Tag Archives: #atoms

Scientists Overhear Two Atoms Chatting (Physics)

How materials behave depends on the interactions between countless atoms. You could see this as a giant group chat in which atoms are continuously exchanging quantum information. Researchers from Delft University of Technology in collaboration with RWTH Aachen University and the Research Center Jülich have now been able to intercept a chat between two atoms. They present their findings in Science on 28 May.

Atoms, of course, don’t really talk. But they can feel each other. This is particularly the case for magnetic atoms. “Each atom carries a small magnetic moment called spin. These spins influence each other, like compass needles do when you bring them close together. If you give one of them a push, they will start moving together in a very specific way,” explains Sander Otte, leader of the team that performed the research. “But according to the laws of quantum mechanics, each spin can be simultaneously point in various directions, forming a superposition. This means that actual transfer of quantum information takes place between the atoms, like some sort of conversation.”

Sharp needle

On a large scale, this kind of exchange of information between atoms can lead to fascinating phenomena. A classic example is superconductivity: the effect where some materials lose all electrical resistivity below a critical temperature. While well understood for the simplest cases, nobody knows exactly how this effect comes about in many complex materials. But it’s certain that magnetic quantum interactions play a key role. For the purpose of trying to explaining phenomena like this, scientists are very interested in being able to intercept these exchanges; to overhear the conversations between atoms.

In Otte’s team they go about this rather directly: they literally put two atoms next to each other to see what happens. This is possible by virtue of a scanning tunneling microscope: a device in which a sharp needle can probe atoms one-by-one and can even rearrange them. The researchers used this device to place two titanium atoms at a distance of just over one nanometer – one millionth of a millimeter – apart. At that distance, the atoms are just able to feel each other’s spin. If you would now twist one of the two spins, the conversation would start by itself.

Usually, this twist is performed by sending very precise radio signals to the atoms. This so-called spin resonance technique – which is quite reminiscent of the working principle of an MRI scanner found in hospitals – is used successfully in research on quantum bits. This tool is also available to the Delft team, but it has a disadvantage. “It is simply too slow,” says PhD student Lukas Veldman, lead author on the Science publication. “You have barely started twisting the one spin before the other starts to rotate along. This way you can never investigate what happens upon placing the two spins in opposite directions.”

Unorthodox approach

So the researchers tried something unorthodox: they rapidly inverted the spin of one of the two atoms with a sudden burst of electric current. To their surprise, this drastic approach resulted in a beautiful quantum interaction, exactly by the book. During the pulse, electrons collide with the atom, causing its spin to rotate. Otte: “But we always assumed that during this process, the delicate quantum information – the so-called coherence – was lost. After all, the electrons are incoherent: the history of each electron prior to the collision is slightly different and this chaos is transferred to the atom’s spin, destroying any coherence.”

The fact that this now seems not to be true was cause for some debate. Apparently, each random electron, regardless of its past, can initiate a coherent superposition: a specific combination of elementary quantum states which is fully known and which forms the basis for almost any form of quantum technology.

Perfect superposition

“The crux is that it depends on the question you ask,” argues Markus Ternes, co-author from the RWTH Aachen University and the Research Center Jülich. “The electron inverts the spin of one atom causing it to point, say, to the left. You could view this as a measurement, erasing all quantum memory. But from the point of view of the combined system comprising both atoms, the resulting situation is not so mundane at all. For the two atoms together, the new state constitutes a perfect superposition, enabling the exchange of information between them. Crucially for this to happen is that both spins become entangled: a peculiar quantum state in which they share more information about each other than classically possible.”

The discovery can be of importance to research on quantum bits. Perhaps also in that research you could get away with being slightly less careful when initializing quantum states. But for Otte and his team it is mostly the starting point of even more beautiful experiments. Veldman: “here we used two atoms, but what happens when you use three? Or ten, or a thousand? Nobody can predict that, as computing power falls short for such numbers. Perhaps one day we will be able to listen to quantum conversations that nobody could ever hear before.”

Featured image: Artist’s impression of the experiment, where an electric pulse is applied to a titanium atom. As a result, its magnetic moment suddenly flips around. A neighbouring titanium atom (right) reacts to this motion, but can’t keep pace with the fast movement. As such, an exchange of magnetic quantum information between the atoms is initiated. © TU Delft/Scixel


Reference: Lukas M. Veldman, Laëtitia Farinacci, Rasa Rejali, Rik Broekhoven, Jérémie Gobeil, David Coffey, Markus Ternes, Alexander F. Otte, “Free coherent evolution of a coupled atomic spin system initialized by electron scattering”, Science  28 May 2021: Vol. 372, Issue 6545, pp. 964-968 DOI: 10.1126/science.abg8223


Provided by Delft University of Technology

Researchers are Developing A New Type of Sensor that is Highly Sensitive to Atoms and Molecules (Physics)

Physicists and chemists have produced a highly sensitive sensor / This is made possible by a new type of heterostructure made from “atomically precise” graphene nanostructures / Publication in Nature Communications

An international research team headed by the University of Cologne has succeeded for the first time in connecting several “atomically precise” nanostrips made of graphene, a modification of carbon, to form complex structures. The scientists were able to integrate the strips into an electronic component. In this way, they have created a new type of sensor that is highly sensitive to atoms and molecules. The results of their research are published under the title “Tunneling current modulation in atomically precise graphene nanoribbon heterojunctions” in the renowned journal Nature Communications. This work was created in close cooperation between the II. Physikalisches Institut, the Department of Chemistry at the University of Cologne and research groups from Montreal, Novosibirsk

The graphene nanostrips are only one nanometer – a millionth of a millimeter – wide. Graphene consists of only a single layer of carbon atoms and is considered to be the thinnest material in the world. In 2010 researchers from Manchester succeeded in producing monatomic layers of graphene for the first time, for which they received the Nobel Prize. “The graphene nano-strips used to manufacture the sensor are each between seven and fourteen carbon atoms wide and around 50 nanometers long. The special thing is that their edges are free of defects. They are therefore referred to as ‘atomically precise’ nanostrips, ”explains Dr. Boris Senkovskiy from the 2nd Physics Institute at the University of Cologne. The researchers have now succeeded to connect several of these strips with their short end and thus create more complex heterostructures. These heterostructures have properties of semiconductors.

The heterostructures were examined by means of angle-resolved photoemission, optical spectroscopy and scanning tunneling microscopy. In the next step, the generated heterostructures were integrated into an electronic component. The electrical current that flows through the nanorestrip heterostructure is determined by the quantum mechanical tunnel effect. This states that under certain conditions electrons can overcome existing energy barriers in atoms by “tunneling”, so that a current flow then occurs, although the barrier is higher than the available energy of the electron.

The researchers have succeeded in building a new type of sensor for the adsorption of atoms and molecules from the nanorestrip heterostructure. The tunnel current through the heterostructure is particularly sensitive to adsorbates that accumulate on surfaces. This means that the current strength changes when atoms or molecules, for example from gases, accumulate on the surface of the sensor. “The sensor prototype we built has excellent properties. Among other things, it is particularly sensitive and can be used to measure even the smallest amounts of adsorbate, ”says Professor Dr. Alexander Grüneis, head of a working group at the 2nd Physics Institute.

Featured image: Schematic illustration of the aligned GNR heterojunctions integrated into the device. Lateral fusion of 7-AGNRs leads to the formation of quasi-metallic 14-AGNR and 21-AGNRs. When the source–drain contacts are fabricated, the remaining 7-AGNR segments act as tunneling barriers. Red arrows indicate different paths for charge transport © authors


Publication:
Nat. Commun. 12, 2542 (2021),
https://doi.org/10.1038/s41467-021-22774-0


Provided by University of Cologne

How Acidic Are Atoms? (Chemistry)

The acidity of molecules can be easily determined, but until now it was not possible to measure this important property for atoms on a surface. With a new microscopy technique from the Vienna University of Technology (TU Wien), this has now been achieved.

The degree of acidity or alkalinity of a substance is crucial for its chemical behavior. The decisive factor is the so-called proton affinity, which indicates how easily an entity accepts or releases a single proton. While it is easy to measure this for molecules, it has not been possible for surfaces. This is important because atoms on surfaces have very different proton affinities, depending on where they sit. Researchers at TU Wien have now succeeded in making this important physical quantity experimentally accessible for the first time: Using a specially modified atomic force microscope, it is possible to study the proton affinity of individual atoms. This should help to analyze catalysts on an atomic scale. The results have been published in the scientific journal Nature.

Precision instead of average

“All previous measurements of surface acidity had one severe drawback,” says Prof. Ulrike Diebold from the Institute of Applied Physics at TU Wien. “Although the surface atoms behave chemically differently, one could only ever measure the average value.”

Thus it is not known which atoms contributed to chemical reactions, and to what extent, which makes it impossible to adjust the atomic scale of the surface to favor certain chemical reactions. But that is exactly what is needed, for example, when looking for more effective catalysts for hydrogen production.

“We analyzed surfaces made of indium oxide. They are particularly interesting because there are five different types of OH groups with different properties on the surface,” says Margareta Wagner, who carried out these measurements in Prof. Diebold’s lab.

Ulrike Diebold, Margareta Wagner, Michael Schmid, Bernd Meyer, Martin Setvin (left to right) © TU Wien

With a special trick it was possible to study these OH groups individually: The researchers placed a single OH group at the tip of an atomic force microscope. This tip was then positioned specifically over one particular atom on the surface. A force then acts between the OH group on the tip and the OH group directly below it on the indium oxide surface, and this force depends sensitively on the distance between them.

“We vary the distance between the tip and the surface and measure how this changes the force,” explains Margareta Wagner. “This gives us a characteristic force curve for each OH group on the surface of a material.” The shape of this force curve provides information about how well the respective oxygen atoms on the indium oxide surface hold their protons – or how easily they will release them.

In order to obtain an actual value for the proton affinity, theoretical work was necessary. This was carried out by Bernd Meyer at the Friedrich-Alexander-University Erlangen-Nürnberg, Germany. In elaborate computer simulations it was possible to show how the force curve of the atomic force microscope can be translated in a simple and precise way into those quantities that are needed in chemistry.

Nanostructure determines the quality of catalysts

“This is quite crucial for the further development of catalysts,” says Bernd Meyer. “We know that atoms of the same type behave quite differently depending on their atomic neighbors and the way they are incorporated into the surface.” For example, it can make a big difference whether the surface is perfectly smooth or has steps on an atomic scale. Atoms with a smaller number of neighbors sit at such step edges, and they can potentially significantly improve or worsen chemical reactions.

“With our functionalized scanning force microscope tip, we can now precisely investigate such questions for the first time,” says Ulrike Diebold. “This means that we no longer have to rely on trial and error, but can precisely understand and improve chemical properties of surfaces.”

Featured image: Using the modified tip of an atomic force microscope, individual atoms in the surface can be probed. © TU Wien


Reference: Wagner, M., Meyer, B., Setvin, M. et al. Direct assessment of the acidity of individual surface hydroxyls. Nature 592, 722–725 (2021). https://doi.org/10.1038/s41586-021-03432-3


Provided by Tu Wein

Little Swirling Mysteries: Uncovering Dynamics of Ultrasmall, Ultrafast Groups of Atoms (Material Science)

Exploring and manipulating the behavior of polar vortices in material may lead to new technology for faster data transfer and storage. Researchers used the Advanced Photon Source at Argonne and the Linac Coherent Light Source at SLAC to learn more.

Our high-speed, high-bandwidth world constantly requires new ways to process and store information. Semiconductors and magnetic materials have made up the bulk of data storage devices for decades. In recent years, however, researchers and engineers have turned to ferroelectric materials, a type of crystal that can be manipulated with electricity.

In 2016, the study of ferroelectrics got more interesting with the discovery of polar vortices — essentially spiral-shaped groupings of atoms — within the structure of the material. Now a team of researchers led by the U.S. Department of Energy’s (DOE) Argonne National Laboratory has uncovered new insights into the behavior of these vortices, insights that may be the first step toward using them for fast, versatile data processing and storage.

“You don’t want something that does what a transistor does, because we have transistors already. So you look for new phenomena. What aspects can they bring? We look for objects with faster speed. This is what inspires people. How can we do something different?”

— John Freeland, senior physicist, Argonne National Laboratory

What is so important about the behavior of groups of atoms in these materials? For one thing, these polar vortices are intriguing new discoveries, even when they are just sitting still. For another, this new research, published as a cover story in Nature, reveals how they move. This new type of spiral-patterned atomic motion can be coaxed into occurring, and can be manipulated. That’s good news for this material’s potential use in future data processing and storage devices.

“Although the motion of individual atoms alone may not be too exciting, these motions join together to create something new — an example of what scientists refer to as emergent phenomena — which may host capabilities we could not imagine before,” said Haidan Wen, a physicist in Argonne’s X-ray Science Division (XSD).

These vortices are indeed small — about five or six nanometers wide, thousands of times smaller than the width of a human hair, or about twice as wide as a single strand of DNA. Their dynamics, however, cannot be seen in a typical laboratory environment. They need to be excited into action by applying an ultrafast electric field.

All of which makes them difficult to observe and to characterize. Wen and his colleague, John Freeland, a senior physicist in Argonne’s XSD, have spent years studying these vortices, first with the ultrabright X-rays of the Advanced Photon Source (APS) at Argonne, and most recently with the free-electron laser capabilities of the LINAC Coherent Light Source (LCLS) at DOE’s SLAC National Accelerator Laboratory. Both the APS and LCLS are DOE Office of Science User Facilities.

Using the APS, researchers were able to use lasers to create a new state of matter and obtain a comprehensive picture of its structure using X-ray diffraction. In 2019, the team, led jointly by Argonne and The Pennsylvania State University, reported their findings in a Nature Materials cover story, most notably that the vortices can be manipulated with light pulses. Data was taken at several APS beamlines: 7-ID-C, 11-ID-D, 33-BM and 33-ID-C.

“Although this new state of matter, a so called supercrystal, does not exist naturally, it can be created by illuminating carefully engineered thin layers of two distinct materials using light,” said Venkatraman Gopalan, professor of materials science and engineering and physics at Penn State.

“A lot of work went into measuring the motion of a tiny object,” Freeland said. “The question was, how do we see these phenomena with X-rays? We could see that there was something interesting with the system, something we might be able to characterize with ultrafast timescale probes.”

The APS was able to take snapshots of these vortices at nanosecond time scales — a hundred million times faster than it takes to blink your eyes — but the research team discovered this was not fast enough.

“We knew something exciting must be happening that we couldn’t detect,” Wen said. “The APS experiments helped us pinpoint where we want to measure, at faster time scales that we were not able to access at the APS. But LCLS, our sister facility at SLAC, provides the exact tools needed to solve this puzzle.” 

With their prior research in hand, Wen and Freeland joined colleagues from SLAC and DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab) — Gopalan and Long-Qing Chen of Pennsylvania State University; Jirka Hlinka, head of the Department of Dielectrics at the Institute of Physics of the Czech Academy of Sciences; Paul Evans of the University of Wisconsin, Madison; and their teams — to design a new experiment that would be able to tell them how these atoms behave, and whether that behavior could be controlled. Using what they learned at APS, the team — including the lead authors of the new paper, Qian Li and Vladimir Stoica, both post-doctoral researchers at the APS at the time of this work — pursued further investigations at the LCLS at SLAC.

“LCLS uses X-ray beams to take snapshots of what atoms are doing at timescales not accessible to conventional X-ray apparatus,” said Aaron Lindenberg, associate professor of materials science and engineering and photon sciences at Stanford University and SLAC. “X-ray scattering can map out structures, but it takes a machine like LCLS to see where the atoms are and to track how they are dynamically moving at unimaginably fast speeds.”

Using a new ferroelectric material designed by Ramamoorthy Ramesh and Lane Martin at Berkeley Lab, the team was able to excite a group of atoms into swirling motion by an electric field at terahertz frequencies, the frequency that’s roughly 1,000 times faster than the processor in your cell phone. They were able to then capture images of those spins at femtosecond timescales. A femtosecond is a quadrillionth of a second — it’s such a short period of time that light can only travel about the length of a small bacteria before it’s over.

With this level of precision, the research team saw a new type of motion they had not seen before.

“Despite theorists having been interested in this type of motion, the exact dynamical properties of polar vortices remained nebulous until the completion of this experiment,” Hlinka said. “The experimental findings helped theorists to refine the model, providing a microscopic insight in the experimental observations. It was a real adventure to reveal this sort of concerted atomic dance.”

This discovery opens up a new set of questions that will take further experiments to answer, and planned upgrades of both the APS and LCLS light sources will help push this research further. LCLS-II, now under construction, will increase its X-ray pulses from 120 to 1 million per second, enabling scientists to look at the dynamics of materials with unprecedented accuracy.

And the APS Upgrade, which will replace the current electron storage ring with a state-of-the-art model that will increase the brightness of the coherent X-rays up to 500 times, will enable researchers to image small objects like these vortices with nanometer resolution. 

Researchers can already see the possible applications of this knowledge. The fact that these materials can be tuned by applying small changes opens up a wide range of possibilities, Lindenberg said.

“From a fundamental perspective we are seeing a new type of matter,” he said. “From a technological perspective of information storage, we want to take advantage of what is happening at these frequencies for high-speed, high-bandwidth storage technology. I am excited about controlling the properties of this material, and this experiment shows possible ways of doing this in a dynamical sense, faster than we thought possible.”

Wen and Freeland agreed, noting that these materials may have applications that no one has thought of yet.

“You don’t want something that does what a transistor does, because we have transistors already,” Freeland said. “So you look for new phenomena. What aspects can they bring? We look for objects with faster speed. This is what inspires people. How can we do something different?”

Featured image: Artist’s conception of polar vortices moving in ferroelectric material. These small groupings of atoms must be excited with high-frequency electric fields to move, but studying their behavior may lead to new innovations in data storage and processing. (Image by Ellen Weiss/Argonne National Laboratory.)


Reference: Stoica, V.A., Laanait, N., Dai, C. et al. Optical creation of a supercrystal with three-dimensional nanoscale periodicity. Nat. Mater. 18, 377–383 (2019). https://doi.org/10.1038/s41563-019-0311-x


Provided by Argonne National University

Size Matters When it Comes To Atomic Properties (Chemistry)

A study from Chalmers University of Technology, Sweden, has yielded new answers to fundamental questions about the relationship between the size of an atom and its other properties, such as electronegativity and energy. The results pave the way for advances in future material development. For the first time, it is now possible under certain conditions to devise exact equations for such relationships.

“Knowledge of the size of atoms and their properties is vital for explaining chemical reactivity, structure and the properties of molecules and materials of all kinds. This is fundamental research that is necessary for us to make important advances,” explains Martin Rahm, the main author of the study and research leader from the Department of Chemistry and Chemical Engineering at Chalmers University of Technology.

The researchers behind the study, consisting of colleagues from the University of Parma, Italy, as well as the Department of Physics at Chalmers University of Technology, have previously worked with quantum mechanical calculations to show how the properties of atoms change under high pressure. These results were presented in scientific articles in the Journal of the American Chemical Society and ChemPhysChem.

The new study, published in the journal Chemical Science, constitutes the next step in their important work, exploring the relationship between the radius of an atom and its electronegativity – a vital piece of chemical knowledge that has been sought since the 1950s.

Establishing useful new equations

By studying how compression affects individual atoms, the researchers have been able to derive a set of equations that explain how changes in one property – an atom’s size – can be translated and understood as changes in other properties – the total energy and the electronegativity of an atom. The derivation has been made for special pressures, at which the atoms can take one of two well-defined energies, two radii and two electronegativities.

“Knowledge of the size of atoms and their properties is vital for explaining chemical reactivity, structure and the properties of molecules and materials of all kinds. This is fundamental research that is necessary for us to make important advances,” explains Martin Rahm, the main author of the study and research leader from the Department of Chemistry and Chemical Engineering at Chalmers University of Technology. © Johan Bodell/Chalmers University of Technology

“This equation can, for example, help to explain how an increase in an atom’s oxidation state also increases its electronegativity and vice versa, in the case of a decrease in oxidation state,” says Martin Rahm.

A key question for the science of unexplored materials

One aim of the study has been to help identify new opportunities and possibilities for the production of materials under high pressure. At the centre of the earth, the pressure can reach hundreds of gigapascals – and such conditions are achievable in laboratory settings today. Examples of areas where pressure is used today include the synthesis of superconductors, materials which can conduct electric current without resistance. But the researchers see many further possibilities ahead.

“Pressure is a largely unexplored dimension within materials science, and the interest in new phenomena and material properties that can be realised using compression is growing,” says Martin Rahm.

Creating the database they themselves wished for

The large amounts of data that the researchers have computed through their work have now been summarised into a database, and made available as a user-friendly web application. This development was sponsored by Chalmers Area of Advance Materials and made possible through a collaboration with the research group of Paul Erhart at the Department of Physics at Chalmers.

In the web application, users can now easily explore what the periodic table looks like at different pressures. In the latest scientific publication, the researchers provide an example for how this tool can be used to provide new insight into chemistry. The properties of iron and silicon – two common elements found in the earth’s crust, mantle and core – are compared, revealing large differences at different pressures.

“The database is something I have been missing for many years. Our hope is that it will prove to be a helpful tool, and be used by many different chemists and materials researchers who study and work with high pressures. We have already used it to guide theoretical searches for new transition metal fluorides,” says Martin Rahm.

Featured image: An illustration of potassium atoms undergoing changes in fundamental characteristics such as radius, energy and electronegativity as they are compressed by surrounding neon atoms. © Neuroncollective, Daniel Spacek, Pavel Travnicek


Provided by Chalmers University of Technology

Catching Electrons in Action in an Antiferromagnetic Nanowire (Physics)

The electron is one of the fundamental particles in nature we read about in school. Its behavior holds clues to new ways to store digital data.

In a study published in Nano Letters, physicists from Michigan Technological University explore alternative materials to improve capacity and shrink the size of digital data storage technologies. Ranjit Pati, professor of physics at Michigan Tech, led the study and explains the physics behind his team’s new nanowire design.

“Thanks to a property called spin, electrons behave like tiny magnets,” Pati said. “Similar to how a bar magnet’s magnetization is dipolar, pointing from south to north, the electrons in a material have magnetic dipole moment vectors that describe the material’s magnetization.”

When these vectors are in random orientation, the material is nonmagnetic. When they are parallel to each other, it’s called ferromagnetism and antiparallel alignments are antiferromagnetism. Current data storage technology is based on ferromagnetic materials, where the data are stored in small ferromagnetic domains. This is why a strong enough magnet can mess up a mobile phone or other electronic storage.

Data Storage Challenges

Depending on the direction of magnetization (whether pointing up or down), data are recorded as bits (either a 1 or 0) in ferromagnetic domains. However, there are two bottlenecks, and both hinge on proximity. First, bring an external magnet too close, and its magnetic field could alter the direction of magnetic moments in the domain and damage the storage device. And, second, the domains each have a magnetic field of their own, so they can’t be too close to each other either. The challenge with smaller, more flexible, more versatile electronics is that they demand devices that make it harder to keep ferromagnetic domains safely apart.

“Ultrahigh-density data packing would be a daunting task with ferromagnetic memory domains,” Pati said. “Antiferromagnetic materials, on the other hand, are free from these issues.”

On their own antiferromagnetic materials aren’t great for electronic devices, but they’re not influenced by outside magnetic fields. This ability to resist magnetic manipulation started getting more attention from the research community and Pati’s team used a predictive quantum many-body theory that considers electron-electron interactions. The team found that chromium-doped nanowires with a germanium core and silicon shell can be an antiferromagnetic semiconductor.

Antiferromagnetism

Several research groups have recently demonstrated manipulation of individual magnetic states in antiferromagnetic materials using electrical current and lasers. They observed spin dynamics in the terahertz frequency — much faster than the frequency used in our current data storage devices. This observation has opened up a plethora of research interests in antiferromagnetism and could lead to faster, higher-capacity data storage.

“In our recent work, we have successfully harnessed the intriguing features of an antiferromagnet into a low-dimensional, complementary metal-oxide compatible semiconductor (CMOS) nanowire without destroying the semiconducting property of the nanowire,” Pati said. “This opens up possibilities for smaller and smarter electronics with higher capacity data storage and manipulation.”

Pati adds that the most exciting part of the research for his team was uncovering the mechanism that dictates antiferromagnetism. The mechanism is called superexchange and it controls the spin of electrons and the antiparallel alignment that makes them antiferromagnetic. In the team’s nanowire, germanium electrons act as a go-between, an exchanger, between unconnected chromium atoms. 

“The interaction between the magnetic states of the chromium atoms is mediated by the intermediate atoms they are bonded to. It is a cooperative magnetic phenomenon,” Pati said. “In a simple way, let us say there are two people A and B: They are far apart and cannot communicate directly. But A has a friend C and B has a friend D. C and D are close friends. So, A and B can interact indirectly through C and D.”

Better understanding how electrons communicate between atomic friends enables more experiments to test the potential of materials like chromium-doped nanowires. Better understanding the germanium-silicon nanowire material’s antiferromagnetic nature is what boosts potential for smaller, smarter, higher capacity electronics.

Featured image: Chromium-doped (purple) nanowires with a germanium (yellow atoms) core and silicon shell (yellow-red connections) can be an antiferromagnetic semiconductor. © MTU


Reference: Sandip Aryal, Durga Paudyal, and Ranjit Pati, “Cr-Doped Ge-Core/Si-Shell Nanowire: An Antiferromagnetic Semiconductor”, Nano Lett. 2021, 21, 4, 1856–1862. Doi: https://doi.org/10.1021/acs.nanolett.0c04971


Provided by Michigan Technological University


About Michigan Technological University

Michigan Technological University is a public research university, home to more than 7,000 students from 54 countries. Founded in 1885, the University offers more than 120 undergraduate and graduate degree programs in science and technology, engineering, forestry, business and economics, health professions, humanities, mathematics, and social sciences. Our campus in Michigan’s Upper Peninsula overlooks the Keweenaw Waterway and is just a few miles from Lake Superior.

Extinct Atom Reveals The Long-kept Secrets of The Solar System (Astronomy)

Using the extinct niobium-​92 atom, ETH researchers have been able to date events in the early solar system with greater precision than before. The study concludes that supernova explosions must have taken place in the birth environment of our sun.

If an atom of a chemical element has a surplus of protons or neutrons, it becomes unstable. It will shed these additional particles as gamma radiation until it becomes stable again. One such unstable isotope is niobium-​92 (92Nb), which experts also refer to as a radionuclide. Its half-​life of 37 million years is relatively brief, so it went extinct shortly after the formation of the solar system. Today, only its stable daughter isotope, zirconium-​92 (92Zr), bears testimony to the existence of 92Nb.

Yet scientists have continued to make use of the extinct radionuclide in the form of the 92Nb-92Zr chronometer, with which they can date events that took place in the early solar system some 4.57 billion years ago.

Use of the 92Nb-92Zr chronometer has hitherto been limited by a lack of precise information regarding the amount of 92Nb that was present at the birth of the solar system. This compromises its use for dating and determining the production of these radionuclides in stellar environments.

Meteorites hold the key to the distant past

Now a research team from ETH Zurich and the Tokyo Institute of Technology (Tokyo Tech) has greatly improved this chronometer. The researchers achieved this improvement by means of a clever trick: they recovered rare zircon and rutile minerals from meteorites that were fragments of the protoplanet Vesta. These minerals are considered to be the most suitable for determing 92Nb, because they give precise evidence of how common 92Nb was at the time of the meteorite’s formation. Then, with the uranium-​lead dating technique (uranium atoms that decay into lead), the team calculated how abundant 92Nb was at the time the solar system’s formation. By combining the two methods, the researchers succeeded in considerably improving the precision of the 92Nb-92Zr chronometer.

“This improved chronometer is thus a powerful tool for providing precise ages for the formation and development of asteroids and planets – events that happened in the first tens of millions of years after the formation of the solar system,” says Maria Schönbächler, Professor at the Institute of Geochemistry and Petrology at ETH Zurich, who led the study.

Supernovas released niobium-​92

Now that the researchers know more precisely how abundant 92Nb was at the very beginnings of our solar system, they can determine more accurately where these atoms were formed and where the material that makes up our sun and the planets originated.

The research team’s new model suggests that the inner solar system, with the terrestrial planets Earth and Mars, is largely influenced by material ejected by Type Ia supernovae in our Milky Way galaxy. In such stellar explosions, two orbiting stars interact with each other before exploding and releasing stellar material. In contrast, the outer solar system was fed primarily by a core-collapse supernova – probably in the stellar nursery where our sun was born –, in which a massive star collapsed in on itself and exploded violently.

Featured image: The unstable atom 92Nb, which has long since disappeared, provides information about the beginnings of our solar system. (Illustration: Makiko K. Haba)


Reference

Haba MK, Lai Y-J, Wotzlaw J-F, Yamaguchi A, Lugaro M, Schönbächler M. Precise initial abundance of Niobium-92 in the Solar System and implications for p-process nucleosynthesis. PNAS February 23, 2021 118 (8) e2017750118. DOI: 10.1073/pnas.2017750118


Provided by University of Zurich

Bottling The World’s Coldest Plasma (Physics)

Laser-cooled plasma-in-a-bottle could answer questions about the sun, fusion power

Rice University physicists have discovered a way to trap the world’s coldest plasma in a magnetic bottle, a technological achievement that could advance research into clean energy, space weather and astrophysics.

“To understand how the solar wind interacts with the Earth, or to generate clean energy from nuclear fusion, one has to understand how plasma — a soup of electrons and ions — behaves in a magnetic field,” said Rice Dean of Natural Sciences Tom Killian, the corresponding author of a published study about the work in Physical Review Letters.

Using laser-cooled strontium, Killian and graduate students Grant Gorman and MacKenzie Warrens made a plasma about 1 degree above absolute zero, or approximately -272 degrees Celsius, and trapped it briefly with forces from surrounding magnets. It is the first time an ultracold plasma has been magnetically confined, and Killian, who’s studied ultracold plasmas for more than two decades, said it opens the door for studying plasmas in many settings.

“This provides a clean and controllable testbed for studying neutral plasmas in far more complex locations, like the sun’s atmosphere or white dwarf stars,” said Killian, a professor of physics and astronomy. “It’s really helpful to have the plasma so cold and to have these very clean laboratory systems. Starting off with a simple, small, well-controlled, well-understood system allows you to strip away some of the clutter and really isolate the phenomenon you want to see.”

Rice University graduate student Grant Gorman at work in Rice’s Ultracold Atoms and Plasmas Lab. (Photo by Jeff Fitlow/Rice University)

That’s important for study co-author Stephen Bradshaw, a Rice astrophysicist who specializes in studying plasma phenomena on the sun.

“Throughout the sun’s atomosphere, the (strong) magnetic field has the effect of altering everything relative to what you would expect without a magnetic field, but in very subtle and complicated ways that can really trip you up if you don’t have a really good understanding of it,” said Bradshaw, an associate professor of physics and astronomy.

Solar physicists rarely get a clear observation of specific features in the sun’s atmosphere because part of the atmosphere lies between the camera and those features, and unrelated phenomena in the intervening atmosphere obscures what they’d like to observe.

“Unfortunately, because of this line-of-sight problem, observational measurements of plasma properties are associated with quite a lot of uncertainty,” Bradshaw said. “But as we improve our understanding of the phenomena, and crucially, use the laboratory results to test and calibrate our numerical models, then hopefully we can reduce the uncertainty in these measurements.”

Plasma is one of four fundamental states of matter, but unlike solids, liquids and gases, plasmas aren’t generally part of everyday life because they tend to occur in very hot places like the sun, a lightning bolt or candle flame. Like those hot plasmas, Killian’s plasmas are soups of electrons and ions, but they’re made cold by laser-cooling, a technique developed a quarter century ago to trap and slow matter with light.

Killian said the quadrupole magnetic setup that was used to trap the plasma is a standard part of the ultracold setup that his lab and others use to make ultracold plasmas. But finding out how to trap plasma with the magnets was a thorny problem because the magnetic field plays havoc with the optical system that physicists use to look at ultracold plasmas.

“Our diagnostic is laser-induced fluorescence, where we shine a laser beam onto the ions in our plasma, and if the frequency of the beam is just right, the ions will scatter photons very effectively,” he said. “You can take a picture of them and see where the ions are, and you can even measure their velocity by looking at the Doppler shift, just like using a radar gun to see how fast a car is moving. But the magnetic fields actually shift around the resonant frequencies, and we have to disentangle the shifts in the spectrum that are coming from the magnetic field from the Doppler shifts we’re interested in observing.”

Rice University graduate student MacKenzie Warrens adjusts a laser-cooling experiment in Rice’s Ultracold Atoms and Plasmas Lab. © Photo by Jeff Fitlow/Rice University

That complicates experiments significantly, and to make matters even more complicated, the magnetic fields change dramatically throughout the plasma.

“So we have to deal with not just a magnetic field, but a magnetic field that’s varying in space, in a reasonably complicated way, in order to understand the data and figure out what’s happening in the plasma,” Killian said. “We spent a year just trying to figure out what we were seeing once we got the data.”

The plasma behavior in the experiments is also made more complex by the magnetic field. Which is precisely why the trapping technique could be so useful.

“There is a lot of complexity as our plasma expands across these field lines and starts to feel the forces and get trapped,” Killian said. “This is a really common phenomenon, but it’s very complicated and something we really need to understand.”

One example from nature is the solar wind, streams of high-energy plasma from the sun that cause the aurora borealis, or northern lights. When plasma from the solar wind strikes Earth, it interacts with our planet’s magnetic field, and the details of those interactions are still unclear. Another example is fusion energy research, where physicists and engineers hope to recreate the conditions inside the sun to create a vast supply of clean energy.

Killian said the quadrupole magnetic setup that he, Gorman and Warrens used to bottle their ultracold plasmas is similar to designs that fusion energy researchers developed in the 1960s. The plasma for fusion needs to be about 150 million degrees Celsius, and magnetically containing it is a challenge, Bradshaw said, in part because of unanswered questions about how the plasma and magnetic fields interact and influence one another.

“One of the major problems is keeping the magnetic field stable enough for long enough to actually contain the reaction,” Bradshaw said. “As soon as there’s a small sort of perturbation in the magnetic field, it grows and ‘pfft,’ the nuclear reaction is ruined.

“For it to work well, you have to keep things really, really stable,” he said. “And there again, looking at things in a really nice, pristine laboratory plasma could help us better understand how particles interact with the field.”

The research was supported by the Air Force Office of Scientific Research (FA9550-17-1-0391) and the National Science Foundation Graduate Research Fellowship Program (1842494).

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Featured image: Images produced by laser-induced fluorescence show how a rapidly expanding cloud of ultracold plasma (yellow and gold) behaves when confined by a quadrupole magnet. Ultracold plasmas are created in the center of the chamber (left) and expand rapidly, typically dissipating in a few thousandths of a second. Using strong magnetic fields (pink), Rice University physicists trapped and held ultracold plasmas for several hundredths of a second. By studying how plasmas interact with strong magnetic fields in such experiments, researchers hope to answer research questions related to clean fusion energy, solar physics, space weather and more. (Image courtesy of T. Killian/Rice University)


Reference: G. M. Gorman, M. K. Warrens, S. J. Bradshaw, and T. C. Killian, “Magnetic Confinement of an Ultracold Neutral Plasma”, Phys. Rev. Lett. 126, 085002 – Published 25 February 2021. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.085002


Provided by Rice University

Study Reveals Platinum’s Role in Clean Fuel Conversion (Chemistry)

Identifying specific platinum atoms activated in a water gas shift reaction catalyst could guide the design of less costly efficient catalysts

Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory, Stony Brook University (SBU), and other collaborating institutions have uncovered dynamic, atomic-level details of how an important platinum-based catalyst works in the water gas shift reaction. This reaction transforms carbon monoxide (CO) and water (H2O) into carbon dioxide (CO2) and hydrogen gas (H2)—an important step in producing and purifying hydrogen for multiple applications, including use as a clean fuel in fuel-cell vehicles, and in the production of hydrocarbons.

But because platinum is rare and expensive, scientists have been seeking ways to create catalysts that use less of this precious metal. Understanding exactly what the platinum does is an essential step.

The new study, published in Nature Communications, identifies the atoms involved in the catalyst’s active site, resolving earlier conflicting reports about how the catalyst operates. The experiments provide definitive evidence that only certain platinum atoms play an important role in the chemical conversion.

“Part of the challenge is that the catalyst itself has a complex structure,” explained lead author Yuanyuan Li, a research scientist at SBU’s Materials Science and Chemical Engineering Department who has a guest appointment in Brookhaven Lab’s Chemistry Division and works under the guidance of Brookhaven/SBU joint appointee Anatoly Frenkel.

Scientists studying a water gas shift reaction catalyst made of platinum atoms (red and blue) on a cerium oxide (CeOx) surface discovered that only some platinum atoms around the periphery of the nanoparticle (shiny dark red) get activated to take part in the reaction. These activated platinum atoms transfer oxygen from OH groups (originally from water molecules) to carbon monoxide (CO), transforming it to CO2, leaving the H to combine with atomic hydrogen to form H2. Understanding these dynamics may help scientists design catalysts that require fewer platinum atoms.

“The catalyst is made of platinum nanoparticles (clumps of platinum atoms) sitting on a cerium oxide (ceria) surface. Some of those platinum atoms are on the surface of the nanoparticle, some are in the core; some are at the interface with ceria, and some of those are at the perimeter—the outside edges—of that interface,” Li said. “Those positions and how you put the particles on the surface may influence which atoms will interact with the support or with gas molecules, because some are exposed and some are not.”

Earlier experiments had produced conflicting results about whether the reactions occur on the nanoparticles or at single isolated platinum atoms, and whether the active sites are positively or negatively charged or neutral. Details of how the ceria support interacts with the platinum to activate it for catalytic activity were also unclear.

“We wanted to address these questions,” said Li. “To identify the active site and determine what is really happening at this site, it is better if we can investigate this type of catalyst at the atomic level,” she noted.

The team, which included scientists from Brookhaven’s Center for Functional Nanomaterials (CFN) and other institutions throughout the U.S. and in Sweden, used a range of techniques to do just that. They studied the catalyst under reaction conditions and, unexpectedly, captured a peculiar effect that occurred when the catalysts reached their active state in reaction conditions.

Study co-authors Anatoly Frenkel, Lihua Zhang, Sanjaya Senanayake, and Yuanyuan Li outside the Chemistry building at Brookhaven National Laboratory.

“The platinum atoms at the perimeter of the particles were ‘dancing’ in and out of focus in an electron microscopy experiment carried out by our collaborators, while the rest of the atoms were much more stable,” Frenkel said. Such dynamic behavior was not observed when some of the reactants (CO or water) were removed from the stream of reacting molecules.

“We found that only the platinum atoms at the perimeter of the interface between the nanoparticles and ceria support provide the catalytic activity,” Li said. “The dynamic properties at these perimeter sites allow the CO to get oxygen from the water so it can become CO2, and the water (H2O) loses oxygen to become hydrogen.”

Now that the scientists know which platinum atoms play an active role in the catalyst, they may be able to design catalysts that contain only those active platinum atoms.

“We might assume that all the surface platinum atoms are working, but they are not,” Li said. “We don’t need them all, just the active ones. This could help us make the catalyst less expensive by removing the atoms that are not involved in the reaction. We believe that this mechanism can be generalized to other catalytic systems and reactions,” she added.

Experimental details

Electron microscopy “snapshots” at the CFN and at the National Institute of Standards and Technology revealed the dynamic nature of the perimeter platinum atoms. “In some images, the perimeter site is there, you can see it, but in some images it is not there. This is evidence that these atoms are very dynamic, with high mobility,” Li said.

Infrared (IR) spectroscopy studies in Brookhaven’s Chemistry Division revealed that the appearance of the perimeter sites coincided with “oxygen vacancies”—a kind of defect in the cerium oxide surface. These studies also showed that CO tended to migrate across the platinum nanoparticle surface toward the perimeter atoms, and that hydroxy (OH) groups lingered on the ceria support near the perimeter platinum atoms.

“So it seems like the perimeter platinum atoms bring the two reactants, CO and OH (from the water molecules) together,” Li said.

X-ray photoelectron spectroscopy studies in Chemistry revealed that perimeter platinum atoms also became activated—changed from a nonmetallic to a metallic state that could capture oxygen atoms from the OH groups and deliver that oxygen to CO. “This really shows that these activated perimeter platinum sites enable the reaction to take place,” Li said.

A final set of experiments—x-ray absorption spectroscopy studies conducted at the Advanced Photon Source (APS) at DOE’s Argonne National Laboratory—showed the dynamic structural changes of the catalyst.

“We see the structure is changing under reaction conditions,” Li said.

Those studies also revealed an unusually long bond between the platinum atoms and the oxygen on the ceria support, suggesting that something invisible to the x-rays was occupying space between the two.

“We think there is some atomic hydrogen between the nanoparticle and the support. X-rays can’t see light atoms like hydrogen. Under reaction conditions, those atomic hydrogens will recombine to form H2,” she added.

The structural features and details of how the dynamic changes are connected to reactivity will help the scientists understand the working mechanism of this particular catalyst and potentially design ones with better activity at lower cost. The same techniques can also be applied to studies of other catalysts.

Brookhaven Lab’s role in this work was funded by the DOE Office of Science (BES). CFN and APS are DOE Office of Science user facilities. Additional collaborating institutions include the University of Illinois, Arizona State University, the University of Maryland, and the KTH Royal Institute of Technology in Stockholm.

Featured image: Lead author Yuanyuan Li, a research scientist at Stony Brook University’s Materials Science and Chemical Engineering Department who has a guest appointment in Brookhaven Lab’s Chemistry Division, performs an analysis on a sample using an infrared spectrometer.


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Provided by Brookeshaven National Laboratory