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Stacking and Twisting Graphene Unlocks a Rare Form of Magnetism

Posted By Terrance Barkan, Wednesday, October 14, 2020
Since the discovery of graphene more than 15 years ago, researchers have been in a global race to unlock its unique properties. Not only is graphene—a one-atom-thick sheet of carbon arranged in a hexagonal lattice—the strongest, thinnest material known to man, it is also an excellent conductor of heat and electricity.

Now, a team of researchers at Columbia University and the University of Washington has discovered that a variety of exotic electronic states, including a rare form of magnetism, can arise in a three-layer graphene structure.

The findings appear in an article published October 12 in Nature Physics.

The work was inspired by recent studies of twisted monolayers or twisted bilayers of graphene, comprising either two or four total sheets. These materials were found to host an array of unusual electronic states driven by strong interactions between electrons.

“We wondered what would happen if we combined graphene monolayers and bilayers into a twisted three-layer system,” said Cory Dean, a professor of physics at Columbia University and one of the paper’s senior authors. “We found that varying the number of graphene layers endows these composite materials with some exciting new properties that had not been seen before.”

In addition to Dean, Assistant Professor Matthew Yankowitz and Professor Xiaodong Xu, both in the departments of physics and materials science and engineering at University of Washington, are senior authors on the work. Columbia graduate student Shaowen Chen, and University of Washington graduate student Minhao He are the paper’s co-lead authors.

To conduct their experiment, the researchers stacked a monolayer sheet of graphene onto a bilayer sheet and twisted them by about 1 degree. At temperatures a few degrees over absolute zero, the team observed an array of insulating states—which do not conduct electricity—driven by strong interactions between electrons. They also found that these states could be controlled by applying an electric field across the graphene sheets.

“We learned that the direction of an applied electric field matters a lot,” said Yankowitz, who is also a former postdoctoral researcher in Dean’s group.

When the researchers pointed the electric field toward the monolayer graphene sheet, the system resembled twisted bilayer graphene. But when they flipped the direction of the electric field and pointed it toward the bilayer graphene sheet, it mimicked twisted double bilayer graphene—the four-layer structure.

The team also discovered new magnetic states in the system. Unlike conventional magnets, which are driven by a quantum mechanical property of electrons called “spin,” a collective swirling motion of the electrons in the team’s three-layer structure underlies the magnetism, they observed.

This form of magnetism was discovered recently by other researchers in various structures of graphene resting on crystals of boron nitride. The team has now demonstrated that it can also be observed in a simpler system constructed entirely with graphene.

“Pure carbon is not magnetic,” said Yankowitz. “Remarkably, we can engineer this property by arranging our three graphene sheets at just the right twist angles.”

In addition to the magnetism, the study uncovered signs of topology in the structure. Akin to tying different types of knots in a rope, the topological properties of the material may lead to new forms of information storage, which “may be a platform for quantum computation or new types of energy-efficient data storage applications,” Xu said.

For now, they are working on experiments to further understand the fundamental properties of the new states they discovered in this platform. “This is really just the beginning,” said Yankowitz.

Tags:  boron nitride  Columbia University  Cory Dean  Graphene  Matthew Yankowitz  Minhao He  quantum materials  University of Washington  Xiaodong Xu 

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Quantum Confinement of Dirac Quasiparticles in Graphene Patterned with Sub‐Nanometer Precision

Posted By Terrance Barkan, Friday, October 9, 2020
How does the unstoppable stop? A team of scientists, from the Universidad Autonoma de Madrid, Université Grenoble Alpes, International Iberian Nanotechnology Laboratory, and Aalto University, has managed to solve this chimerical challenge using atomic bricks to build walls capable of stopping the ultra-relativistic electrons of graphene. This fact, in addition to the fascination that being able to experiment with ultra-relativistic quantum particles produces, has a fundamental applied relevance. Being able to build walls capable of confining graphene electrons allows this material to be provided with a tunable electronic gap, something key to enable its integration into real electronic devices.

Electrons are subatomic particles responsible for the transport of electricity. When circulating in graphene, electrons behave like ultra-relativistic quantum particles. This is due to the peculiar arrangement, in a honeycomb structure, of the carbon atoms composing this purely two-dimensional material. Therefore, the rules that govern the behavior of electrons in graphene are very special. They must simultaneously obey the laws of quantum mechanics (necessary for objects of very small sizes) and those of ultra-relativistic physics (necessary for objects of negligible mass moving at speeds close to the speed of light). This gives rise, for example, to what is known as the Klein paradox, which implies that these electrons can only be stopped by atomically abrupt walls. Otherwise, the electrons impacting in certain directions would cross these walls regardless of their thickness or height. This unique property means that electrons can circulate freely through graphene, being barely affected by the impurities that may exist in it, which makes this material exceptional for its use in electronic devices. However, so much freedom of movement comes at a price; it is in turn extremely complex to contain the movement of these electrons. This has hitherto prevented the use of quantum confinement of graphene’s electrons to reach the long sought goal of a tunable graphene’s electronic gap.

The work, published this week in Advanced Materials, shows how an international team of researchers has been able to collectively manipulate a large number of hydrogen atoms to create impenetrable walls for graphene electrons. In their experiments, carried out at the Universidad Autonoma de Madrid with a scanning tunneling microscope, they have used these walls to construct, with subnanometric precision, graphene nanostructures of arbitrarily complex shapes, with dimensions ranging from two nanometers to one micron. The developed method allows to erase and rebuild the nanostructures at will, and can be implemented in different types of graphene. The experiments, supported by theoretical calculations, show that the created nanostructures are capable of perfectly confining the graphene electrons. In this way, the researchers have managed to overcome the pressing challenge of opening an electronic gap in graphene with a tunable value, which can be defined by the size and shape of the created nanostructures. In addition, the method opens up a plethora of exciting new possibilities, as the created nanostructures behave like graphene quantum dots that can be selectively coupled, allowing them to be used in quantum simulators to deepen our understanding of quantum matter. [Full article]

Tags:  Aalto University  Electronics  Graphene  International Iberian Nanotechnology Laboratory  Iván Brihuega  José María Gómez‐Rodríguez  quantum dots  quantum materials  Universidad Autonoma de Madrid  Université Grenoble Alpes 

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All together now: Experiments with twisted 2D materials catch electrons behaving collectively

Posted By Terrance Barkan, Thursday, October 8, 2020
Scientists can have ambitious goals: curing disease, exploring distant worlds, clean-energy revolutions. In physics and materials research, some of these ambitious goals are to make ordinary-sounding objects with extraordinary properties: wires that can transport power without any energy loss, or quantum computers that can perform complex calculations that today’s computers cannot achieve. And the emerging workbenches for the experiments that gradually move us toward these goals are 2D materials — sheets of material that are a single layer of atoms thick.

In a paper published Sept. 14 in the journal Nature Physics, a team led by the University of Washington reports that carefully constructed stacks of graphene — a 2D form of carbon — can exhibit highly correlated electron properties. The team also found evidence that this type of collective behavior likely relates to the emergence of exotic magnetic states.

“We’ve created an experimental setup that allows us to manipulate electrons in the graphene layers in a number of exciting new ways,” said co-senior author Matthew Yankowitz, a UW assistant professor of physics and of materials science and engineering, as well as a faculty researcher at the UW Clean Energy Institute.

Yankowitz led the team with co-senior author Xiaodong Xu, a UW professor of physics and of materials science and engineering. Xu is also a faculty researcher with the UW Molecular Engineering and Sciences Institute, the UW Institute for Nano-Engineered Systems and the Clean Energy Institute.

Since 2D materials are one layer of atoms thick, bonds between atoms only form in two dimensions and particles like electrons can only move like pieces on a board game: side-to-side, front-to-back or diagonally, but not up or down. These restrictions can imbue 2D materials with properties that their 3D counterparts lack, and scientists have been probing 2D sheets of different materials to characterize and understand these potentially useful qualities.

But over the past decade, scientists like Yankowitz have also started layering 2D materials — like a stack of pancakes — and have discovered that, if stacked and rotated in a particular configuration and exposed to extremely low temperatures, these layers can exhibit exotic and unexpected properties.

The UW team worked with building blocks of bilayer graphene: two sheets of graphene naturally layered together. They stacked one bilayer on top of another — for a total of four graphene layers — and twisted them so that the layout of carbon atoms between the two bilayers were slightly out of alignment. Past research has shown that introducing these small twist angles between single layers or bilayers of graphene can have big consequences for the behavior of their electrons. With specific configurations of the electric field and charge distribution across the stacked bilayers, electrons display highly correlated behaviors. In other words, they all start doing the same thing — or displaying the same properties — at the same time.

“In these instances, it no longer makes sense to describe what an individual electron is doing, but what all electrons are doing at once,” said Yankowitz.

“It’s like having a room full of people in which a change in any one person’s behavior will cause everyone else to react similarly,” said lead author Minhao He, a UW doctoral student in physics and a former Clean Energy Institute fellow.

Quantum mechanics underlies these correlated properties, and since the stacked graphene bilayers have a density of more than 1012, or one trillion, electrons per square centimeter, a lot of electrons are behaving collectively.

The team sought to unravel some of the mysteries of the correlated states in their experimental setup. At temperatures of just a few degrees above absolute zero, the team discovered that they could “tune” the system into a type of correlated insulating state — where it would conduct no electrical charge. Near these insulating states, the team found pockets of highly conducting states with features resembling superconductivity.

Though other teams have recently reported these states, the origins of these features remained a mystery. But the UW team’s work has found evidence for a possible explanation. They found that these states appeared to be driven by a quantum mechanical property of electrons called “spin” — a type of angular momentum. In regions near the correlated insulating states, they found evidence that all the electron spins spontaneously align. This may indicate that, near the regions showing correlated insulating states, a form of ferromagnetism is emerging — not superconductivity. But additional experiments would need to verify this.

These discoveries are the latest example of the many surprises that are in store when conducting experiments with 2D materials.

“Much of what we’re doing in this line of research is to try to create, understand and control emerging electronic states, which can be either correlated or topological, or possess both properties,” said Xu. “There could be a lot we can do with these states down the road — a form of quantum computing, a new energy-harvesting device, or some new types of sensors, for example — and frankly we won’t know until we try.”

In the meantime, expect stacks, bilayers and twist angles to keep making waves.

Co-authors are UW researchers Yuhao Li and Yang Liu; UW physics doctoral student and Clean Energy Institute fellow Jiaqi Cai; and K. Watanabe and T. Taniguchi with the National Institute for Materials Science in Japan. The research was funded by the UW Molecular Engineering Materials Center, a National Science Foundation Materials Research Science and Engineering Center; the China Scholarship Council; the Ministry of Education, Culture, Sports, Science and Technology of Japan; and the Japan Science and Technology Agency.

Tags:  2D materials  Graphene  Matthew Yankowitz  quantum materials  University of Washington  Xiaodong Xu 

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Sensor with 100,000 times higher sensitivity could bolster thermal imaging

Posted By Terrance Barkan, Monday, October 5, 2020
Army-funded research developed a new microwave radiation sensor with 100,000 times higher sensitivity than currently available commercial sensors. Researchers said better detection of microwave radiation will enable improved thermal imaging, electronic warfare, radio communications and radar.

Researchers published their study in the peer-reviewed journal Nature. The team includes scientists from Harvard University, The Institute of Photonic Sciences, Massachusetts Institute of Technology, Pohang University of Science and Technology, and Raytheon BBN Technologies. The Army, in part, funded the work to fabricate this bolometer by exploiting the giant thermal response of graphene to microwave radiation.

"The microwave bolometer developed under this project is so sensitive that it is capable of detecting a single microwave photon, which is the smallest amount of energy in nature," said Dr. Joe Qiu, program manager for solid-state electronics and electromagnetics, Army Research Office, an element of the U.S. Army Combat Capabilities Development Command's Army Research Laboratory. "This technology will potentially enable new capabilities for applications such as quantum sensing and radar, and ensure the U.S. Army maintains spectral dominance in the foreseeable future."

The graphene bolometer sensor detects electromagnetic radiation by measuring the temperature rise as the photons are absorbed into the sensor. Graphene is a two dimensional, one-atom layer thick material. The researchers achieved a high bolometer sensitivity by incorporating graphene in the microwave antenna.

A key innovation in this advancement is to measure the temperature rise by superconducting Josephson junction while maintaining a high microwave radiation coupling into the graphene through an antenna, researchers said. The coupling efficiency is essential in a high sensitivity detection because "every precious photon counts."

A Josephson junction is a quantum mechanical device which is made of two superconducting electrodes separated by a barrier (thin insulating tunnel barrier, normal metal, semiconductor, ferromagnet, etc.)

In addition to being thin, the electrons in graphene are also in a very special band structure in which the valence and conduction bands meet at only one point, known as Dirac point.

"The density of states vanishes there so that when the electrons receive the photon energy, the temperature rise is high while the heat leakage is small," said Dr. Kin Chung Fong, Raytheon BBN Technologies.

With increased sensitivity of bolometer detectors, this research has found a new pathway to improve the performance of systems detecting electromagnetic signal such as radar, night vision, LIDAR (Light Detection and Ranging), and communication. It could also enable new applications such as quantum information science, thermal imaging as well as the search of dark matter.

The part of the research conducted at MIT included work from the Institute for Soldier Nanotechnologies. The U.S. Army established the institute in 2002 as an interdisciplinary research center to dramatically improve protection, survivability and mission capabilities of the Soldier and of Soldier-supporting platforms and systems.

Tags:  2D materials  CCDC Army Research Laboratory  Graphene  Harvard University  Joe Qiu  Kin Chung Fong  MIT  Pohang University of Science and Technology  quantum materials  Raytheon BBN Technologies  semiconductor  Sensors  U.S. Army Research Laboratory 

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The most sensitive and fastest graphene microwave bolometer

Posted By Terrance Barkan, Friday, October 2, 2020
Bolometers are devices that measure the power of incident electromagnetic radiation thru the heating of materials, which exhibit a temperature-electric resistance dependence. These instruments are among the most sensitive detectors so far used for infrared radiation detection and are key tools for applications that range from advanced thermal imaging, night vision, infrared spectroscopy to observational astronomy, to name a few.

Even though they have proven to be excellent sensors for this specific range of radiation, the challenge lies in attaining high sensitivity, fast response time and strong light absorption, which not always are accomplished all together. Many studies have been conducted to obtain these higher-sensitivity bolometers by searching to reduce the size of the detector and thus increase the thermal response, and in doing so, they have found that graphene seems to be an excellent candidate for this.

If we focus on the infrared range, several experiments have demonstrated that if you take a sheet of graphene and place it in between two layers of superconducting material to create a Josephson junction, you can obtain a single photon detector device. At low temperatures, and in the absence of photons, a superconducting current flows through the device. When a single infrared photon passes through the detector, the heat it generates is enough to warm up the graphene, which alters the Josephson junction such that no superconducting current can flow. So you can actually detect the photons that are passing through the device by measuring the current. This can be done basically because graphene has an almost negligible electronic heat capacity. This means that, contrary to materials that retain heat like water, in the case of graphene a single low-energy photon can heat the detector enough to block the superconducting current, and then dissipate quickly, allowing the detector to rapidly reset, and thus achieving very fast time responses and high sensitivities.

Trying to take a step further and move to higher wavelengths, in a recent study published in Nature, a team of scientists which includes ICFO researcher Dmitri Efetov, together with colleagues from Harvard University, Raytheon BBN Technologies, MIT, and the National Institute for Material Sciences, has been able to develop a graphene-based bolometer that can detect microwave photons at extremely high sensitivities and with fast time responses.

Just like with the infrared range, the team took a sheet of graphene and placed it in between two layers of superconducting material to create a Josephson junction. This time, they went an entirely new route and attached a microwave resonator to generate the microwave photons and by passing these photons through the device, were able to reach an unprecedented detection levels. In particular, they were able to detect single photons with a much lower energy resolution, equivalent to that of a single 32 Ghz photon, and achieve detection readouts 100.000 times faster than the fastest nanowire bolometers constructed so far.

The results achieved in this study mean a major breakthrough in the field of bolometers. Not only has graphene proven to be an ideal material for infrared sensing and imaging, but it has also proven to span to higher wavelengths, reaching the microwave, where it has also shown to attain extremely high sensitivities and ultra-fast read out times.

As Prof. at ICFO Dmitri Efetov comments "such achievements were thought impossible with traditional materials, and graphene did the trick again. This open entirely new avenues for quantum sensors for quantum computation and quantum communication".

Tags:  Dmitri Efetov  Graphene  ICFO  photonics  quantum materials  Sensors 

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New studies on particle entanglement dynamics in graphene for alternative quantum computing protocols

Posted By Graphene Council, Thursday, September 10, 2020
Quantum properties of matter as entanglement, which can allow controlling quantum states of physical systems, are key to the development of quantum computing and higher-performance information processing. Entanglement usually defines a nonlocal correlation between two or more particles, such that the quantum state of each of them cannot be described independently of the state of the others, even when particles become separated by an extremely large distance. Entanglement can be also observed between internal degrees of freedom of a single particle, which are independent parameters describing the state of a system, as physical coordinates define the position of a point in space. The comprehension of these phenomena, called inter- and intra-particle entanglement, can lead to manipulating the quantum states of physical systems, including materials as graphene and topological matter as a whole. 

In a paper recently published in Physical Review B as a Rapid Communication, researchers from the ICN2 Theoretical and Computational Nanoscience group, led by ICREA Prof. Stephan Roche, present a study on the origin, dynamics and magnitude of intra-particle entanglement between various degrees of freedom of electrons propagating in graphene. In particular, they explore the quantum correlations between the spin, defined as the intrinsic angular momentum of particles, and the pseudo-spin, which is a property analogous to spin that emerges in lattice structures and depends on their specific geometrical symmetries.

The authors of this study show that large intra-particle entanglement is a general feature of graphene supported onto a substrate and that its generation and evolution is independent of the initial state of the system. In addition, it may be robust to disorder and dephasing, which means that, if an interaction compromised the intra-particle entanglement, it would regenerate. This research also suggests that the properties of intra-particle entanglement in graphene should be relevant to the dynamic of inter-particle entanglement between pairs of electrons: in fact, the evolution of the first phenomenon is reflected in the second. Because of this, intra-particle entanglement might be detected indirectly in experiments via inter-particle correlations.

These results unveil unexplored paths to understanding and manipulating entanglement phenomena in a family of materials, called Dirac materials, which includes graphene: this name is due to the fact that they are systems that can be described by the Dirac equation of relativistic quantum mechanics. The ability to detect and manipulate entanglement in such materials could become an unprecedented resource for future research on the application of this phenomenon to quantum information processing.

Tags:  Graphene  ICN2  ICREA  quantum materials  Stephan Roche 

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Fast electrical modulation of nanoscale erbium-graphene systems towards quantum technology applications

Posted By Graphene Council, Thursday, August 27, 2020
Quantum technologies promise to revolutionize information technology and communications by taking advantage of some peculiar aspects of quantum physics, such as quantum state superposition and entanglement. Research is moving forward in different directions with the goal of building optimal devices for quantum information processing, secure communication, and high-precision sensing.

Systems based on rare-earth ions, such as erbium, are very relevant to this quest, in particular because they typically have very long decoherence times, which means that quantum states persist longer than in other systems.  Furthermore, erbium emits light at a wavelength of 1.5 micrometers, one of the main bands for optical communications systems. Hybrid systems containing nanoscale rare-earth components may prove highly versatile and useful to meet the needs of various (quantum) optoelectronic applications.

A team of researchers including Dr Klaas-Jan Tielrooij, leader of the Ultrafast Dynamics in Nanoscale Systems group at the ICN2, and scientists from the Institute of Photonic Sciences (ICFO) and the Institut de Recherche de Chimie Paris (IRCP) have combined a 10 nm thin film of an erbium-doped oxide crystal with monolayer graphene. This hybrid system exhibits extremely strong emitter-environment interactions due to the physical closeness of the emitters to graphene, and the strong dipole-dipole coupling to Dirac electrons.

Their study, recently published in Nature Communications, showed that a large fraction of excited erbium ions decays more than a thousand times faster than normal due to the presence of graphene. This implies that more than 99.9% of the energy flows from these excited emitters to graphene through near-field interactions – where the near-field is the region of the electromagnetic field closest to the object that emits the radiation; in this specific case, it means at a distance from the emitter much smaller than the wavelength of the emitted light. The energy that is transferred from excited emitters to graphene leads to either electron-hole pair generation or plasmon launching (see illustration) in graphene, depending on the Fermi energy of graphene.

Moreover, as reported in the paper, the authors were able to efficiently control the near-field interactions of this hybrid system and to modulate them dynamically by applying a small electrical voltage of just a few volts. This is possible because the gate voltage allows for tuning graphene’s Fermi energy over a large range. The emitter-environment interactions were controlled with high modulation frequencies — up to 300 kHz, which is three orders of magnitude higher than the emitter’s normal radiative decay rate.

This fast dynamic modulation can lead to interesting phenomena and applications, such as the emission of single photons with controlled waveform and quantum entanglement generation by collective plasmon emission. The development of hybrid systems enabling fast control over the near-field interactions, as this erbium-graphene platform, also provides an interesting tool to manipulate quantum states in nanoscale solid state devices by means of conventional electronics. Further studies on these structures will certainly open the way to wider applications in optoelectronic, plasmonic and quantum technologies.

Tags:  Graphene  ICN2  Klaas-Jan Tielrooij  optoelectronics  quantum materials 

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Henriksen lands CAREER grant to chase electron effects

Posted By Graphene Council, Thursday, August 20, 2020
Erik Henriksen, assistant professor of physics in Arts & Sciences at Washington University in St. Louis, has been awarded a prestigious Faculty Early Career Development (CAREER) Award by the National Science Foundation. His grant, expected to total $850,000 over the next five years, is for research that explores many-particle interactions in graphene and other single-atom-thick materials.

The behavior of electrons determines the fundamental properties of any material — such as its ability to conduct electricity, or its reflectivity. But these electron interactions are mostly impossible to directly perceive.

“The odd reason that no one has been able to investigate this is that spectroscopic techniques are blind to correlated motion of electrons in most materials,” Henriksen said. “In graphene, amusingly enough, spectroscopy does work and can directly observe the appearance of such many-particle effects.”

Henriksen painstakingly built a unique facility at Washington University that allows him to shine infrared light through graphene under the influence of a strong magnetic field, at extremely low temperatures — revealing the fundamental ways in which electrons jostle with each other as part of a larger system.

“We want to perform spectroscopy of the fractional quantum Hall effect, a remarkable many-particle correlated electron effect discovered in the 1980s,” he said. Henriksen’s graduate adviser Horst L. Stormer was awarded the 1998 Nobel Prize in physics for his role in that discovery. “This effect is characterized by strange features such as apparent fractional electron charges and electrons that bind to magnetic field lines.”

Henriksen also will use funds from his CAREER grant to place atomically small slivers of materials between two mirrors, trapping the light such that it bounces back and forth through the slivers thousands of times.

“Ultimately, this creates novel particles that are a quantum mixture of light and matter: a new form of stuff that doesn’t normally exist,” Henriksen said. “With graphene and our low-temp facility, we can do this in a new regime no one has looked at before.

“Hopefully, this means we’ll find new behaviors,” he said. “Or, at the very least, we can enhance the correlated effects.”

Henriksen plays a key role in the university’s Center for Quantum Sensors. To that end, he also is working to develop novel graphene-based infrared sensors and light emitters operating in the so-called ‘terahertz gap,’ a part of the infrared spectrum that is historically bereft of convenient sources and detectors.

In a forthcoming paper at Physical Review X, a scientific journal of the American Physical Society, Henriksen directly observed many-particle effects and how they show up as unusual gaps in the electronic structure of graphene.

“Many materials can be understood as if the electrons inside were unaware of or unaffected by each other,” Henriksen said. “It’s weird but true. But when you find a material where this is not the case, things get very interesting!”

Tags:  Erik Henriksen  Graphene  quantum materials  Sensors  Washington University 

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Quantum Materials Quest Could Benefit From Graphene That Buckles

Posted By Graphene Council, Friday, August 14, 2020
Graphene, an extremely thin two-dimensional layer of the graphite used in pencils, buckles when cooled while attached to a flat surface, resulting in beautiful pucker patterns that could benefit the search for novel quantum materials and superconductors, according to Rutgers-led research in the journal Nature.

Quantum materials host strongly interacting electrons with special properties, such as entangled trajectories, that could provide building blocks for super-fast quantum computers. They also can become superconductors that could slash energy consumption by making power transmission and electronic devices more efficient.

“The buckling we discovered in graphene mimics the effect of colossally large magnetic fields that are unattainable with today’s magnet technologies, leading to dramatic changes in the material’s electronic properties,” said lead author Eva Y. Andrei, Board of Governors professor in the Department of Physics and Astronomy in the School of Arts and Sciences at Rutgers University–New Brunswick. “Buckling of stiff thin films like graphene laminated on flexible materials is gaining ground as a platform for stretchable electronics with many important applications, including eye-like digital cameras, energy harvesting, skin sensors, health monitoring devices like tiny robots and intelligent surgical gloves. Our discovery opens the way to the development of devices for controlling nano-robots that may one day play a role in biological diagnostics and tissue repair.” 

The scientists studied buckled graphene crystals whose properties change radically when they’re cooled, creating essentially new materials with electrons that slow down, become aware of each other and interact strongly, enabling the emergence of fascinating phenomena such as superconductivity and magnetism, according to Andrei.

Using high-tech imaging and computer simulations, the scientists showed that graphene placed on a flat surface made of niobium diselenide, buckles when cooled to 4 degrees above absolute zero. To the electrons in graphene, the mountain and valley landscape created by the buckling appears as gigantic magnetic fields. These pseudo-magnetic fields are an electronic illusion, but they act as real magnetic fields, according to Andrei.

“Our research demonstrates that buckling in 2D materials can dramatically alter their electronic properties,” she said.

The next steps include developing ways to engineer buckled 2D materials with novel electronic and mechanical properties that could be beneficial in nano-robotics and quantum computing, according to Andrei.

Tags:  2D materials  Eva Y. Andrei  Graphene  quantum materials  Rutgers University 

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Shining a light on the quantum world

Posted By Graphene Council, Wednesday, July 29, 2020
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.

Unfinished business

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.”

Tags:  2D materials  Graphene  Ido Kaminer  Marin Soljačić  Massachusetts Institute of Technology  Nicholas Rivera  quantum materials  Technion-Israel Institute of Technology 

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