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Infinite chains of hydrogen atoms have surprising properties, including a metallic phase

An infinite chain of hydrogen atoms is just about the simplest bulk material imaginable — a never-ending single-file line of protons surrounded by electrons. Yet a new computational study combining four cutting-edge methods finds that the modest material boasts fantastic and surprising quantum properties.

By computing the consequences of changing the spacing between the atoms, an international team of researchers from the Flatiron Institute and the Simons Collaboration on the Many Electron Problem found that the hydrogen chain’s properties can be varied in unexpected and drastic ways. That includes the chain transforming from a magnetic insulator into a metal, the researchers report September 14 in Physical Review X.

The computational methods used in the study present a significant step toward custom-designing materials with sought-after properties, such as the possibility of high-temperature superconductivity in which electrons flow freely through a material without losing energy, says the study’s senior author Shiwei Zhang. Zhang is a senior research scientist at the Center for Computational Quantum Physics (CCQ) at the Simons Foundation’s Flatiron Institute in New York City.

“The main purpose was to apply our tools to a realistic situation,” Zhang says. “Almost as a side product, we discovered all of this interesting physics of the hydrogen chain. We didn’t think that it would be as rich as it turned out to be.”

Zhang, who is also a chancellor professor of physics at the College of William and Mary, co-led the research with Mario Motta of IBM Quantum. Motta serves as first author of the paper alongside Claudio Genovese of the International School for Advanced Studies (SISSA) in Italy, Fengjie Ma of Beijing Normal University, Zhi-Hao Cui of the California Institute of Technology, and Randy Sawaya of the University of California, Irvine. Additional co-authors include CCQ co-director Andrew Millis, CCQ Flatiron Research Fellow Hao Shi and CCQ research scientist Miles Stoudenmire.

The paper’s long author list — 17 co-authors in total — is uncommon for the field, Zhang says. Methods are often developed within individual research groups. The new study brings many methods and research groups together to combine forces and tackle a particularly thorny problem. “The next step in the field is to move toward more realistic problems,” says Zhang, “and there is no shortage of these problems that require collaboration.”

While conventional methods can explain the properties of some materials, other materials, such as infinite hydrogen chains, pose a more daunting computational hurdle. That’s because the behavior of the electrons in those materials is heavily influenced by interactions between electrons. As electrons interact, they become quantum-mechanically entangled with one another. Once entangled, the electrons can no longer be treated individually, even when they are physically separate.

The sheer number of electrons in a bulk material — roughly 100 billion trillion per gram — means that conventional brute force methods can’t even come close to providing a solution. The number of electrons is so large that it’s practically infinite when thinking at the quantum scale.

Thankfully, quantum physicists have developed clever methods of tackling this many-electron problem. The new study combines four such methods: variational Monte Carlo, lattice-regularized diffusion Monte Carlo, auxiliary-field quantum Monte Carlo, and standard and sliced-basis density-matrix renormalization group. Each of these cutting-edge methods has its strengths and weaknesses. Using them in parallel and in concert provides a fuller picture, Zhang says.

Researchers, including authors of the new study, previously used those methods in 2017 to compute the amount of energy each atom in a hydrogen chain has as a function of the chain’s spacing. This computation, known as the equation of state, doesn’t provide a complete picture of the chain’s properties. By further honing their methods, the researchers did just that.

At large separations, the researchers found that the electrons remain confined to their respective protons. Even at such large distances, the electrons still ‘know’ about each other and become entangled. Because the electrons can’t hop from atom to atom as easily, the chain acts as an electrical insulator.

As the atoms move closer together, the electrons try to form molecules of two hydrogen atoms each. Because the protons are fixed in place, these molecules can’t form. Instead, the electrons ‘wave’ to one another, as Zhang puts it. Electrons will lean toward an adjacent atom. In this phase, if you find an electron leaning toward one of its neighbors, you’ll find that neighboring electron responding in return. This pattern of pairs of electrons leaning toward each other will continue in both directions.

Moving the hydrogen atoms even closer together, the researchers discovered that the hydrogen chain transformed from an insulator into a metal with electrons moving freely between atoms. Under a simple model of interacting particles known as the one-dimensional Hubbard model, this transition shouldn’t happen, as electrons should electrically repel each other enough to restrict movement. In the 1960s, British physicist Nevill Mott predicted the existence of an insulator-to-metal transition based on a mechanism involving so-called excitons, each consisting of an electron trying to break free of its atom and the hole it leaves behind. Mott proposed an abrupt transition driven by the breakup of these excitons — something the new hydrogen chain study didn’t see.

Instead, the researchers discovered a more nuanced insulator-to-metal transition. As the atoms move closer together, electrons gradually get peeled off the tightly bound inner core around the proton line and become a thin `vapor’ only loosely bound to the line and displaying interesting magnetic structures.

The infinite hydrogen chain will be a key benchmark in the future in the development of computational methods, Zhang says. Scientists can model the chain using their methods and check their results for accuracy and efficiency against the new study.

The new work is a leap forward in the quest to utilize computational methods to model realistic materials, the researchers say. In the 1960s, British physicist Neil Ashcroft proposed that metallic hydrogen, for instance, might be a high-temperature superconductor. While the one-dimensional hydrogen chain doesn’t exist in nature (it would crumple into a three-dimensional structure), the researchers say that the lessons they learned are a crucial step forward in the development of the methods and physical understanding needed to tackle even more realistic materials.

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New anode material could lead to safer fast-charging batteries

Scientists at UC San Diego have discovered a new anode material that enables lithium-ion batteries to be safely recharged within minutes for thousands of cycles. Known as a disordered rocksalt, the new anode is made up of earth-abundant lithium, vanadium and oxygen atoms arranged in a similar way as ordinary kitchen table salt, but randomly. It is promising for commercial applications where both high energy density and high power are desired, such as electric cars, vacuum cleaners or drills.

The study, jointly led by nanoengineers in the labs of Professors Ping Liu and Shyue Ping Ong, was published in Nature on September 2.

Currently, two materials are used as anodes in most commercially available lithium-ion batteries that power items like cell phones, laptops and electric vehicles. The most common, a graphite anode, is extremely energy dense — a lithium ion battery with a graphite anode can power a car for hundreds of miles without needing to be recharged. However, recharging a graphite anode too quickly can result in fire and explosions due to a process called lithium metal plating. A safer alternative, the lithium titanate anode, can be recharged rapidly but results in a significant decrease in energy density, which means the battery needs to be recharged more frequently.

This new disordered rocksalt anode — Li3V2O5 — sits in an important middle ground: it is safer to use than graphite, yet offers a battery with at least 71% more energy than lithium titanate.

“The capacity and energy will be a little bit lower than graphite, but it’s faster, safer and has a longer life. It has a much lower voltage and therefore much improved energy density over current commercialized fast charging lithium-titanate anodes,” said Haodong Liu, a postdoctoral scholar in Professor Ping Liu’s lab and first author of the paper. “So with this material we can make fast-charging, safe batteries with a long life, without sacrificing too much energy density.”

The researchers formed a company called Tyfast in order to commercialize this discovery. The startup’s first markets will be electric buses and power tools, since the characteristics of the Li3V2O5 disordered rocksalt make it ideal for use in devices where recharging can be easily scheduled.

Researchers in Professor Liu’s lab plan to continue developing this lithium-vanadium oxide anode material, while also optimizing other battery components to develop a commercially viable full cell.

“For a long time, the battery community has been looking for an anode material operating at a potential just above graphite to enable safe, fast charging lithium-ion batteries. This material fills an important knowledge and application gap,” said Ping Liu. “We are excited for its commercial potential since the material can be a drop-in solution for today’s lithium-ion battery manufacturing process.”

Why try this material?

Researchers first experimented with disordered rocksalt as a battery cathode six years ago. Since then, much work has been done to turn the material into an efficient cathode. Haodong Liu said the UC San Diego team decided to test the material as an anode based on a hunch.

“When people use it as a cathode they have to discharge the material to 1.5 volts,” he said. “But when we looked at the structure of the cathode material at 1.5 volts, we thought this material has a special structure that may be able to host more lithium ions — that means it can go to even lower voltage to work as an anode.”

In the study, the team found that their disordered rocksalt anode could reversibly cycle two lithium ions at an average voltage of 0.6 V — higher than the 0.1 V of graphite, eliminating lithium metal plating at a high charge rate which makes the battery safer, but lower than the 1.5 V at which lithium-titanate intercalates lithium, and therefore storing much more energy.

The researchers showed that the Li3V2O5 anode can be cycled for over 6,000 cycles with negligible capacity decay, and can charge and discharge energy rapidly, delivering over 40 percent of its capacity in 20 seconds. The low voltage and high rate of energy transfer are due to a unique redistributive lithium intercalation mechanism with low energy barriers.

Postdoctoral scholar Zhuoying Zhu, from Professor Shyue Ping Ong’s Materials Virtual Lab, performed theoretical calculations to understand why the disordered rocksalt Li3V2O5 anode works as well as it does.

“We discovered that Li3V2O5 operates via a charging mechanism that is different from other electrode materials. The lithium ions rearrange themselves in a way that results in both low voltage as well as fast lithium diffusion,” said Zhuoying Zhu.

“We believe there are other electrode materials waiting to be discovered that operate on a similar mechanism,” added Ong.

The experimental studies at UC San Diego were funded by awards from the UC San Diego startup fund to Ping Liu, while the theoretical studies were funded by the Department of Energy and the National Science Foundation’s Data Infrastructure Building Blocks (DIBBS) Local Spectroscopy Data Infrastructure program, and used resources at the San Diego Supercomputer Center provided under the Extreme Science and Engineering Discovery Environment (XSEDE).

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New X-ray detection technology developed

Florida State University researchers have developed a new material that could be used to make flexible X-ray detectors that are less harmful to the environment and cost less than existing technologies.

The team led by Biwu Ma, a professor in the Department of Chemistry and Biochemistry, created X-ray scintillators that use an environmentally friendly material. Their research was published in the journal Nature Communications .

“Developing low-cost scintillation materials that can be easily manufactured and that perform well remains a great challenge,” Ma said. “This work paves the way for exploring new approaches to create these important devices.”

Biwu Ma, professor in the Department of Chemistry and Biochemistry X-ray scintillators convert the radiation of an X-ray into visible light, and they are a common type of X-ray detector. When you visit the dentist or the airport, scintillators are used to take images of your teeth or scan your luggage.

Various materials have been used to make X-ray scintillators, but they can be difficult or expensive to manufacture. Some recent developments use compounds that include lead, but the toxicity of lead could be a concern.

Ma’s team found a different solution. They used the compound organic manganese halide to create scintillators that don’t use lead or heavy metals. The compound can be used to make a powder that performs very well for imaging and can be combined with a polymer to create a flexible composite that can be used as a scintillator. That flexibility broadens the potential use of this technology.

“Researchers have made scintillators with a variety of compounds, but this technology offers something that combines low cost with high performance and environmentally friendly materials,” Ma said. “When you also consider the ability to make flexible scintillators, it’s a promising avenue to explore.”

Ma recently received a GAP Commercialization Investment Program grant from the FSU Office of the Vice President for Research to develop this technology. The grants help faculty members turn their research into possible commercial products.

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New evidence for quantum fluctuations near a quantum critical point in a superconductor

Among all the curious states of matter that can coexist in a quantum material, jostling for preeminence as temperature, electron density and other factors change, some scientists think a particularly weird juxtaposition exists at a single intersection of factors, called the quantum critical point or QCP.

“Quantum critical points are a very hot issue and interesting for many problems,” says Wei-Sheng Lee, a staff scientist at the Department of Energy’s SLAC National Accelerator Laboratory and investigator with the Stanford Institute for Materials and Energy Sciences (SIMES). “Some suggest that they’re even analogous to black holes in the sense that they are singularities — point-like intersections between different states of matter in a quantum material — where you can get all sorts of very strange electron behavior as you approach them.”

Lee and his collaborators reported in Nature Physics today that they have found strong evidence that QCPs and their associated fluctuations exist. They used a technique called resonant inelastic X-ray scattering (RIXS) to probe the electronic behavior of a copper oxide material, or cuprate, that conducts electricity with perfect efficiency at relatively high temperatures.

These so-called high-temperature superconductors are a bustling field of research because they could give rise to zero-waste transmission of energy, energy-efficient transportation systems and other futuristic technologies, although no one knows the underlying microscopic mechanism behind high-temperature superconductivity yet. Whether QCPs exist in cuprates is also a hotly debated issue.

In experiments at the UK’s Diamond Light Source, the team chilled the cuprate to temperatures below 90 kelvins (minus 183 degrees Celsius), where it became superconducting. They focused their attention on what’s known as charge order — alternating stripes in the material where electrons and their negative charges are denser or more sparse.

The scientists excited the cuprate with X-rays and measured the X-ray light that scattered into the RIXS detector. This allowed them to map out how the excitations propagated through the material in the form of subtle vibrations, or phonons, in the material’s atomic lattice, which are hard to measure and require very high-resolution tools.

At the same time, the X-rays and the phonons can excite electrons in the charge order stripes, causing the stripes to fluctuate. Since the data obtained by RIXS reflects the coupling between the behavior of the charge stripes and the behavior of the phonons, observing the phonons allowed the researchers to measure the behavior of the charge order stripes, too.

What the scientists expected to see is that when the charge order stripes grew weaker, their excitations would also fade away. “But what we observed was very strange,” Lee said. “We saw that when charge order became weaker in the superconducting state, the charge order excitations became stronger. This is a paradox because they should go hand in hand, and that’s what people find in other charge order systems.”

He added, “To my knowledge this is the first experiment about charge order that has shown this behavior. Some have suggested that this is what happens when a system is near a quantum critical point, where quantum fluctuations become so strong that they melt the charge order, much like heating ice increases thermal vibrations in its rigid atomic lattice and melts it into water. The difference is that quantum melting, in principle, occurs at zero temperature.” In this case, Lee said, the unexpectedly strong charge order excitations seen with RIXS were manifestations of those quantum fluctuations.

Lee said the team is now studying these phenomena at a wider range of temperatures and at different levels of doping — where compounds are added to change the density of freely moving electrons in the material — to see if they can nail down exactly where the quantum critical point could be in this material.

Thomas Devereaux, a theorist at SIMES and senior author of the report, noted that many phases of matter can be intertwined in cuprates and other quantum materials.

“Superconducting and magnetic states, charge order stripes and so on are so entangled that you can be in all of them at the same time,” he said. “But we’re stuck in our classical way of thinking that they have to be either one way or another.”

Here, he said, “We have an effect, and Wei-Sheng is trying to measure it in detail, trying to see what’s going on.”

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A colorful detector: Crystalline material reversibly changes color when absorbing water

Researchers at the University of Tsukuba have developed a new kind of color-shifting crystalline material that can be used to indicate the presence of water. The change in hue is dramatic enough to be gauged by the unaided human eye. This work could lead to the creation of highly sensitive “vapochromic” sensors that can show if a particular gas or water vapor is present without the need for external power.

Chemical sensors are important to many industrial processes. To ensure safety and efficiency, factories often need to be monitored for potentially toxic gasses or even excess humidity. Sensors for water vapor are particularly important, but may have limited lifetimes or require external power. To address this, scientists at the University of Tsukuba have invented a new crystalline material that changes color when exposed to water vapor. Inside the crystal, long branching molecules called dendrimers are held together by van der Waals forces.

“The aromatic carbazole dendrimers containing carbon rings are anchored to a dibenzophenazine core,” explains senior author Professor Yohei Yamamoto. “Interestingly, even though van der Waals forces are usually considered to be relatively weak, the crystal stays together during operation.”

The research team also extensively characterized the new material. In addition to studying the color in both the hydrated and dehydrated states using spectroscopy, the scientists used techniques including single-crystal and powder X-ray diffraction analysis, as well as thermogravimetric analysis. On the basis of the experimental results and theoretical density functional theory calculations, they were able to determine the molecular mechanism responsible for the different appearances under different water concentrations. The color-shifting properties of the crystal come from conformation changes in the dendrimers. Upon exposure to water vapor, the planes of the outermost carbazole units in the crystal twist simultaneously. This motion changes the energies of the electronic orbitals, which causes the electrons to absorb different colors of light.

“We believe that our findings will lead to the further exploration of van der Waals porous crystals, much like metal-organic frameworks that have found a place in chemistry,” Professor Yamamoto says. “This work can lead to a new class of gas sensors that can work in difficult to reach locations, because they do not require external power.”

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Scientists get atomistic picture of platinum catalyst degradation

Degradation of platinum, used as a key electrode material in the hydrogen economy, severely shortens the lifetime of electrochemical energy conversion devices, such as fuel cells. For the first time, scientists elucidated the movements of the platinum atoms that lead to catalyst surface degradation. Their results are published today in Nature Catalysis.

For more than half a century, platinum has been known as one of the best catalysts for oxygen reduction, one of the key reactions in fuel cells. However, it is difficult to meet the catalysts’ long-term high activity and stability needed for the massive deployment of the hydrogen technology in the transportation sector.

Scientists led by Kiel University (Germany), in collaboration with the ESRF, University of Victoria (Canada), University of Barcelona (Spain) and Forschungszentrum Jülich (Germany), have now found out why and how platinum degrades. “We have come up with an atomistic picture to explain it,” says Olaf Magnussen, professor at Kiel University and corresponding author of the article.

In order to achieve this, the team went to ESRF’s beamline ID31 to study the different facets of platinum electrodes in electrolyte solution. They discovered how atoms arrange themselves and move on the surface during the processes of oxidation, the main reaction responsible for platinum dissolution.

The findings open doors to atomistic engineering: “With this new knowledge, we can imagine targeting certain shapes and surface arrangements of nanoparticles to enhance the stability of the catalyst. We can also find how the atoms move, so we could potentially add surface additives to suppress atoms moving the wrong way,” explains Jakub Drnec, scientist at beamline ID31 and co-author of the study.

The fact that the experiments took place under electrochemical conditions similar to what happens in the actual device was key to translate the findings into fuel cell technology. “Because platinum surface rapidly changes during oxidation, these measurements became possible only thanks to a new, very fast technique for surface structure characterization. This method, high-energy surface X-ray diffraction, has been co-developed at the ESRF” explains Timo Fuchs, from Kiel University and co-author of the study. “And it is, in fact, the only technique that can provide this kind of information in the real environment,” he adds. This is the first publication where atomic movements were determined by the technique under such conditions.

This research owes its success to the combination of the X-ray measurements at the ESRF with highly sensitive dissolution measurements performed at Forschungszentrum Jülich and advanced computer simulations. “Only such a combination of different characterization techniques and theoretical calculations provides a full picture of what goes on with the atoms at the nanoscale level in a platinum catalyst,” notes Federico Calle-Vallejo from University of Barcelona, in charge of the simulations.

The next step for the team is to continue experiments that provide insight into the degradation mechanisms of further model facets mimicking edges and corners on catalyst particles. These results will provide a map of platinum stability under reaction conditions and allow researchers to develop rational strategies for the design of more stable catalysts in the future.

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Surface deep: Light-responsive top layer of plastic film induces movement

Azobenzene-containing plastic film is a peculiar material; its surface can change shape when exposed to light, making it a valuable component in modern technologies/devices like TV screens and solar cells. Scientists now show that only a thin, topmost layer of the light-dependent azobenzene-containing plastic film needs to be light-sensitive, rather than the entire film, opening up new ways to potentially reduce production costs and revolutionize its use.

So far, it had been widely accepted that the light-sensitive nature of this material extends throughout the whole film, but scientists did not understand what was causing the shape-shifting movement. A group of scientists led by Dr Takahiro Seki of Nagoya University, Japan, set out to figure out exactly how this happens; they have published their findings in the journal Scientific Reports.

They cite a well-studied phenomenon called Marangoni flow as their inspiration: owing to this phenomenon, differences in “surface tension” (the property by which the particles in the outermost layer of liquids are always attracted inwards, creating a boundary for the liquid) cause many soft, plastic films to move in a peculiar pattern. The most famous example of this phenomenon is the formation of “wine legs” or droplets of liquid evaporating and streaking down the surfaces of wine glasses.

They decided to test whether ultraviolet light triggered changes in the surface tension of azobenzene plastic film, and whether those changes resulted in the film moving. They chose to first cover azobenzene film with a very thin top layer that was light-sensitive, then exposed this film to UV radiation. Next, they did the same with film that was covered in a top layer unresponsive to light. To their excitement, the scientists found surface structural changes in the film with a light-sensitive top layer, but not in the film with a “light-insensitive” top layer. “This is the first time anyone has demonstrated that only the light responsiveness of a very thin ‘nanometer’ level layer is needed for azobenzene-containing film to alter its surface morphology under UV,” said Dr Seki.

An important observation of this study is that the movement of the material isn’t dependent on “light polarization,” or the direction in which light waves travel. If it were, that would suggest that there is another force on the molecular level affecting the whole film. Instead, Dr Seki concludes that it is probably the changes in chemical structure at the surface induced by the UV radiation that changes surface tension, inducing movement to the top of the film.

Describing the wider ramifications of their results, Dr Seki states: “We are only at the cusp of developing this discovery onto an industrial scale, but you can imagine how needing only a very small amount of light-sensitive material can reduce costs. Many optical devices like photocopiers, printers, and monitors depend on the light-based surface change in azobenzene polymer film. Based on our findings, azobenzene film can also act as an “actuator” (that part in a device that moves other parts) in nanomachinery.”

These newly discovered properties have vast implications, from improving the economics of production and lowering material prices, to advancing the field of nanotechnology itself.

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When Dirac meets frustrated magnetism

The fields of condensed matter physics and material science are intimately linked because new physics is often discovered in materials with special arrangements of atoms. Crystals, which have repeating units of atoms in space, can have special patterns which result in exotic physical properties. Particularly exciting are materials which host multiple types of exotic properties because they give scientists the opportunity to study how those properties interact with and influence each other. The combinations can give rise to unexpected phenomena and fuel years of basic and technological research.

In a new study published in Science Advances this week, an international team of scientists from the USA, Columbia, Czech Republic, England, and led by Dr. Mazhar N. Ali at the Max Planck Institute of Microstructure Physics in Germany, has shown that a new material, KV3Sb5, has a never-seen-before combination of properties that results in one of the largest anomalous Hall effects (AHEs) ever observed; 15,500 siemens per centimeter at 2 Kelvin.

Discovered in the lab of co-author Prof. Tyrel McQueen at Johns Hopkins University, KV3Sb5 combines four properties into one material: Dirac physics, metallic frustrated magnetism, 2D exfoliability (like graphene), and chemical stability.

Dirac physics, in this context, relates to the fact that the electrons in KV3Sb5 aren’t just your normal run-of-the-mill electrons; they are moving extremely fast with very low effective mass. This means that they are acting “light-like”; their velocities are becoming comparable to the speed of light and they are behaving as though they have only a small fraction of the mass which they should have. This results in the material being highly metallic and was first shown in graphene about 15 years ago.

The “frustrated magnetism” arises when the magnetic moments in a material (imagine little bar magnets which try to turn each other and line up North to South when you bring them together) are arranged in special geometries, like triangular nets. This scenario can make it hard for the bar magnets to line up in way that they all cancel each other out and are stable. Materials exhibiting this property are rare, especially metallic ones. Most frustrated magnet materials are electrical insulators, meaning that their electrons are immobile. “Metallic frustrated magnets have been highly sought after for several decades. They have been predicted to house unconventional superconductivity, Majorana fermions, be useful for quantum computing, and more,” commented Dr. Ali.

Structurally, KV3Sb5 has a 2D, layered structure where triangular vanadium and antimony layers loosely stack on top of potassium layers. This allowed the authors to simply use tape to peel off a few layers (a.k.a. flakes) at a time. “This was very important because it allowed us to use electron-beam lithography (like photo-lithography which is used to make computer chips, but using electrons rather than photons) to make tiny devices out of the flakes and measure properties which people can’t easily measure in bulk.” remarked lead author Shuo-Ying Yang, from the Max Planck Institute of Microstructure Physics. “We were excited to find that the flakes were quite stable to the fabrication process, which makes it relatively easy to work with and explore lots of properties.”

Armed with this combination of properties, the team first chose to look for an anomalous Hall effect (AHE) in the material. This phenomenon is where electrons in a material with an applied electric field (but no magnetic field) can get deflected by 90 degrees by various mechanisms. “It had been theorized that metals with triangular spin arrangements could host a significant extrinsic effect, so it was a good place to start,” noted Yang. Using angle resolved photoelectron spectroscopy, microdevice fabrication, and a low temperature electronic property measurement system, Shuo-Ying and co-lead author Yaojia Wang (Max Planck Institute of Microstructure Physics) were able to observe one of the largest AHE’s ever seen.

The AHE can be broken into two general categories: intrinsic and extrinsic. “The intrinsic mechanism is like if a football player made a pass to their teammate by bending the ball, or electron, around some defenders (without it colliding with them),” explained Ali. “Extrinsic is like the ball bouncing off of a defender, or magnetic scattering center, and going to the side after the collision. Many extrinsically dominated materials have a random arrangement of defenders on the field, or magnetic scattering centers randomly diluted throughout the crystal. KV3Sb5 is special in that it has groups of 3 magnetic scattering centers arranged in a triangular net. In this scenario, the ball scatters off of the cluster of defenders, rather than a single one, and is more likely to go to the side than if just one was in the way.” This is essentially the theorized spin-cluster skew scattering AHE mechanism which was demonstrated by the authors in this material. “However the condition with which the incoming ball hits the cluster seems to matter; you or I kicking the ball isn’t the same as if, say, Christiano Ronaldo kicked the ball,” added Ali. “When Ronaldo kicks it, it is moving way faster and bounces off of the cluster with way more velocity, moving to the side faster than if just any average person had kicked it. This is, loosely speaking, the difference between the Dirac quasiparticles (Ronaldo) in this material vs normal electrons (average person) and is related to why we see such a large AHE,” Ali laughingly explained.

These results may also help scientists identify other materials with this combination of ingredients. “Importantly, the same physics governing this AHE could also drive a very large spin Hall effect (SHE) — where instead of generating an orthogonal charge current, an orthogonal spin current is generated,” remarked Wang. “This is important for next-generation computing technologies based on an electron’s spin rather than its charge.”

“This is a new playground material for us: metallic Dirac physics, frustrated magnetism, exfoliatable, and chemically stable all in one. There is a lot of opportunity to explore fun, weird phenomena, like unconventional superconductivity and more,” said Ali, excitedly.

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Physicists find misaligned carbon sheets yield unparalleled properties

A material composed of two one-atom-thick layers of carbon has grabbed the attention of physicists worldwide for its intriguing — and potentially exploitable — conductive properties.

Dr. Fan Zhang, assistant professor of physics in the School of Natural Sciences and Mathematics at The University of Texas at Dallas, and physics doctoral student Qiyue Wang published an article in June with Dr. Fengnian Xia’s group at Yale University in Nature Photonics that describes how the ability of twisted bilayer graphene to conduct electrical current changes in response to mid-infrared light.

From One to Two Layers

Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern, where each hexagon is formed by six carbon atoms at its vertices. Since graphene’s first isolation in 2004, its unique properties have been intensely studied by scientists for potential use in advanced computers, materials and devices.

If two sheets of graphene are stacked on top of one another, and one layer is rotated so that the layers are slightly out of alignment, the resulting physical configuration, called twisted bilayer graphene, yields electronic properties that differ significantly from those exhibited by a single layer alone or by two aligned layers.

“Graphene has been of interest for about 15 years,” Zhang said. “A single layer is interesting to study, but if we have two layers, their interaction should render much richer and more interesting physics. This is why we want to study bilayer graphene systems.”

A New Field Emerges

When the graphene layers are misaligned, a new periodic design in the mesh emerges, called a moiré pattern. The moiré pattern is also a hexagon, but it can be made up of more than 10,000 carbon atoms.

“The angle at which the two layers of graphene are misaligned — the twist angle — is critically important to the material’s electronic properties,” Wang said. “The smaller the twist angle, the larger the moiré periodicity.”

The unusual effects of specific twist angles on electron behavior were first proposed in a 2011 article by Dr. Allan MacDonald, professor of physics at UT Austin, and Dr. Rafi Bistritzer. Zhang witnessed the birth of this field as a doctoral student in MacDonald’s group.

“At that time, others really paid no attention to the theory, but now it has become arguably the hottest topic in physics,” Zhang said.

In that 2011 research MacDonald and Bistritzer predicted that electrons’ kinetic energy can vanish in a graphene bilayer misaligned by the so-called “magic angle” of 1.1 degrees. In 2018, researchers at the Massachusetts Institute of Technology proved this theory, finding that offsetting two graphene layers by 1.1 degrees produced a two-dimensional superconductor, a material that conducts electrical current with no resistance and no energy loss.

In a 2019 article in Science Advances, Zhang and Wang, together with Dr. Jeanie Lau’s group at The Ohio State University, showed that when offset by 0.93 degrees, twisted bilayer graphene exhibits both superconducting and insulating states, thereby widening the magic angle significantly.

“In our previous work, we saw superconductivity as well as insulation. That’s what’s making the study of twisted bilayer graphene such a hot field — superconductivity. The fact that you can manipulate pure carbon to superconduct is amazing and unprecedented,” Wang said.

New UT Dallas Findings

In his most recent research in Nature Photonics, Zhang and his collaborators at Yale investigated whether and how twisted bilayer graphene interacts with mid-infrared light, which humans can’t see but can detect as heat. “Interactions between light and matter are useful in many devices — for example, converting sunlight into electrical power,” Wang said. “Almost every object emits infrared light, including people, and this light can be detected with devices.”

Zhang is a theoretical physicist, so he and Wang set out to determine how mid-infrared light might affect the conductance of electrons in twisted bilayer graphene. Their work involved calculating the light absorption based on the moiré pattern’s band structure, a concept that determines how electrons move in a material quantum mechanically.

“There are standard ways to calculate the band structure and light absorption in a regular crystal, but this is an artificial crystal, so we had to come up with a new method,” Wang said. Using resources of the Texas Advanced Computing Center, a supercomputer facility on the UT Austin campus, Wang calculated the band structure and showed how the material absorbs light.

The Yale group fabricated devices and ran experiments showing that the mid-infrared photoresponse — the increase in conductance due to the light shining — was unusually strong and largest at the twist angle of 1.8 degrees. The strong photoresponse vanished for a twist angle less than 0.5 degrees.

“Our theoretical results not only matched well with the experimental findings, but also pointed to a mechanism that is fundamentally connected to the period of moiré pattern, which itself is connected to the twist angle between the two graphene layers,” Zhang said.

Next Step

“The twist angle is clearly very important in determining the properties of twisted bilayer graphene,” Zhang added. “The question arises: Can we apply this to tune other two-dimensional materials to get unprecedented features? Also, can we combine the photoresponse and the superconductivity in twisted bilayer graphene? For example, can shining a light induce or somehow modulate superconductivity? That will be very interesting to study.”

“This new breakthrough will potentially enable a new class of infrared detectors based on graphene with high sensitivity,” said Dr. Joe Qiu, program manager for solid-state electronics and electromagnetics at the U.S. Army Research Office (ARO), an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “These new detectors will potentially impact applications such as night vision, which is of critical importance for the U.S. Army.”

In addition to the Yale researchers, other authors included scientists from the National Institute for Materials Science in Japan. The ARO, the National Science Foundation and the Office of Naval Research supported the study.

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How a crystalline sponge sheds water molecules

In a new study, scientists answer this question in detail for a porous, crystalline material made from metal and organic building blocks — specifically, cobalt(II) sulfate heptahydrate, 5-aminoisophthalic acid and 4,4′-bipyridine.

Using advanced techniques, researchers studied how this crystalline sponge changed shape as it went from a hydrated state to a dehydrated state. The observations were elaborate, allowing the team to “see” when and how three individual water molecules left the material as it dried out.

Crystalline sponges of this kind belong to a class of materials called metal-organic frameworks (MOFs), which hold potential for applications such as trapping pollutants or storing fuel at low pressures.

“This was a really nice, detailed example of using dynamic in-situ x-ray diffraction to study the transformation of a MOF crystal,” says Jason Benedict, PhD, associate professor of chemistry in the University at Buffalo College of Arts and Sciences. “We initiate a reaction — a dehydration. Then we monitor it with x-rays, solving crystal structures, and we can actually watch how this material transforms from the fully hydrated phase to the fully dehydrated phase.

“In this case, the hydrated crystal holds three independent water molecules, and the question was basically, how do you go from three to zero? Do these water molecules leave one at time? Do they all leave at once?

“And we discovered that what happens is that one water molecule leaves really quickly, which causes the crystal lattice to compress and twist, and the other two molecules wind up leaving together. They leak out at the same time, and that causes the lattice to untwist but stay compressed. All of that motion that I’m describing — you wouldn’t have any insight into that kind of motion in the absence of these sort of experiments that we are performing.”

The research was published online on June 23 in the journal Structural Dynamics. Benedict led the study with first authors Ian M. Walton and Jordan M. Cox, UB chemistry PhD graduates. Other scientists from UB and the University of Chicago also contributed to the project.

Understanding how the structures of MOFs morph — step by step — during processes like dehydration is interesting from the standpoint of basic science, Benedict says. But such knowledge could also aid efforts to design new crystalline sponges. As Benedict explains, the more researchers can learn about the properties of such materials, the easier it will be to tailor-make novel MOFs geared toward specific tasks.

The technique the team developed and employed to study the crystal’s transformation provides scientists with a powerful tool to advance research of this kind.

“Scientists often study dynamic crystals in an environment that is static,” says co-author Travis Mitchell, a chemistry PhD student in Benedict’s lab. “This greatly limits the scope of their observations to before and after a particular process takes place. Our findings show that observing dynamic crystals in an environment that is also dynamic allows scientists to make observations while a particular process is taking place. Our group developed a device that allows us to control the environment relative to the crystal: We are able to continuously flow fluid around the crystal as we are collecting data, which provides us with information about how and why these dynamic crystals transform.”

The study was supported by the National Science Foundation (NSF) and U.S. Department of Energy, including through the NSF’s ChemMatCARS facility, where much of the experimental work took place.

“These types of experiments often take days to perform on a laboratory diffractometer,” Mitchell says. “Fortunately, our group was able to perform these experiments using synchrotron radiation at NSF’s ChemMatCARS. With synchrotron radiation, we were able to make measurements in a matter of hours.”

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Materials provided by University at Buffalo. Original written by Charlotte Hsu. Note: Content may be edited for style and length.

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