviram. It visualizes, in great detail, what happens on the level of a single atomic, super-lattice period that is induced by the two rotated layers. The double layers of graphene-with-a-twist devices were prepared for the experiment at MIT and sent to Zeldov’s lab.
The first thing the researchers noted in their measurements was that the electrons followed “preferred” narrow paths through the material. These paths resembled quantum “edge states” that Zeldov and his team had identified in their previous experiments in graphene; but as opposed to those experiments, where the edge states were actually on the edges, here they were running right through the middle. How and why did these strange edge states form?
The solution to this puzzle, says Zeldov, is that these currents still flow along edges, but in this case the edges are the boundaries of patches within the material, each made up of different twist angles. The researchers found that even if one aims for a specific twist-angle when fabricating the device, there will be random strains and stresses so that the twist angle varies throughout -- creating a complex structure rather than a uniform one. By tracing the exact positions of the edge states, the researchers were in fact able to construct spatial maps of the local twist angle with unprecedented resolution and accuracy. These new maps revealed an intricate landscape consisting of valleys, peaks and saddle points, and a network of sharp jumps.
“Close to the magic angle, the electronic properties of the material depend strongly on the exact twist angle. That means that regions with different twist angles should really be thought of as different materials that are somehow attached together,” explains Aviram. This new perspective has far reaching implications. The researchers in Zeldov’s group showed that gradients in the twist angle lead to the formation of strong internal electric fields that do not behave as would be expected, given the metallic nature of the material. Moreover, these fields can be tuned and amplified significantly simply by changing the density of the electrons. Unlike the electric fields produced by the more familiar “charge disorder,” these electric fields reflect a fundamentally new type of disorder – “twist-angle disorder” – a phenomenon that affects the very properties of its electrons, causing them to alter their mass as they traverse the different regions of the material.
Qantum Hall edge states surprisingly appear in the bulk of magic angle graphene rather than along the edges of the device (black outline). Each edge state consists of a pair of red and blue colors indicating counterpropagating persistent currents. A scanning nanoSQUID-on-tip was used to directly image the currents through their magnetic field imprint.
The nature of this new kind of disorder gave the researchers some clues as to how edge states form in the interior of the sample. “You can think of the twisted material as a series of egg cartons with different periodicities placed side by side,” says Aviram. “The edge states run in the narrow areas that separate those ‘cartons’ – this is where intense in-plane electric fields exist, pointing from one egg carton to the next.”
Scientists at the University of Bath have taken an important step towards understanding the interaction between layers of atomically thin materials arranged in stacks. They hope their research will speed up the discovery of new, artificial materials, leading to the design of electronic components that are far tinier and more efficient than anything known today.
Smaller is always better in the world of electronic circuitry, but there’s a limit to how far you can shrink a silicon component without it overheating and falling apart, and we’re close to reaching it. The researchers are investigating a group of atomically thin materials that can be assembled into stacks. The properties of any final material depend both on the choice of raw materials and on the angle at which one layer is arranged on top of another.
Dr Marcin Mucha-Kruczynski who led the research from the Department of Physics, said: “We’ve found a way to determine how strongly atoms in different layers of a stack are coupled to each other, and we’ve demonstrated the application of our idea to a structure made of graphene layers.”
The Bath research, published in Nature Communications, is based on earlier work into graphene – a crystal characterised by thin sheets of carbon atoms arranged in a honeycomb design. In 2018, scientists at the Massachusetts Institute of Technology (MIT) found that when two layers of graphene are stacked and then twisted relative to each other by the ‘magic’ angle of 1.1°, they produce a material with superconductive properties. This was the first time scientists had created a super-conducting material made purely from carbon. However, these properties disappeared with the smallest change of angle between the two layers of graphene.
Since the MIT discovery, scientists around the world have been attempting to apply this ‘stacking and twisting’ phenomenon to other ultra-thin materials, placing together two or more atomically different structures in the hope of forming entirely new materials with special qualities.
“In nature, you can’t find materials where each atomic layer is different,” said Dr Mucha-Kruczynski. “What’s more, two materials can normally only be put together in one specific fashion because chemical bonds need to form between layers. But for materials like graphene, only the chemical bonds between atoms on the same plane are strong. The forces between planes – known as van der Waals interactions – are weak, and this allows for layers of material to be twisted with respect to each other.”
The challenge for scientists now is to make the process of discovering new, layered materials as efficient as possible. By finding a formula that allows them to predict the outcome when two or more materials are stacked, they will be able to streamline their research enormously.
It is in this area that Dr Mucha-Kruczynski and his collaborators at the University of Oxford, Peking University and ELETTRA Synchrotron in Italy expect to make a difference.
“The number of combinations of materials and the number of angles at which they can be twisted is too large to try out in the lab, so what we can predict is important,” said Dr Mucha-Kruczynski.
The researchers have shown that the interaction between two layers can be determined by studying a three-layer structure where two layers are assembled as you might find in nature, while the third is twisted. They used angle-resolved photoemission spectroscopy – a process in which powerful light ejects electrons from the sample so that the energy and momentum from the electrons can be measured, thus providing insight into properties of the material – to determine how strongly two carbon atoms at a given distance from each other are coupled. They have also demonstrated that their result can be used to predict properties of other stacks made of the same layers, even if the twists between layers are different.
The list of known atomically thin materials like graphene is growing all the time. It already includes dozens of entries displaying a vast range of properties, from insulation to superconductivity, transparency to optical activity, brittleness to flexibility. The latest discovery provides a method for experimentally determining the interaction between layers of any of these materials. This is essential for predicting the properties of more complicated stacks and for the efficient design of new devices.
Dr Mucha-Kruczynski believes it could be 10 years before new stacked and twisted materials find a practical, everyday application. “It took a decade for graphene to move from the laboratory to something useful in the usual sense, so with a hint of optimism, I expect a similar timeline to apply to new materials,” he said.
Building on the results of his latest study, Dr Mucha-Kruczynski and his team are now focusing on twisted stacks made from layers of transition metal dichalcogenides (a large group of materials featuring two very different types of atoms – a metal and a chalcogen, such as sulphur). Some of these stacks have shown fascinating electronic behaviour which the scientists are not yet able to explain.
“Because we’re dealing with two radically different materials, studying these stacks is complicated,” explained Dr Mucha-Kruczynski. “However, we're hopeful that in time we'll be able to predict the properties of various stacks, and design new multifunctional materials.”
In the universe, there is the world we can see with the naked eye: trees, planes in the sky, dishes in the sink. But there are other worlds that reveal themselves with the help of a magnifying glass, telescope, or microscope. With these, we can see up into the universe or down into the smallest particles that make it up. The smallest of these is a world populated by particles smaller than an atom: the quantum world.
Physicists who probe this world study how these subatomic particles interact with one another, often in ways not predicted by behavior at the atomic or molecular level. One such physicist is Nicholas Rivera, who studies light-matter interactions at the quantum level.
In the quantum world, light is two things: both a wave and a small particle called a photon. “I was always fascinated with light, especially the quantum nature of light,” says Rivera, a Department of Physics graduate student in Professor Marin Soljačić’s group.
According to Rivera, there is still a lot we don’t know about quantum light, and uncovering these unknowns may prove useful for a number of applications. “It’s connected to a lot of interesting problems,” says Rivera, such as how to make better quantum computers and lasers at new frequencies like ultraviolet and X-ray. It’s this dual nature of the work — with fundamental questions coupled with practical solutions — that attracted Rivera to his current area of research.
Rivera joined Soljačić’s group in 2013, when he was an undergraduate at MIT. Since then his research has focused on how light and matter interact at the most elementary level, between quanta of light, also called photons, and electrons of matter. These interactions are governed by the laws of quantum electrodynamics and involve the emission of photons by electrons that hop up and down energy levels. This may sound simple, but it is surprisingly difficult because light and matter are operating on two different size scales, which often means these interactions are inefficient. One specific goal of Rivera’s work is to improve that efficiency.
“The atom is this tiny thing, a 10th of a nanometer large,” says Rivera. But when light takes the form of a wave, its wavelengths are much larger than an atom. “The idea is that, because of this mismatch, many of the possible ways that an electron could release a photon are just too slow to be observable.” Rivera uses theory to figure out how light and matter could be manipulated to allow for new types of interactions and ways to intentionally change the quantum state of light.
Inefficient interactions are often thought of as “forbidden” because, in normal circumstances, they would take billions of years to happen. “The forbidden light-matter interactions project is something we have been thinking about for many years, but we didn’t have a suitable material-system platform for it,” says Soljačić. In 2015, graphene plasmons arrived on the scene, and forbidden interactions could be explored.
Graphene is an ultra-thin 2D material, and plasmons are another quantum-scale particle related to the oscillation of electrons. In these ultra-thin materials, light can be “shrunk” so that the wavelengths are closer to the scale of the electrons, making forbidden interactions possible.
Rivera’s first paper on this topic, published the summer after he graduated with his bachelor’s degree in 2016, was the start of his longstanding collaboration with Ido Kaminer, an assistant professor at the Technion-Israel Institute of Technology. But Rivera wasn’t done with light-matter interactions. “There were so many other directions that one could go with that work, and I really wanted the ability to probe all of them,” Rivera says, and he decided to stay in Soljačić’s group for his PhD.
A natural match
That first collaboration with Kaminer, who was then a postdoc in Soljačić’s group, was a pivotal moment in Rivera’s career as a physicist. “I was working on a different project with Marin, but then he invited me to his office with Ido and told me about the project that would become the 2016 paper,” says Rivera. According to Soljačić, putting Kaminer and Rivera together “was a natural match.”
Kaminer moved to the Technion in 2018, which was when Rivera took his first trip to Haifa, Israel, with funds provided by MISTI-Israel, a program within the MIT International Science and Technology Initiatives (MISTI). There, he gave a seminar and met with students and professors. “That visit seeded some projects that we’re still working on today,” says Rivera, such as a project where vacuum forces were used to generate X-ray photons.
With the help of lasers and optical materials, it’s relatively easy to generate photons of visible light, but making X-ray photons is much harder. “We don’t have lasers the same way we do for visible light, and we don’t have as many materials to manipulate X-rays,” says Rivera. The search for new strategies for generating X-ray photons is important, Rivera says, because these photons can help scientists explore physics at the atomic scale.
This past January, Rivera visited Israel for the third time. On these trips, “[we make] progress on the collaborations we have with the students, and also brainstorm new projects,” says Rivera. According to Kaminer, the in-person brainstorming is vital when coming up with new ideas. “Such creative ideas are, in the end, the most important part of our work as scientists,” Kaminer explains. During each visit, Rivera and Kaminer sketch out a research plan for the next six months to year, such as continuing to investigate new ways to control and generate quantum sources of X-ray photons.
When investigating the theory of light-matter interactions, the potential applications are never far from Rivera’s mind. “We’re trying to think about applications that could potentially be realized next year and in the next five years, but even potentially further down the line.”
For Rivera, being able to be in the same place as his collaborators is a major boon, and he doubts the continued collaboration with Kaminer would be as active if he hadn’t taken that first trip to Haifa in 2018. “And the hummus isn’t bad,” he jokes.
When Soljačić introduced Rivera and Kaminer five years ago, neither expected that the collaboration would still be going strong. “It’s hard to anticipate what collaborations will be successful in the long term,” says Kaminer. “But more important than the collaboration is the friendship,” he adds.
The deeper Rivera explores the quantum aspects of light-matter interactions, the more potential avenues of exploration open up. “It just keeps branching,” says Rivera. And he envisions himself continuing to visit Kaminer in Israel, no matter where his research takes him next. “It’s a lifelong collaboration at this point.”
In 2018 it was discovered that two layers of graphene twisted one with respect to the other by a “magic” angle show a variety of interesting quantum phases, including superconductivity, magnetism and insulating behaviours. Now a team of researchers from the Weizmann Institute of Science led by Prof. Shahal Ilani of the Condensed Matter Physics Department, in collaboration with Prof. Pablo Jarillo-Herrero’s group at MIT, have discovered that these quantum phases descend from a previously unknown high-energy “parent state,” with an unusual breaking of symmetry.
Graphene is a flat crystal of carbon, just one atom thick. When two sheets of this material are placed on top of each other, misaligned at small angle, a periodic “moiré” pattern appears. This pattern provides an artificial lattice for the electrons in the material. In this twisted bilayer system the electrons come in four “flavours”: spins “up” or “down,” combined with two “valleys” that originate in the graphene’s hexagonal lattice. As a result, each moiré site can hold up to four electrons, one of each flavour.
While researchers already knew that the system behaves as a simple insulator when all the moiré sites are completely full (four electrons per site), Jarillo-Herrero and his colleagues discovered to their surprise, in 2018, that at a specific “magic" angle, the twisted system also becomes insulating at other integer fillings (two or three electrons per moiré site). This behaviour, exhibited by magic-angle twisted bilayer graphene (MATBG), cannot be explained by single particle physics, and is often described as a “correlated Mott insulator.” Even more surprising was the discovery of exotic superconductivity close to these fillings. These findings led to a flurry of research activity aiming to answer the big question: what is the nature of the new exotic states discovered in MATBG and similar twisted systems?
Imaging magic-angle graphene electrons with a carbon nanotube detector
The Weizmann team set out to understand how interacting electrons behave in MATBG using a unique type of microscope that utilizes a carbon nanotube single-electron transistor, positioned at the edge of a scanning probe cantilever. This instrument can image, in real space, the electric potential produced by electrons in a material with extreme sensitivity.
“Using this tool, we could image for the first time the ‘compressibility' of the electrons in this system – that is, how hard it is to squeeze additional electrons into a given point in space,” explains Ilani. “Roughly speaking, the compressibility of electrons reflects the phase they are in: In an insulator, electrons are incompressible, whereas in a metal they are highly compressible.”
Compressibility also reveals the “effective mass” of electrons. For example, in regular graphene the electrons are extremely “light,” and thus behave like independent particles that practically ignore the presence of their fellow electrons. In magic-angle graphene, on the other hand, electrons are believed to be extremely “heavy” and their behaviour is thus dominated by interactions with other electrons ‒ a fact that many researchers attribute to the exotic phases found in this material. The Weizmann team therefore expected the compressibility to show a very simple pattern as a function of electron filling: interchanging between a highly-compressible metal with heavy electrons and incompressible Mott insulators that appear at each integer moiré lattice filling.
To their surprise, they observed a vastly different pattern. Instead of a symmetric transition from metal to insulator and back to metal, they observed a sharp, asymmetric jump in the electronic compressibility near the integer fillings.
"This means that the nature of the carriers before and after this transition is markedly different," says study lead author Uri Zondiner. "Before the transition the carriers are extremely heavy, and after it they seem to be extremely light, reminiscent of the ‘Dirac electrons’ that are present in graphene."
The same behaviour was seen to repeat near every integer filling, where heavy carriers abruptly gave way and light Dirac-like electrons re-emerged.
But how can such an abrupt change in the nature of the carriers be understood? To address this question, the team worked together with Weizmann theorists Profs. Erez Berg, Yuval Oreg and Ady Stern, and Dr. Raquel Quiroez; as well as Prof. Felix von-Oppen of Freie Universität Berlin. They constructed a simple model, revealing that electrons fill the energy bands in MATBG in a highly unusual “Sisyphean” manner: when electrons start filling from the “Dirac point” (the point at which the valence and conduction bands just touch each other), they behave normally, being distributed equally among the four possible flavours. “However, when the filling nears that of an integer number of electrons per moiré superlattice site, a dramatic phase transition occurs,” explains study lead author Asaf Rozen. “In this transition, one flavour ‘grabs’ all the carriers from its peers, ‘resetting’ them back to the charge-neutral Dirac point.”
“Left with no electrons, the three remaining flavours need to start refilling again from scratch. They do so until another phase transition occurs, where this time one of the remaining three flavours grabs all the carriers from its peers, pushing them back to square one. Electrons thus need to climb a mountain like Sisyphus, being constantly pushed back to the starting point in which they revert to the behavior of light Dirac electrons,” says Rozen. While this system is in a highly symmetric state at low carrier fillings, in which all the electronic flavours are equally populated, with further filling it experiences a cascade of symmetry-breaking phase transitions that repeatedly reduce its symmetry.
A “parent state”
“What is most surprising is that the phase transitions and Dirac revivals that we discovered appear at temperatures well above the onset of the superconducting and correlated insulating states observed so far,” says Ilani. “This indicates that the broken symmetry state we have seen is, in fact, the ‘parent state’ out of which the more fragile superconducting and correlated insulating ground states emerge.”
The peculiar way in which the symmetry is broken has important implications for the nature of the insulating and superconducting states in this twisted system.
“For example, it is well known that stronger superconductivity arises when electrons are heavier. Our experiment, however, demonstrates the exact opposite: superconductivity appears in this magic-angle graphene system after a phase transition has revived the light Dirac electrons. How this happens, and what it tells us about the nature of superconductivity in this system compared to other more conventional forms of superconductivity remain interesting open questions,” says Zondiner.
A similar cascade of phase transitions was reported in another paper published in the same Nature issue by Prof. Ali Yazdani and colleagues at Princeton University. "The Princeton team studied MATBG using a completely different experimental technique, based on a highly-sensitive scanning tunneling microscope, so it is very reassuring to see that complementary techniques lead to analogous observations,” says Ilani.
The Weizmann and MIT researchers say they will now use their scanning nanotube single-electron-transistor platform to answer these and other basic questions about electrons in various twisted-layer systems: What is the relationship between the compressibility of electrons and their apparent transport properties? What is the nature of the correlated states that form in these systems at low temperatures? And what are the fundamental quasiparticles that make up these states?
An exotic physical phenomenon known as a Kohn anomaly has been found for the first time in an unexpected type of material by researchers at MIT and elsewhere. They say the finding could provide new insights into certain fundamental processes that help determine why metals and other materials display the complex electronic properties that underlie much of today’s technology.
The way electrons interact with phonons — which are essentially vibrations passing through a crystalline material — determines the physical processes that take place inside many electronic devices. These interactions affect the way metals resist electric current, the temperature at which some materials suddenly become superconductors, and the very low temperature requirements for quantum computers, among many other processes.
But electron-phonon interactions have been difficult to study in detail because they are generally very weak. The new study has found a new, stronger kind of unusual electron-phonon interaction: The researchers induced a Kohn anomaly, which was previously thought to exist only in metals, in an exotic material called a topological Weyl semimetal. The finding could help shed light on important aspects of the complex interplay between electrons and phonons, they say.
The new finding, based on both theoretical predictions and experimental observation, is described this week in the journal Physical Review Letters, in a paper by MIT graduate students Thanh Nguyen and Nina Andrejevic, postdoc Ricardo Pablo-Pedro, Research Scientist Fei Han, Professor Mingda Li, and 14 others at MIT and several other universities and national laboratories.
Kohn anomalies, first discovered in the 1950s by physicist Walter Kohn, reflect a sudden change, sometimes described as a kind of kink or wiggle, in the graph describing a physical parameter called the electron response function. This discontinuity in an otherwise smooth curve reflects a sudden change of the capability of electrons for shielding phonons. This can give rise to instabilities in the propagation of electrons through the material, and can lead to many new electronic properties.
These anomalies have been observed before in certain metals and in other highly electrically conductive materials such as graphene, but had never been seen or predicted before in a “topological material,” whose electrical behaviors are robust against perturbation. In this case, a kind of topological material called a Weyl semimetal, specifically tantalum phosphide, was found to be capable of exhibiting this unusual anomaly. Unlike in conventional metals, where a property called the Fermi surface drives the formation of the Kohn anomaly, in this material, the Weyl points serve as the driving force.
Because electron-phonon couplings are taking place practically everywhere all the time, they can be a major source of disturbance in delicate physical systems such as those used to represent data in quantum computers. Measuring the strength of these interactions, which is key to knowing how to protect such quantum-based technologies, has been very difficult, but this new finding, Li says, provides a way of making such measurements. “The Kohn anomaly can be used to quantify how strong the electron-phonon coupling can be,” he says.
To measure the interactions, the team made use of advanced neutron and X-ray scattering probes at three national laboratories — Argonne National Laboratory, Oak Ridge National Laboratory, and the National Institute of Standards and Technology — to probe the behavior of the tantalum phosphide material. “We predicted that there is a Kohn anomaly in the material just based on pure theory,” Li explains, Using their calculations, “we could guide the experiments to the point where we want to search for the phenomenon, and we see a very good agreement between theory and the experiments.”
Martin Greven, a professor of physics at the University of Minnesota who was not involved in this research, says this work “has impressive breadth and depth, spanning both sophisticated theory and scattering experiments. It breaks new ground in condensed matter physics, in that it establishes a new kind of Kohn anomaly.”
A better understanding of the electron-phonon couplings could help lead the way to developing such materials as better high-temperature superconductors or fault-tolerant quantum computers, the researchers say. This new tool could be used to probe material properties in search of those that remain relatively unaffected at higher temperatures.
Brent Fultz, a professor of materials science and applied physics at Caltech, who was also not involved in this work, adds that “perhaps these effects will help the development of materials with new thermal or electronic properties, but since they are so new, we need time to think about what they can do.”
Nguyen, the paper’s lead author, says he thinks this work helps to demonstrate the sometimes overlooked importance of phonons in the behavior of topological materials. Materials such as these, whose surface electrical properties are different from those of the bulk material, are a hot area of current research. “I think this could lead us to further understand processes that would underlie some of these materials that hold a lot of promise for the future,” says Andrejevic, who along with Han was a co-lead author on the paper.
“Although electron-phonon interaction is long known to exist, the experimental prediction and observation of these interactions is exceedingly rare,” says professor of physics and astronomy Pengcheng Dai at Rice University, who also was not involved in this work. These results, he says, “provide an excellent demonstration of the power of combined theory and experiments as a way to extend our understanding of these exotic materials.”
Researchers from Skoltech, Aalto University and Massachusetts Institute of Technology have designed a high-performance, low-cost, environmentally friendly, and stretchable supercapacitor that can potentially be used in wearable electronics. The paper was published in the Journal of Energy Storage.
Supercapacitors, with their high power density, fast charge-discharge rates, long cycle life, and cost-effectiveness, are a promising power source for everything from mobile and wearable electronics to electric vehicles. However, combining high energy density, safety, and eco-friendliness in one supercapacitor suitable for small devices has been rather challenging.
"Usually, organic solvents are used to increase the energy density. These are hazardous, not environmentally friendly, and they reduce the power density compared to aqueous electrolytes with higher conductivity," says Professor Tanja Kallio from Aalto University, a co-author of the paper.
The researchers proposed a new design for a "green" and simple-to-fabricate supercapacitor. It consists of a solid-state material based on nitrogen-doped graphene flake electrodes distributed in the NaCl-containing hydrogel electrolyte. This structure is sandwiched between two single-walled carbon nanotube film current collectors, which provides stretchability. Hydrogel in the supercapacitor design enables compact packing and high energy density and allows them to use the environmentally friendly electrolyte.
The scientists managed to improve the volumetric capacitive performance, high energy density and power density for the prototype over analogous supercapacitors described in previous research. "We fabricated a prototype with unchanged performance under the 50% strain after a thousand stretching cycles. To ensure lower cost and better environmental performance, we used a NaCl-based electrolyte. Still the fabrication cost can be lowered down by implementation of 3D printing or other advanced fabrication techniques," concluded Skoltech professor Albert Nasibulin.
Place a single sheet of carbon atop another at a slight angle and remarkable properties emerge, including the highly prized resistance-free flow of current known as superconductivity.
Now a team of researchers at Princeton University has looked for the origins of this unusual behavior in a material known as magic-angle twisted bilayer graphene, and detected signatures of a cascade of energy transitions that could help explain how superconductivity arises in this material. The study was published June 11 in the journal Nature.
“This study shows that the electrons in magic-angle graphene are in a highly correlated state even before the material becomes superconducting,” said Ali Yazdani, Class of 1909 Professor of Physics, the leader of the team that made the discovery. “The sudden shift of energies when we add or remove an electron in this experiment provides a direct measurement of the strength of the interaction between the electrons.”
This is significant because these energy jumps provide a window into the collective behaviors of electrons, such as superconductivity, that emerge in magic-angle twisted bilayer graphene, a material composed of two layers of graphene in which the top sheet is rotated by a slight angle relative to the other.
In everyday metals, electrons can move freely through the material, but collisions among electrons and from the vibration of atoms give rise to resistance and the loss of some electrical energy as heat – which is why electronic devices get warm during use.
In superconducting materials, electrons cooperate. “The electrons are kind of dancing with each other,” said Biao Lian, a postdoctoral research associate in the Princeton Center for Theoretical Science who will become an assistant professor of physics this fall, and one of the co-first authors of the study. “They have to collaborate to go into such a remarkable state.”
By some measures, magic-angle graphene, discovered two years ago by Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology (MIT), is one of the strongest superconductors ever discovered. Superconductivity is relatively robust in this system even though it occurs when there are very few freely moving electrons.
The researchers set out to explore how the unique crystal structure of magic-angle graphene enables collective behaviors. Electrons not only have a negative charge, but also two other characteristics: angular momentum or “spin,” and possible movements in the crystal structure known as “valley” states. Combinations of spin and valley make up the various “flavors” of electrons.
The team particularly wanted to know how these flavors affect collective behaviors, so they conducted their experiments at temperatures just slightly above the point at which the electrons become strongly interacting, which the researchers likened to the parent phase of the behaviors.
“We measured the force between the electrons in the material at higher temperatures in the hopes that understanding this force will help us understand the superconductor that it becomes at lower temperatures,” said Dillon Wong, a postdoctoral research fellow in the Princeton Center for Complex Materials and a co-first author.
They used a tool called a scanning tunneling microscope, in which a conductive metal tip can add or remove an electron from magic-angle graphene and detect the resulting energy state of that electron.
Researchers used scanning tunneling microscopy to detect electrons in the material known as magic-angle bilayer graphene. Image by the Yazdani lab at Princeton University. Because strongly interacting electrons resist the addition of a new electron, it costs some energy to add the additional electron. The researchers can measure this energy and from it determine the strength of the interaction force.
“I’m literally putting an electron in and seeing how much energy it costs to shove this electron into the cooperative bath,” said Kevin Nuckolls, a graduate student in the Department of Physics, also a co-first author.
The team found that the addition of each electron caused a jump in the amount of energy needed to add another one – which would not have been the case if the electrons were able to go into the crystal and then move freely among the atoms. The resulting cascade of energy transitions resulted from an energy jump for each of the electrons’ flavors – since electrons need to assume the lowest energy state possible while also not being of the same energy and same flavor as other electrons at the same location in the crystal.
A key question in the field is how the strength of interactions between electrons compares to the energy levels that the electrons would have had in the absence of such interactions. In most common and low-temperature superconductors, this is a small correction, but in rare high-temperature superconductors, the interactions among electrons are believed to change the energy levels of the electrons dramatically. Superconductivity in the presence of such a dramatic influence of interactions among electrons is very poorly understood.
A cascade of changes in the electronic properties of magic-angle graphene are observed by high-resolution scanning tunneling microscopy as a function of applied voltage, which tunes the electron filling between fully occupied (v = 4) and empty (v = -4). Image by the Yazdani lab at Princeton University. Published in Nature. The quantitative measurements of the sudden shifts detected by the researchers confirms the picture that magic-angle graphene belongs to the class of superconductors with strong interaction among the electrons.
Graphene is a single-atom-thin layer of carbon atoms, which, due to the chemical properties of carbon, arrange themselves in a flat honeycomb lattice. The researchers obtain graphene by taking a thin block of graphite – the same pure carbon used in pencils – and removing the top layer using sticky tape.
They then stack two atom-thin layers and rotate the top layer by exactly 1.1 degrees – the magic angle. Doing this causes the material to become superconducting, or attain unusual insulating or magnetic properties.
“If you’re at 1.2 degrees, it’s bad. It’s, it’s just a bland metal. There’s nothing interesting happening. But if you’re at 1.1 degrees, you see all this interesting behavior,” Nuckolls said.
This misalignment creates an arrangement known as a moiré pattern for its resemblance to a French fabric.
To conduct the experiments, the researchers built a scanning tunneling microscope in the basement of Princeton’s physics building, Jadwin Hall. So tall that it occupies two floors, the microscope sits atop a granite slab, which floats on air springs. “We need to isolate the equipment very precisely because it is extremely sensitive to vibrations,” said Myungchul Oh, a postdoctoral research associate and co-first author. Dillon Wong, Kevin Nuckolls, Myungchul Oh, and Biao Lian contributed equally to the work.
Additional contributions were made by Yonglong Xie, who earned his Ph.D. in 2019 and is now a postdoctoral researcher at Harvard University; Sangjun Jeon, who is now an assistant professor at Chung-Ang University in Seoul; Kenji Watanabe and Takashi Taniguchi of the National Institute for Material Science (NIMS) in Japan; and Princeton Professor of Physics B. Andrei Bernevig.
A similar cascade of electronic phase transitions was noted in a paper published simultaneously in Nature on June 11 by a team led by Shahal Ilani at the Weizmann Institute of Science in Israel and featuring Jarillo-Herrero and colleagues at MIT, Takashi Taniguchi and Kenji Watanabe of NIMS Japan, and researchers at the Free University of Berlin.
“The Weizmann team observed the same transitions as we did with a completely different technique,” Yazdani said. “It is nice to see that their data are compatible with both our measurements and our interpretation.”
The study, “Cascade of electronic transitions in magic-angle twisted bilayer graphene,” by Dillon Wong, Kevin P. Nuckolls, Myungchul Oh, Biao Lian, Yonglong Xie, Sangjun Jeon, Kenji Watanabe, Takashi Taniguchi, B. Andrei Bernevig, and Ali Yazdani, was published June 11 in the journal Nature. [DOI 10.1038/s41586-020-2339-0]
The UT Austin Portugal program, a 13-year-old innovation partnership between the university and the Portuguese government, received $23.6 million in funding to pursue 11 R&D projects as part of a major technology initiative from Portugal’s Ministry of Science, Technology and Higher Education.
The projects fall under four major categories: nanomaterials, earth-space interactions, medical physics and advanced computing. The teams will spend the next three years developing their projects, which could transform industries like automotive, space, health care and data science.
“Ranging from electromagnetic interference shielding nanomaterials, to in-beam time-of-flight positron emission tomography for proton radiation therapy, all the way to an ocean and climate change monitoring constellation based on radar altimeter data combined with gravity and ocean temperature and salinity measurements, the spread, number, and quality of the UT Austin Portugal joint strategic projects selected for funding within the recent competitive solicitation set forth by the Foundation for Science and Technology and National Innovation Agency are truly outstanding,” said Manuel Heitor, Portugal’s Minister of Science, Technology and Higher Education. “I look forward to witnessing the results of such collaborative research between Portuguese and UT researchers.”
The call for proposals included just three universities: The University of Texas at Austin, Carnegie Mellon University and the Massachusetts Institute of Technology. UT won the majority of the investment dollars, about 40% of the funding, and saw the most projects funded among the three engineering powerhouses.
“We had anticipated four to five projects would be selected for strategic grant awards and were astounded when we learned 11 had been selected by the evaluation panel in Portugal,” said John Ekerdt, Cockrell School associate dean for research and principal investigator for UT Austin Portugal. “This is a testament to the outstanding faculty and quality projects they proposed with collaborators in Portugal and to the close ties that have been forged between UT researchers and faculty and counterparts in Portugal.”
“The performance of the UT Austin Portugal program in the 2019 call for strategic projects has been remarkable,” said Marco Bravo, executive director of the UT Austin Portugal program. “Eleven of 14 project proposals submitted by the UT Austin Portugal research consortia were approved for funding through an independent assessment process. Overall, UT Austin Portugal saw 11 of its groundbreaking, industry-led proposals approved out of a total of 25 projects approved at this solicitation that included proposals from two other international partnerships, corresponding to nearly $24 million over three years. That’s 40% of total funding to UT Austin Portugal projects, the largest share of research dollars available. UT Austin researchers are to be congratulated on this effort.”
The UT Austin Portugal program dates back to 2007, and it is one of several partnerships between the Portuguese government and research institutions. The goal is to elevate science and technology in Portugal while fostering strong partnerships to help universities continue to innovate. The partnership with UT was extended in 2018, continuing the alliance until at least 2030.
“Of the three international partnerships with American universities sponsored by the Portuguese Foundation for Science and Technology in Portugal, the partnership with UT Austin had the best performance in this call, which was designed and launched on the Portuguese side,” said José Manuel Mendonça, national director of the program. “The 11 approved projects represent a proposal success rate of almost 80% for the UT Austin Portugal Program. The approved projects will, undoubtedly, contribute to promoting and strengthening collaborations with UT Austin in high-level R&D matters with immediate transposition to various sectors of economic activity, several of which are critical to Portugal's competitive position at an international level.”
About a third of the funds for UT’s projects come from the university, with the rest coming from a combination of public and private Portuguese entities. Each project team in Portugal is led by a Portuguese company. The UT side includes 21 faculty members and one from the MD Anderson Cancer Center.
Here is a look at the UT projects:
Shielding electronic devices from electromagnetic interference
This project proposes to use the “wonder material” graphene to improve on methods to combat electromagnetic interference, which can disrupt circuits and cause devices to fail. The team plans to create two composites with electromagnetic interference shielding capabilities and fabricate a solution to protect electric wires used in the automotive industry.
UT Austin Faculty: Deji Akinwande, Cockrell School of Engineering, Department of Electrical and Computer Engineering; Brian Korgel, Cockrell School of Engineering, McKetta Department of Chemical Engineering
New lasers for next-generation biomedical imaging
The use of multiphoton microscopy to examine cell behavior in live tissue over time has become an important research tool for learning more about brains and tumors. This project aims to increase the speed and depth of this form of imaging and diagnostics through the development and application of ultrashort laser pulses.
UT Austin Faculty: Andrew Dunn, Cockrell School of Engineering, Department of Biomedical Engineering; Adela Ben-Yakar, Cockrell School of Engineering, Walker Department of Mechanical Engineering
Nano-satellites for gravitational field assessment
Researchers propose to develop a nano-satellite prototype for studying gravitational fields. The project will also develop a platform for future nano-satellite capabilities, including Earth observation, communications and exploration missions.
UT Austin Faculty: Byron Tapley, Cockrell School of Engineering, Department of Aerospace Engineering and Engineering Mechanics, and the Center for Space Research; Brandon Jones, Cockrell School of Engineering, Department of Aerospace Engineering and Engineering Mechanics, and the Texas Spacecraft Laboratory
Software to match big data with high-performance computing
The advancement of technology has generated huge troves of data, which requires stronger computing power to process and analyze all that information. This project aims to create a software bundle to help companies pair their big data operations with high-performance computing, which includes tools for managing challenges such as computing and research storage.
UT Austin Faculty: Vijay Chidambaram, College of Nature Sciences, Department of Computer Science; Todd Evans, Texas Advanced Computing Center
Sensors for monitoring cancer patients
This project will develop a biosensor that can be injected into prostate cancer patients after surgery. The minimally invasive sensor would allow medical personnel to monitor high-risk patients remotely and look for the development of early tumors, with the potential to increase the predictive value of cancer screenings.
UT Austin Faculty: Thomas Milner, Cockrell School of Engineering, Department of Biomedical Engineering; James Tunnell, Cockrell School of Engineering, Department of Biomedical Engineering
Wearable rehabilitation devices
Researchers will develop a series of nano-sensors embedded into clothing that administer electrostimulation to people suffering from a lack of mobility and motor deficiency. The sensors could be monitored remotely by health professionals, creating a mobile rehabilitation option for people who have trouble getting to a doctor’s office consistently or want greater freedom to complete treatment anywhere. The team envisions its project as a tool mostly for elderly people, but it has applications for training high-level athletes as well.
UT Austin Faculty: George Biros, Cockrell School of Engineering, Walker Department of Mechanical Engineering, and the Oden Institute for Computational Engineering and Sciences; Michael Cullinan, Cockrell School of Engineering, Walker Department of Mechanical Engineering
Software for gathering better data on manufacturing
Getting reliable data on manufacturing processes proves challenging due to issues with placing sensors in the right spots and retaining strong connectivity. Thin films loaded with small sensors that can be applied directly to the equipment represent a promising solution; however, installation has proved difficult. This project proposes a new set of software to make it easier to layer these films on top of equipment by providing necessary data to avoid mechanical problems during installation.
UT Austin Faculty: Rui Huang, Cockrell School of Engineering, Department of Aerospace Engineering and Engineering Mechanics, Center for Mechanics of Solids, Structures and Materials; Kenneth M. Liechti, Cockrell School of Engineering, Department of Aerospace Engineering and Engineering Mechanics, Center for Mechanics of Solids, Structures and Materials
A new way to measure next-generation cancer therapy
Proton radiation therapy, the use of protons rather than X-rays to treat cancer patients, is on the rise, but measuring the distance protons travel proves problematic. Typically, it takes a ring of detectors surrounding the patient to get accurate measurements, but that poses geometric challenges. This project proposes to develop a new type of Positron Emission Tomography scan, which shows how tissues and organs are functioning to better understand the range of protons and whether they are traveling to the right spots to attack the cancer.
UT Faculty: Karol Lang, College of Natural Sciences, Department of Physics; Narayan Sahoo, University of Texas MD Anderson Cancer Center, Department of Radiation Physics
Satellite constellations for monitoring climate change
This project aims to develop the next generation of radar altimeter instruments — which measure the distance between an aircraft and the terrain below it — and a series of small satellites that can understand long-term variability in local, regional and global climate created by changes in sea levels due to water temperature. The project also includes a data processing and visualization system using advanced modeling, estimation techniques, statistical and scientific machine learning methods and error analysis.
UT Austin Faculty: Byron Tapley, Cockrell School of Engineering, Department of Aerospace Engineering and Engineering Mechanics Department, and the Center for Space Research; Patrick Heimbach, Jackson School of Geosciences, Department of Geological Sciences, and the Oden Institute for Computational Engineering and Sciences
Improving cutting tools for airline and automotive components
Fabricating parts of cars and planes is hard on cutting tools and tends to ware them down. This project aims to develop coatings that better protect and extend the lifespan of these crucial pieces of equipment. The team also plans to develop simulation programs to improve cutting tools’ performance.
UT Austin Faculty: Gregory J. Rodin, Cockrell School of Engineering, Department of Aerospace Engineering and Engineering Mechanics, and the Oden Institute for Computational Engineering and Sciences; Filippo Mangolini, Cockrell School of Engineering, Walker Department of Mechanical Engineering
An alternative to traditional water treatment options
Traditional water treatment tech struggles to efficiently remove high amounts of pollutants from some types of surface and groundwater. This team is looking to use metallic nanoparticles to clean water by improving a process called catalytic hydrogenation, which involves adding hydrogen via a metallic catalyst.
UT Austin Faculty: Charles J. Werth, Cockrell School of Engineering, Department of Civil, Architectural, and Environmental Engineering; Simon M. Humphrey, College of Natural Sciences, Department of Chemistry
A new way of making large sheets of high-quality, atomically thin graphene could lead to ultra-lightweight, flexible solar cells, and to new classes of light-emitting devices and other thin-film electronics.
The new manufacturing process, which was developed at MIT and should be relatively easy to scale up for industrial production, involves an intermediate “buffer” layer of material that is key to the technique’s success. The buffer allows the ultrathin graphene sheet, less than a nanometer (billionth of a meter) thick, to be easily lifted off from its substrate, allowing for rapid roll-to-roll manufacturing.
The process is detailed in a paper published yesterday in Advanced Functional Materials, by MIT postdocs Giovanni Azzellino and Mahdi Tavakoli; professors Jing Kong, Tomas Palacios, and Markus Buehler; and five others at MIT.
Finding a way to make thin, large-area, transparent electrodes that are stable in open air has been a major quest in thin-film electronics in recent years, for a variety of applications in optoelectronic devices — things that either emit light, like computer and smartphone screens, or harvest it, like solar cells. Today’s standard for such applications is indium tin oxide (ITO), a material based on rare and expensive chemical elements.
Many research groups have worked on finding a replacement for ITO, focusing on both organic and inorganic candidate materials. Graphene, a form of pure carbon whose atoms are arranged in a flat hexagonal array, has extremely good electrical and mechanical properties, yet it is vanishingly thin, physically flexible, and made from an abundant, inexpensive material. Furthermore, it can be easily grown in the form of large sheets by chemical vapor deposition (CVD), using copper as a seed layer, as Kong’s group has demonstrated. However, for device applications, the trickiest part has been finding ways to release the CVD-grown graphene from its native copper substrate.
This release, known as graphene transfer process, tends to result in a web of tears, wrinkles, and defects in the sheets, which disrupts the film continuity and therefore drastically reduces their electrical conductivity. But with the new technology, Azzellino says, “now we are able to reliably manufacture large-area graphene sheets, transfer them onto whatever substrate we want, and the way we transfer them does not affect the electrical and mechanical properties of the pristine graphene.”
The key is the buffer layer, made of a polymer material called parylene, that conforms at the atomic level to the graphene sheets on which it is deployed. Like graphene, parylene is produced by CVD, which simplifies the manufacturing process and scalability.
As a demonstration of this technology, the team made proof-of-concept solar cells, adopting a thin-film polymeric solar cell material, along with the newly formed graphene layer for one of the cell’s two electrodes, and a parylene layer that also serves as a device substrate. They measured an optical transmittance close to 90 percent for the graphene film under visible light.
The prototyped graphene-based solar cell improves by roughly 36 times the delivered power per weight, compared to ITO-based state-of-the-art devices. It also uses 1/200 the amount of material per unit area for the transparent electrode. And, there is a further fundamental advantage compared to ITO: “Graphene comes for almost free,” Azzellino says.
“Ultra-lightweight graphene-based devices can pave the way to a new generation of applications,” he says. “So if you think about portable devices, the power per weight becomes a very important figure of merit. What if we could deploy a transparent solar cell on your tablet that is able to power up the tablet itself?” Though some further development would be needed, such applications should ultimately be feasible with this new method, he says.
The buffer material, parylene, is widely used in the microelectronics industry, usually to encapsulate and protect electronic devices. So the supply chains and equipment for using the material already are widespread, Azzellino says. Of the three existing types of parylene, the team’s tests showed that one of them, which contains more chlorine atoms, was by far the most effective for this application.
The atomic proximity of chlorine-rich parylene to the underlying graphene as the layers are sandwiched together provides a further advantage, by offering a kind of “doping” for graphene, finally providing a more reliable and nondestructive approach for conductivity improvement of large-area graphene, unlike many others that have been tested and reported so far.
“The graphene and the parylene films are always face-to-face,” Azzellino says. “So basically, the doping action is always there, and therefore the advantage is permanent.”
The research team also included Marek Hempel, Ang-Yu Lu, Francisco Martin-Martinez, Jiayuan Zhao and Jingjie Yeo, all at MIT. The work was supported by Eni SpA through the MIT Energy Initiative, the U.S. Army Research Office through the Institute for Soldier Nanotechnologies, and the Office of Naval Research.
When ultrathin layered materials are coupled with other quantum materials having different properties, the resulting interface could produce a new quantum phenomenon — and new properties of the hybrid system could be unprecedented. This rich interface phenomenon is the topic of new investigation by Jagadeesh Moodera and his group at MIT's Plasma Science and Fusion Center.
“Surface and interface play pivotal roles in many of the recently discovered quantum phenomena in condensed matter physics,” Moodera points out. “Investigating the complex interface behavior when two quantum systems are coupled is a treasure island to be explored for new discoveries and for advancing the field.”
Moodera’s group has extensive experience studying quantum interfaces, having discovered in 2016 that coupling ultrathin layers of topological insulators (TI) — where electrons flow freely but only on the surface — with ferromagnetic or superconducting layers dramatically affects the behavior of each layer. Most recently they've explored the surface superconductivity in nanostructures of gold in proximity to the superconductor vanadium and a ferromagnetic insulator, in the quest to create the enigmatic Majorana fermion pair.
New multiyear funding and an equipment grant from the U.S. Department of Defense (DoD) Army Research Office will support novel work exploring the behavior that arises at the interface of quantum materials, and will help uncover ways to tune these new properties to develop future quantum electronics. Working closely with Argonne, Brookhaven, and Oak Ridge national laboratories advanced facilities, Moodera's group will explore interface effects, such as “interfacial exchange coupling,” with the goal of creating energy-efficient quantum devices.
As part of the DoD project, they will work with scientists at the Army Research Lab (ARL) in Maryland to build an ultraclean, atomically groomed multifunctional hybrid materials platform to probe the interplay between various quantum phenomena. They will experiment with tuning the interface to create quantum materials for building devices that could, for example, operate faster and use less energy. Moodera anticipates that some results may lead them into unexpected territory, possibly guiding them to even more surprising observations in quantum materials.
“If we can build new devices with two-dimensional quantum materials that have desirable properties, such as graphene or TI, and change their state for memory and logic with electric fields rather than actual flowing current, we will have gained a big advantage over conventional electronics.”
This could lead to greatly improved quantum electronics, including quantum sensors, memories, and interconnects. It could benefit computer microprocessors, which contain hundreds of millions of transistors that are affected by heat generated in them by conventional electronics, and powerful computer data storage banks, which consume about 2-3 percent of all the electrical power in the country.
Moodera looks forward to working with postdocs Hang Chi (visiting from ARL) and Yunbo Ou, along with instrument scientist Valeria Lauter from Oak Ridge National Laboratory, visiting scientist from Northeastern University Don Heiman, and other worldwide collaborators. He is excited about the possibilities their new research will uncover.
“Our DoD-supported project allows us to explore exciting physics and to address important scientific questions at the atomic scale, advancing experimental knowledge and theoretical understanding,” he says. “We aim to build a safer, sustainable, and energy-efficient future quantum information system, one layer at a time.”