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Stopping the unstoppable with atomic bricks

Posted By Graphene Council, Monday, June 29, 2020
Graphene's unique 2D structure means that electrons travel through it differently to most other materials. One consequence of this unique transport is that applying a voltage to them doesn't stop the electrons like it does in most other materials. This is a problem because to make useful applications out of graphene and its unique electrons like quantum computers, it is necessary to be able to stop and control graphene electrons.

An interdisciplinary team of scientists from the Universidad Autonoma de Madrid (Spain), Université Grenoble Alpes (France), International Iberian Nanotechnology Laboratory (Portugal) and Aalto University has managed to solve this long-standing problem. They combined experimental researchers including Eva Cortés del Río, Pierre Mallet, Héctor González‐Herrero, José María Gómez‐Rodríguez, Jean‐Yves Veuillen and Iván Brihuega with theorists, including Joaquín Fernández-Rossier and Jose Lado, assistant Professor in the department of Applied Physics at Aalto.

The experimental team used atomic bricks to build walls capable of stopping the graphene electrons. This was achieved by creating atomic walls that confined the electrons, leading to structures whose spectrum was then compared with theoretical predictions, demonstrating that electrons were confined. In particular, it was obtained that the engineered structures gave rise to nearly perfect confinement of electrons, as demonstrated from the emergence of sharp quantum well resonances with a remarkably long lifetime.

The work, published this week in Advanced Materials, demonstrates that impenetrable walls for graphene electrons can be created by collective manipulation of a large number of hydrogen atoms. In the experiments, a scanning tunnelling microscope was used to construct artificial walls with sub nanometric precision. This led to graphene nanostructures of arbitrarily complex shapes, with dimensions ranging from two nanometres to one micron.

Importantly, the developed method is non-destructive, allowing to erase and rebuild the nanostructures at will, providing an unprecedented degree of control to create artificial graphene devices. The experiments demonstrate that the engineered nanostructures are capable of perfectly confining the graphene electrons in these artificially designed structures, overcoming the critical challenge imposed by Klein tunnelling. Ultimately, this opens up a plethora of exciting new possibilities, as the created nanostructures realize graphene quantum dots that can be selectively coupled, opening ground-breaking possibilities for artificially designed quantum matter.

Tags:  2D materials  Aalto University  Graphene  Jose Lado  nanostructures  quantum materials  Universidad Autonoma de Madrid 

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Newly observed phenomenon could lead to new quantum devices

Posted By Graphene Council, Monday, June 15, 2020
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.”

Tags:  Brent Fultz  Caltech  Graphene  Martin Greven  Massachusetts Institute of Technology  Mingda Li  quantum materials  University of Minnesota 

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New method predicts spin dynamics of materials for quantum computing

Posted By Graphene Council, Thursday, June 4, 2020
Researchers at UC Santa Cruz have developed a theoretical foundation and new computational tools for predicting a material's spin dynamics, a key property for building solid-state quantum computing platforms and other applications of spintronics.

Spin is a fundamental property of electrons and other particles, and the rapidly growing field of spintronics uses spin states in a manner analogous to the use of electrical charge in electronics. Spin can be used as the basis for qubits (quantum bits) and single-photon emitters in applications of quantum information science, including quantum computation, communication, and sensing.

Qubits can be made from any quantum system that has two states, but the challenge is to maintain quantum coherence (a relationship between quantum states) long enough to allow manipulation of the qubits. Decoherence means a loss of information from the system, and spin qubits can lose coherence by interacting with their environment through, for example, lattice vibrations within the material.

"The key property for quantum information science is the lifetime of the spin states, known as the spin relaxation and decoherence time," said Yuan Ping, assistant professor of chemistry at UC Santa Cruz. "For quantum information applications, we need materials with long spin relaxation times."

In a paper published June 3 in Nature Communications, Ping and her coauthors at UCSC and Rensselaer Polytechnic Institute present a new theoretical framework and computational tools for accurately predicting the spin relaxation time of any material, which was not previously possible.

"These days, people just make a material and try it to see whether it works. Now we have the predictive capability from quantum mechanics that will allow us to design materials with the properties we want for applications in quantum information science," she said. "And if you have a promising material, this can tell you how to change it to make it better."

The researchers established methods for determining spin dynamics from first principles, meaning that no empirical parameters from experimental measurements are needed to do the calculations. They also showed that their approach is generalizable to different types of materials with vastly different crystal symmetries and electronic structures.

For example, they predicted accurately the spin relaxation time of centrosymmetric materials such as silicon, ferromagnetic iron, and graphene, as well as non-centrosymmetric materials such as molybdenum disulfide and gallium nitride, highlighting the predictive power of their method for a broad range of quantum materials.

By enabling the rational design of materials, instead of searching blindly and testing a wide range of materials experimentally, these new methods could enable rapid advances in the field of quantum information technologies.

Tags:  Electronics  Graphene  quantum materials  Rensselaer Polytechnic Institute  UC Santa Cruz  Yuan Ping 

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Joined nano-triangles pave the way to magnetic carbon materials

Posted By Graphene Council, Tuesday, June 2, 2020

Graphene triangles with an edge length of only a few atoms behave like peculiar quantum magnets. When two of these nano-triangles are joined, a "quantum entanglement" of their magnetic moments takes place: the structure becomes antiferromagnetic. This could be a breakthrough for future magnetic materials, and another step towards spintronics. An international group led by Empa researchers recently published the results in the journal "Angewandte Chemie".

The "miracle material" graphene - a two-dimensional honeycomb structure made of carbon atoms with a thickness of only one atom - has numerous outstanding properties. These include enormous mechanical resistance and extraordinary electronic and optical properties. Last year a team led by the Empa researcher was able to show that it can even be magnetic: they succeeded in synthesizing a molecule in the shape of a bowtie, which has special magnetic properties.

Now another breakthrough has been made in this direction. Theoretical work from 2007 predicted that graphene could exhibit magnetic behaviour if it were cut into tiny triangles. Over the last three years, several teams, including the Empa team, have succeeded in producing the so-called triangulenes, consisting of only a few dozen carbon atoms, by chemical synthesis under ultra-high vacuum.

On the track of magnetism with the scanning tunneling microscope
However, their magnetism had remained undiscovered until now. First, the presence of unpaired spins, which make triangulenes magnetic in the first place, also make them extremely reactive. Secondly, even with stable molecules, it is extremely difficult to prove the magnetism of such a tiny piece of matter. But now an international group of scientists from Empa, the Technical University of Dresden, the University of Alicante and the International Iberian Nanotechnology Laboratory in Portugal has succeeded in doing just that.

The breakthrough was made possible by a powerful tool for investigating matter at the atomic level - the scanning tunneling microscope (STM). The STM makes it possible to conduct electrical currents through individual atoms or nanostructures deposited on a conductive substrate. So far, however, individual triangulenes had only provided indirect evidence of their magnetic nature.
Double triangle with quantum entanglement
Now, however, the researchers have examined molecules in which two triangulenes are joined by a single carbon-carbon bond (so-called triangulene dimers). These structures provided direct evidence of the magnetic nature of triangulenes. This is because theory says the following: if two triangulenes are joined, not only is their magnetism preserved; their magnetic moments should also form a "quantum entangled" state. This means that the spins - the tiny magnetic moments - of their unpaired electrons should point in opposite directions. This state is known as the antiferromagnetic (or spin-0) state.

In addition, the theory also predicted that it should be possible to excite the triangulene dimers to a state in which their spins are no longer perfectly aligned (spin-1 state). The energy required to cause this excitation, the so-called exchange energy, reflects the strength with which the spins of the two triangulenes in the dimers are bound in the antiferromagnetic state. And indeed in their experiments, the researchers found that the triangulene dimer can be excited to the spin 1 state by injecting electrons with an energy of 14 meV.

Organic magnetic materials for spintronics
The scientists also synthesized a second triangulene dimer in which the triangulene units were not directly connected by a carbon-carbon single bond, but by a "spacer", a hexagonal carbon ring. The researchers expected that this larger connecting element between the triangulene units would significantly reduce the exchange energy. And this is exactly what the experiments showed: the exchange energy was now only 2 meV - 85% less than with the directly connected triangulenes.

These results are relevant not only because they provide direct evidence for the long-awaited magnetism in triangulenes, but also because they show how these remarkable nanosystems can be combined to form larger structures with quantum entangled magnetic states. In the future, such new (and purely organic) magnetic materials could not only be used in technologies such as spin-based information processing, which promise faster computers with lower power consumption, or in quantum technologies; but they could also provide fertile ground for the study of exotic physical phenomena.

Tags:  2D materials  Electronics  Empa  Graphene  quantum materials 

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The Physics behind Twisted Bilayer Graphene

Posted By Graphene Council, Tuesday, May 26, 2020
An international team of researchers, including ICFO Prof. Dmitri Efetov, give a thorough review in Nature Physics on the status and prospects of the physics behind stacked monolayers of 2D materials.

Strongly correlated quantum materials are excellent testbeds for complex quantum phases of matter since they have shown to give rise to spectacular phenomenology, from high-temperature superconductivity to the emergence of states with long-range quantum entanglement. However, their complexity and the vast amount of system variables have so far hindered scientists to obtain a more complete, thorough understanding of its microscopic mechanisms.

Van der Waals heterostructures consist of individual layers of two-dimensional atomic materials such as graphene, hexagonal boron nitride, and transition metal chalcogenides, which are vertically stacked on top of each other. As the relative angle between the crystals can be freely chosen, this creates a new capability in physics – a concept which is called Twistronics. Scientists have recently found that by overlaying two individual layers of these 2D materials, complex heterostructures moiré structures can be created, which host an amazing realm of unexplored and undiscovered physical phenomena.

Recently and considered one of major scientific achievements in these last two years, researchers found that, when tuning one of these systems, specifically in a twisted bi-layer graphene system, it was possible to drive the system from exhibiting strongly correlated states to presenting clear superconductivity features. That is, by changing the electrical charge carrier density within the device with a nearby capacitor, the material could be tuned from behaving as an insulator, to behaving as a superconductor, or even an exotic orbital magnet with non-trivial topological texture – a phase never observed before. Sor far, there is still no theoretical approach that can precisely explain such complex and exceptionally rich physics, and in particular, how these all these states may be linked or connected with each other and why they occur in such order.

In a recent study published in Nature Physics, researchers Leon Balents, Cory R. Dean, ICFO Prof. Dmitri K. Efetov and Andrea F. Young give a thorough report on the status and the prospects of these systems and the physics that arises from them. They focus on understanding the patterns that are created when two individual monolayers of these materials, called moiré patterns, are stacked in a specific way, discuss the engineering in van der Waals heterostructures as well as explore how different phenomena emerge from the moiré flat bands that are formed.

Flat bands are advantageous because they guarantee a large density of states, which amplifies the effects of interactions. Bearing this in mind, the researchers have focused their study on moiré systems in Twisted Bi Layer Graphene (tBLG) in particular. They have studied various systems in which a small mismatch in periodicity of the pattern, introduced either by lattice mismatch or rotational misalignment, results in different physical scenarios, may it be correlating states, insulating states, superconductivity, etc. From these, they search, among other things, to understand and find answers to what may be the nature of the insulating states, what is the origin and nature of the observed superconductivity? How strong are analogies to other correlated electronic systems, such as high-temperature superconducting cuprates? This study is a step forward in understanding and setting the basis for the theory that may be capable of explaining in a future all the rich and very complex physics behind these novel materials and systems. Being able to control and manipulate such systems, and really understanding what is occurring inside them, will be a major advancement, if not a revolutionary shift, in the engineering of these materials and the development of applications for future innovative and disruptive technologies.

Tags:  2D Materials  Andrea F. Young  Cory R. Dean  Dmitri K. Efetov  Graphene  Hexagonal boron nitride  Leon Balents  quantum materials 

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Exploring new paths to future quantum electronics

Posted By Graphene Council, Tuesday, May 12, 2020
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.”

Tags:  Electronics  Graphene  Jagadeesh Moodera  Massachusetts Institute of Technology  quantum materials 

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People in Graphene - David Graves to Head New Research at PPPL for Plasma Applications in Industry and Quantum Information Science

Posted By Graphene Council, Monday, May 11, 2020
Updated: Tuesday, May 12, 2020

David Graves, an internationally-known chemical engineer, has been named to lead a new research enterprise that will explore plasma applications in nanotechnology for everything from semiconductor manufacturing to the next generation of super-fast quantum computers.

Graves, a professor at the University of California, Berkeley, since 1986, is an expert in plasma applications in semiconductor manufacturing. He will become the Princeton Plasma Physics Laboratory’s (PPPL) first associate laboratory director for Low-Temperature Plasma Surface Interactions, effective June 1. He will likely begin his new position from his home in Lafayette, California, in the East Bay region of San Francisco.

He will lead a collaborative research effort to not only understand and measure how plasma is used in the manufacture of computer chips, but also to explore how plasma could be used to help fabricate powerful quantum computing devices over the next decade.

“This is the apex of our thrust into becoming a multipurpose lab,” said Steve Cowley, PPPL director, who recruited Graves. “Working with Princeton University, and with industry and the U.S. Department of Energy (DOE), we are going to make a big push to do research that will help us understand how you can manufacture at the scale of a nanometer.” A nanometer, one-billionth of a meter, is about ten thousand times less than the width of a human hair.

The new initiative  will draw on PPPL’s expertise in low temperature plasmas, diagnostics, and modeling. At the same time, it will work closely with plasma semiconductor equipment industries and will collaborate with Princeton University experts in various departments, including chemical and biological engineering, electrical engineering, materials science, and physics.  In particular, collaborations with PRISM (the Princeton Institute for the Science and Technology of Materials) are planned, Cowley said. “I want to see us more tightly bound to the University in some areas because that way we get cross-fertilization,” he said.
Graves will also have an appointment as professor in the Princeton University Department of Chemical and Biological Engineering, starting July 1. He is retiring from his position at Berkeley at the end of this semester. He is currently writing a book (“Plasma Biology”) on plasma applications in biology and medicine. He said he changed his retirement plans to take the position at PPPL and Princeton University. “This seemed like a great opportunity,” Graves said. “There’s a lot we can do at a national laboratory where there’s bigger scale, world-class colleagues, powerful computers and other world-class facilities.”

“Exciting new direction for the Lab”

Graves is already working with Jon Menard, PPPL deputy director for research, on the strategic plan for the new research initiative  over the next five years. “It’s a really exciting new direction for the Lab that will build upon our unique expertise in diagnosing and simulating low-temperature plasmas,” Menard said. “It also brings us much closer to the university and industry, which is great for everyone.”

The staff will grow over the next five years and PPPL is recruiting for an expert in nano-fabrication and quantum devices. The first planned research would use converted PPPL laboratory space fitted with equipment provided by industry. Subsequent work  would use laboratory space at PRISM on Princeton University’s campus.  In the longer term, researchers in the growing group  would have brand new laboratory and office space as a central part the Princeton Plasma Innovation Center (PPIC), a new building planned at PPPL.

Physicists Yevgeny Raitses, principal investigator for the Princeton Collaborative Low Temperature Plasma Research Facility (PCRF) and head of the Laboratory for Plasma Nanosynthesis, and Igor Kavanovich, co-principal investigator of PCRF, are both internationally-known experts in low temperature plasmas who have forged recent partnerships between PPPL and various industry partners. The new initiative builds on their work, Cowley said.

A priority research area
Research aimed at developing quantum information science (QIS) is a priority for the DOE. Quantum computers could be very powerful in solving complex scientific problems, including simulating quantum behavior in material or chemical systems. QIS could also have applications in quantum communication, especially in encryption, and quantum sensing. It could potentially have an impact in areas such as national security. “A key question is whether plasma-based fabrication tools commonly used today will play a role in fabricating quantum devices in the future,” Menard said. “There are huge implications in that area,” Menard said. “We want to be part of that.”

Graves is an expert on applying molecular dynamics simulations to low temperature plasma-surface interactions. These simulations are used to understand how plasma-generated ions, atoms and molecules interact with various surfaces. He has extensive research experience in academia and industry in plasma-related semiconductor manufacturing. That expertise will be useful for understanding how to make “very fine structures and circuits” at the nanometer, sub-nanometer and even atom-by-atom level, Menard said. “David’s going to bring a lot of modeling and fundamental understanding to that process. That, paired with our expertise and measurement capabilities, should make us unique in the U.S. in terms of what we can do in this area.”

Tags:  David Graves  Graphene  quantum materials  Semiconductor  Steve Cowley  University of California 

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Researchers map tiny twists in “magic-angle” graphene

Posted By Graphene Council, Monday, May 11, 2020
Made of a single layer of carbon atoms linked in a hexagonal honeycomb pattern, graphene’s structure is simple and seemingly delicate. Since its discovery in 2004, scientists have found that graphene is in fact exceptionally strong. And although graphene is not a metal, it conducts electricity at ultrahigh speeds, better than most metals.

In 2018, MIT scientists led by Pablo Jarillo-Herrero and Yuan Cao discovered that when two sheets of graphene are stacked together at a slightly offset “magic” angle, the new “twisted” graphene structure can become either an insulator, completely blocking electricity from flowing through the material, or paradoxically, a superconductor, able to let electrons fly through without resistance. It was a monumental discovery that helped launch a new field known as “twistronics,” the study of electronic behavior in twisted graphene and other materials.

Now the MIT team reports their latest advancements in graphene twistronics, in two papers published this week in the journal Nature.

In the first study, the researchers, along with collaborators at the Weizmann Institute of Science, have imaged and mapped an entire twisted graphene structure for the first time, at a resolution fine enough that they are able to see very slight variations in local twist angle across the entire structure.

The results revealed regions within the structure where the angle between the graphene layers veered slightly away from the average offset of 1.1 degrees.

The team detected these variations at an ultrahigh angular resolution of 0.002 degree. That’s equivalent to being able to see the angle of an apple against the horizon from a mile away.

They found that structures with a narrower range of angle variations had more pronounced exotic properties, such as insulation and superconductivity, versus structures with a wider range of twist angles.

“This is the first time an entire device has been mapped out to see what is the twist angle at a given region in the device,” says Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT. “And we see that you can have a little bit of variation and still show superconductivity and other exotic physics, but it can’t be too much. We now have characterized how much twist variation you can have, and what is the degradation effect of having too much.”

In the second study, the team report creating a new twisted graphene structure with not two, but four layers of graphene. They observed that the new four-layer magic-angle structure is more sensitive to certain electric and magnetic fields compared to its two-layer predecessor. This suggests that researchers may be able to more easily and controllably study the exotic properties of magic-angle graphene in four-layer systems.

“These two studies are aiming to better understand the puzzling physical behavior of magic-angle twistronics devices,” says Cao, a graduate student at MIT. “Once understood, physicists believe these devices could help design and engineer a new generation of high-temperature superconductors, topological devices for quantum information processing, and low-energy technologies.”

Like wrinkles in plastic wrap

Since Jarillo-Herrero and his group first discovered magic-angle graphene, others have jumped at the chance to observe and measure its properties. Several groups have imaged magic-angle structures, using scanning tunneling microscopy, or STM, a technique that scans a surface at the atomic level. However, researchers have only been able to scan small patches of magic-angle graphene, spanning at most a few hundred square nanometers, using this approach.

“Going over an entire micron-scale structure to look at millions of atoms is something that STM is not best suited for,” Jarillo-Herrero says. “In principle it could be done, but would take an enormous amount of time.”

So the group consulted with researchers at the Weizmann Institute for Science, who had developed a scanning technique they call “scanning nano-SQUID,” where SQUID stands for Superconducting Quantum Interference Device. Conventional SQUIDs resemble a small bisected ring, the two halves of which are made of superconducting material and joined together by two junctions. Fit around the tip of a device similar to an STM, a SQUID can measure a sample’s magnetic field flowing through the ring at a microscopic scale. The Weizmann Institute researchers scaled down the SQUID design to sense magnetic fields at the nanoscale.

When magic-angle graphene is placed in a small magnetic field, it generates persistent currents across the structure, due to the formation of what are known as “Landau levels.” These Landau levels, and hence the persistent currents, are very sensitive to the local twist angle, for instance, resulting in a magnetic field with a different magnitude, depending on the precise value of the local twist angle. In this way, the nano-SQUID technique can detect regions with tiny offsets from 1.1 degrees.

“It turned out to be an amazing technique that can pick up miniscule angle variations of 0.002 degrees away from 1.1 degrees,” Jarillo-Herrero says. “This was very good for mapping magic-angle graphene.”

The group used the technique to map two magic-angle structures: one with a narrow range of twist variations, and another with a broader range.

“We placed one sheet of graphene on top of another, similar to placing plastic wrap on top of plastic wrap,” Jarillo-Herrero says. “You would expect there would be wrinkles, and regions where the two sheets would be a bit twisted, some less twisted, just as we see in graphene.”

They found that the structure with a narrower range of twist variations had more pronounced properties of exotic physics, such as superconductivity, compared with the structure with more twist variations.

“Now that we can directly see these local twist variations, it might be interesting to study how to engineer variations in twist angles to achieve different quantum phases in a device,” Cao says.

Tunable physics

Over the past two years, researchers have experimented with different configurations of graphene and other materials to see whether twisting them at certain angles would bring out exotic physical behavior. Jarillo-Herrero’s group wondered whether the fascinating physics of magic-angle graphene would hold up if they expanded the structure, to offset not two, but four graphene layers.

Since graphene’s discovery nearly 15 years ago, a huge amount of information has been revealed about its properties, not just as a single sheet, but also stacked and aligned in multiple layers — a configuration that is similar to what you find in graphite, or pencil lead.

“Bilayer graphene — two layers at a 0-degree angle from each-other — is a system whose properties we understand well,” Jarillo-Herrero says. “Theoretical calculations have shown that in a bilayer-on-top-of-bilayer structure, the range of angles over which interesting physics would happen is larger. So this type of structure might be more forgiving in terms of making devices.”

Partly inspired by this theoretical possibility, the researchers fabricated a new magic-angle structure, offsetting one graphene bilayer with another bilayer by 1.1 degrees. They then connected the new “double-layer” twisted structure to a battery, applied a voltage, and measured the current that flowed through the device as they placed the structure under various conditions, such as a magnetic field, and a perpendicular electric field.

Just like magic-angle structures made from two layers of graphene, the new four-layered structure showed an exotic insulating behavior. But uniquely, the researchers were able to tune this insulating property up and down with an electric field — something that’s not possible with two-layered magic-angle graphene.

“This system is highly tunable, meaning we have a lot of control, which will allow us to study things we cannot understand with monolayer magic-angle graphene,” Cao says.

“It’s still very early in the field,” Jarillo-Herrero says. “For the moment, the physics community is still fascinated just by the phenomena of it. People fantasize about what type of devices we could make but realize it’s still too early and we have so much yet to learn about these systems.”

Tags:  Electronics  Graphene  Massachusetts Institute of Technology  Pablo Jarillo-Herrero  quantum materials  Yuan Cao 

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Engineering Edge States of Graphene Nanoribbons for Narrow-Band Photoluminescence

Posted By Graphene Council, Monday, May 11, 2020
A bottom-up approach for coupling graphene nanodots (GND) covalently at the edges of graphene nanoribbons (GNR) to create quantum-well-like states for well-defined narrow-band light emission.

Significance and Impact
This work establishes a new strategy to achieve narrow-band emission by engineering interface states of mixed-dimensional GNR-GND heterojunctions with atomic precision.

Research Details
– Covalent heterostructures are formed by fusing GNDs to the edges of GNRs via controlled on-surface reactions of molecular precursors.

– Scanning tunneling microscopy (STM) reveals the quantum-well-like electronic states and photoluminescence (PL) spectra show a defined optical transition energy with an ultra-narrow linewidth.

Tags:  Graphene  Graphene Nanoribbons  quantum materials 

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A twist connecting magnetism and electronic-band topology

Posted By Graphene Council, Wednesday, April 8, 2020

Materials that combine topological electronic properties and quantum magnetism are of high current interest, for the quantum many-body physics that can unfold in them and for possible applications in electronic components. For one such material, ETH physicists have now established the microscopic mechanism linking magnetism and electronic-band topology.

Dirac matter is an intriguing class of materials with rather peculiar properties: electrons in these materials behave as if they had no mass. The most prominent Dirac material is graphene, but further members have been discovered during the past 15 years or so. Each one of them serves as a rich playground for exploring ‘exotic’ electronic behaviours, some of which hold the promise to enable novel components for electronics. However, even if Dirac matter and other so-called topological materials — in which electrons behave in similarly unexpected ways — are among the currently most intensively studied condensed-matter systems, there are only very few examples where the topology of the electronic bands is connected in a well-defined manner to the magnetic properties of the materials. One material in which such interplay between topological electronic states and magnetism has been observed is CaMnBi2, but the mechanism connecting the two remained unclear.

Writing in Physical Review Letters [1], postdoc Run Yang and PhD student Matteo Corasaniti from the Optical Spectroscopy group of Prof. Leonardo Degiorgi at the Laboratory for Solid State Physics, working with colleagues at Brookhaven National Laboratory (US) and the Chinese Academy of Sciences in Beijing, now report a comprehensive study in which they provide clear evidence that it is a slight nudge on the magnetic moments, known as spin canting, that provokes substantial changes in the electronic band structure.

Compass points to the right direction on a bumpy road

CaMnBi2 and the related compound SrMnBi2 have recently attracted attention as they display quantum magnetism — the manganese ions are antiferromagnetically ordered at around room temperature and below — and at the same time they host Dirac electrons. That there is interplay between the two properties has been suspected for some while, not least as at ~50 K there appears an unexpected ‘bump’ in the conduction properties at these materials. But the precise nature of this anomaly was still poorly understood until now.

In earlier work studying optical properties of CaMnBi2 [2], Corasaniti, Yang and co-workers had established already a link to the electronic properties of the material. They used in particular the fact that the bump-like anomaly in the transport properties can be shifted in temperature by replacing a proportion of the calcium atoms with sodium atoms. To get now to the microscopic origins of the observed behaviour, they studied samples with different sodium dopings by torque magnetometry. In this technique, the torque on a magnetic sample is measured when it is exposed to a suitably strong field, similarly as a compass needle aligns with the Earth magnetic field. And this approach proved to point the team to the origins of the anomaly.

A firm link between magnetic and electronic properties

In their magnetic-torque experiments, the researchers found that at temperatures where no anomaly is observed in the electronic transport measurements, the magnetic behaviour is such as one would expect for an antiferromagnet. This was not the case anymore at lower temperatures, where the anomaly is present. There, a ferromagnetic component appeared, which can be explained by a projection of magnetic moments onto the plane orthogonal to the easy spin c-axis of the original antiferromagnetic order (see the figure). This phenomenon is known as spin-canting, induced by a so-called super-exchange mechanism.

These two sets of experiments — optical and torque measurements — were supported by dedicated first-principles calculations. In particular, for the case where spin canting was included in the calculations, a peculiar hybridization between the manganese and bismuth atoms was found to mediate the interlayer magnetic coupling and to govern the electronic properties in the material. Taken together, the study therefore establishes that sought-after direct link between the magnetic properties and changes to the electronic band structure, reflected in the bump anomaly of the transport properties.

With such detailed understanding on board, the door is now open to exploring not only the electronic properties of CaMnBi2 and related compounds, but also the possibilities arising from the connection between magnetic properties and topological states in these intriguing forms of matter.

Tags:  Electronics  ETH Zurich  Graphene  Leonardo Degiorgi  quantum materials 

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