At small scales, tug-of-war between electrons can lead to magnetism under surprising circumstances

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A quantum dots two mobile electrons will actually influence the manganese spins differently. That's because while one mobile electron prefers to stay in the middle of the quantum dot, the other prefers to locate further toward the edges. As a result, manganese atoms in different parts of the quantum dot receive different messages about which way to align their spins. In the "tug-of-war" that ensues, the mobile electron that interacts more intensely with the manganese atoms "wins," aligning more spins and causing the quantum dot, as a whole, to be magnetic. Credit: University at Buffalo

(PhysOrg.com) -- At the smallest scales, magnetism may not work quite the way scientists expected, according to a recent paper in Physical Review Letters by Rafal Oszwaldowski and Igor Zutic of the University at Buffalo and Andre Petukhov of the South Dakota School of Mines and Technology.

The three physicists have proposed that it would be possible to create a quantum dot -- a kind of nanoparticle -- that is magnetic under surprising circumstances.

Magnetism is determined by a property all electrons possess: spin. Individual spins are akin to tiny bar magnets, which have north and south poles. Electrons can have an "up" or "down" spin, and a material is magnetic when most of its electrons have the same spin.

Mobile electrons can act as "magnetic messengers," using their own spin to align the spins of nearby atoms. If two mobile electrons with opposite spins are in an area, conventional wisdom says that their influences should cancel out, leaving a material without magnetic properties.

But the UB-South Dakota team has proposed that at very small scales, magnetism may be more nuanced than that. It is possible, the physicists say, to observe a peculiar form of magnetism in quantum dots whose mobile electrons have opposing spins.

In their Physical Review Letters article, the researchers describe a theoretical scenario involving a quantum dot that contains two free-floating, mobile electrons with opposite spins, along with manganese atoms fixed at precise locations within the quantum dot.

The quantum dot's mobile electrons act as "magnetic messengers," using their own spins to align the spins of nearby manganese atoms.

Under these circumstances, conventional thinking would predict a stalemate: Each mobile electron exerts an equal influence over spins of manganese atoms, so neither is able to "win."

Through complex calculations, however, Oszwaldowski, ?utic and Petukhov show that the quantum dot's two mobile electrons will actually influence the manganese spins differently.

That's because while one mobile electron prefers to stay in the middle of the quantum dot, the other prefers to locate further toward the edges. As a result, manganese atoms in different parts of the quantum dot receive different messages about which way to align their spins.

In the "tug-of-war" that ensues, the mobile electron that interacts more intensely with the manganese atoms "wins," aligning more spins and causing the quantum dot, as a whole, to be magnetic. (For a visual representation of this tug-of-war, see Figure 1.)

This prediction, if proven, could "completely alter the basic notions that we have about magnetic interactions," ?utic says.

"When you have two mobile electrons with opposite spins, the assumption is that there is a nice balance of up and down spins, and therefore, there is no magnetic message, or nothing that could be sent to align nearby manganese spins," he says. "But what we are saying is that it is actually a tug of war. The building blocks of magnetism are still mysterious and hold many surprises."

Scientists including UB Professor Athos Petrou, UB College of Arts and Sciences Dean Bruce McCombe and UB Vice President for Research Alexander Cartwright have demonstrated experimentally that in a quantum dot with just one mobile electron, the mobile electron will act as a magnetic messenger, robustly aligning the spins of adjacent manganese atoms.

Now, Petrou and his collaborators are interested in taking their research a step further and testing the tug-of-war prediction for two-electron quantum dots, ?utic says.

?utic adds that learning more about magnetism is important as society continues to find novel uses for magnets, which could advance technologies including lasers, medical imaging devices and, importantly, computers.

He explains the promise of magnet- or spin-based computing technology -- called "spintronics" -- by contrasting it with conventional electronics. Modern, electronic gadgets record and read data as a blueprint of ones and zeros that are represented, in circuits, by the presence or absence of electrons. Processing information requires moving electrons, which consumes energy and produces heat.

Spintronic gadgets, in contrast, store and process data by exploiting electrons' "up" and "down" spins, which can stand for the ones and zeros devices read. Future energy-saving improvements in data processing could include devices that process information by "flipping" spin instead of shuttling electrons around.

Studying how magnetism works on a small scale is particularly important, ?utic says, because "we would like to pack more information into less space."

And, of course, unraveling the mysteries of magnetism is satisfying for other, simpler reasons.

"Magnets have been fascinating people for thousands of years," ?utic says. "Some of this fascination was not always related to how you can make a better compass or a better computer hard drive. It was just peculiar that you have materials that attract one another, and you wanted to know why."

Provided by University at Buffalo (news : web)

At small scales, tug-of-war between electrons can lead to magnetism under surprising circumstances

Enlarge

A quantum dots two mobile electrons will actually influence the manganese spins differently. That's because while one mobile electron prefers to stay in the middle of the quantum dot, the other prefers to locate further toward the edges. As a result, manganese atoms in different parts of the quantum dot receive different messages about which way to align their spins. In the "tug-of-war" that ensues, the mobile electron that interacts more intensely with the manganese atoms "wins," aligning more spins and causing the quantum dot, as a whole, to be magnetic. Credit: University at Buffalo

(PhysOrg.com) -- At the smallest scales, magnetism may not work quite the way scientists expected, according to a recent paper in Physical Review Letters by Rafal Oszwaldowski and Igor Zutic of the University at Buffalo and Andre Petukhov of the South Dakota School of Mines and Technology.

The three physicists have proposed that it would be possible to create a quantum dot -- a kind of nanoparticle -- that is magnetic under surprising circumstances.

Magnetism is determined by a property all electrons possess: spin. Individual spins are akin to tiny bar magnets, which have north and south poles. Electrons can have an "up" or "down" spin, and a material is magnetic when most of its electrons have the same spin.

Mobile electrons can act as "magnetic messengers," using their own spin to align the spins of nearby atoms. If two mobile electrons with opposite spins are in an area, conventional wisdom says that their influences should cancel out, leaving a material without magnetic properties.

But the UB-South Dakota team has proposed that at very small scales, magnetism may be more nuanced than that. It is possible, the physicists say, to observe a peculiar form of magnetism in quantum dots whose mobile electrons have opposing spins.

In their Physical Review Letters article, the researchers describe a theoretical scenario involving a quantum dot that contains two free-floating, mobile electrons with opposite spins, along with manganese atoms fixed at precise locations within the quantum dot.

The quantum dot's mobile electrons act as "magnetic messengers," using their own spins to align the spins of nearby manganese atoms.

Under these circumstances, conventional thinking would predict a stalemate: Each mobile electron exerts an equal influence over spins of manganese atoms, so neither is able to "win."

Through complex calculations, however, Oszwaldowski, ?utic and Petukhov show that the quantum dot's two mobile electrons will actually influence the manganese spins differently.

That's because while one mobile electron prefers to stay in the middle of the quantum dot, the other prefers to locate further toward the edges. As a result, manganese atoms in different parts of the quantum dot receive different messages about which way to align their spins.

In the "tug-of-war" that ensues, the mobile electron that interacts more intensely with the manganese atoms "wins," aligning more spins and causing the quantum dot, as a whole, to be magnetic. (For a visual representation of this tug-of-war, see Figure 1.)

This prediction, if proven, could "completely alter the basic notions that we have about magnetic interactions," ?utic says.

"When you have two mobile electrons with opposite spins, the assumption is that there is a nice balance of up and down spins, and therefore, there is no magnetic message, or nothing that could be sent to align nearby manganese spins," he says. "But what we are saying is that it is actually a tug of war. The building blocks of magnetism are still mysterious and hold many surprises."

Scientists including UB Professor Athos Petrou, UB College of Arts and Sciences Dean Bruce McCombe and UB Vice President for Research Alexander Cartwright have demonstrated experimentally that in a quantum dot with just one mobile electron, the mobile electron will act as a magnetic messenger, robustly aligning the spins of adjacent manganese atoms.

Now, Petrou and his collaborators are interested in taking their research a step further and testing the tug-of-war prediction for two-electron quantum dots, ?utic says.

?utic adds that learning more about magnetism is important as society continues to find novel uses for magnets, which could advance technologies including lasers, medical imaging devices and, importantly, computers.

He explains the promise of magnet- or spin-based computing technology -- called "spintronics" -- by contrasting it with conventional electronics. Modern, electronic gadgets record and read data as a blueprint of ones and zeros that are represented, in circuits, by the presence or absence of electrons. Processing information requires moving electrons, which consumes energy and produces heat.

Spintronic gadgets, in contrast, store and process data by exploiting electrons' "up" and "down" spins, which can stand for the ones and zeros devices read. Future energy-saving improvements in data processing could include devices that process information by "flipping" spin instead of shuttling electrons around.

Studying how magnetism works on a small scale is particularly important, ?utic says, because "we would like to pack more information into less space."

And, of course, unraveling the mysteries of magnetism is satisfying for other, simpler reasons.

"Magnets have been fascinating people for thousands of years," ?utic says. "Some of this fascination was not always related to how you can make a better compass or a better computer hard drive. It was just peculiar that you have materials that attract one another, and you wanted to know why."

Provided by University at Buffalo (news : web)

Nanometer-scale layers between materials have both solid and liquid characteristics

 

Gold crystals equilibrated on a polycrystalline sapphire(aluminum oxide) surface.

Researchers at the Technion have discovered the nature of nanometer-thick layers between different materials and found that they have both solid and liquid properties. By doing so, the researchers made a crucial addition to Gibbs' theory which describes the fundamental aspects of the thermodynamics of interfaces.

The recently published paper in Science experimentally demonstrates that the formation of a very thin layer (of the order of one nanometer in thickness) at interfaces reduces the interface energy, and promotes adhesion and interface stability.  The thin layer is not a conventional state of matter, in that it is neither a solid nor a liquid, but rather something in between.
 
The results could enable scientists to improve the resilience of the bond between ceramic materials and metals, two types of materials that ?do not like? to come into contact.  The many real-world applications include cutting tools for metal-working; composites for brake pads; the joins between metal conducting wires and chips in computers; and the application of protective ceramic coatings on jet engine blades.

?Until now, no one had been able to understand why these thin layers exist, or if they were a temporary or an equilibrium state,? explains Prof. Wayne D. Kaplan, Dean of the Department of Materials Engineering at the Technion.  ?While their existence at interfaces between ceramic crystals and on the surface of ice was known, there was an ongoing debate about the cause of this phenomenon and its properties."
 
Through a long series of experiments, Dr. Mor Baram proved that a thin layer exists at the interface between metals and ceramic materials, which reduces the interface energy.  The research was Dr. Baram?s doctoral work, and was carried out under the supervision of Prof. Kaplan in cooperation with Dr. Dominique Chatain of CNRS in France.

?This phenomenon enables us to ice-skate, reduces the mechanical properties of ceramic materials at high temperatures, affects the morphology of crystals in modern polycrystalline engineering materials, and contributes to the stability of modern micro-electronic devices,? says Prof. Kaplan.
 
The team conducted novel experiments at the Technion using the new ?Titan? electron microscope and a focused ion beam (FIB). This included plating sapphire with a thin (0.6 micron thick) film of gold (for comparison, a single strand of hair is 80-100 microns thick), and heating the samples until they reached equilibrium (i.e. until the gold film broke-up into billions of tiny gold crystals on the sapphire).  The researchers also included a source of elements on the sapphire known to play a role in forming the layer between different materials (in this case, silicon and calcium). As the samples reached equilibrium, the calcium and silicon moved to the interface between the gold and sapphire, and a thin (0.0012 microns, or 1.2 nanometers thick) layer was created.
 
The researchers were then able to successfully measure the energy stored between the gold and sapphire in the presence of the thin layer.  By doing so, they proved that its presence decreases the energy of the interface, and therefore improves its stability. 

'Molecular torch' between carbon nanotubes emits electroluminescence

'Molecular torch' between carbon nanotubes emits electroluminescence

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For the first time, scientists observed electroluminescence from a molecule lodged in a gap between carbon nanotubes. Image credit: Karlsruhe Institute of Technology.

(PhysOrg.com) -- A single molecule bridging a "broken" single-walled carbon nanotube (CNT) is barely visible through a powerful scanning electron microscope, but the precisely assembled system can act as a functional solid-state electronics device. These CNT-molecule-CNT junctions have been developed only in the past few years, and measuring their optical characteristics has been a difficult task. In a new study, scientists have observed for the first time that the molecule between the nanotubes can emit light due to an electric current passing through it, a phenomenon called electroluminescence.

In their study, scientists Christoph W. Marquardt from the Karlsruhe Institute of Technology in Karlsruhe, Germany, and coauthors from the University of Basel in Basel, Switzerland; the Poznan University of Economics in Poznan, Poland; and the DFG Center for Functional Nanostructures in Karlsruhe, Germany, have published their study in a recent issue of Nature Nanotechnology.

As the scientists explained, the carbon nanotubes contain a pair of metallic electrodes. Through electrical breakdown, the scientists could create a gap of just a few nanometers between the electrodes. The gap’s position and size of less than 10 nm had to be controlled with nanoscale precision in order to allow for a current. The researchers then assembled a molecule having a 6-nm-long rod-like structure and electrical characteristics that enabled it to be electrostatically trapped in the gap, completing the “circuit” between the electrodes. They predicted that the electrode gap could host no more than one to three of these molecules.

When applying a voltage to the electrodes, the scientists observed bright spots of electroluminescence, and they could control the electroluminescence by switching the voltage on and off. The scientists could determine that the light was coming from the molecule between the electrodes by overlaying an image captured previously with external illumination. The researchers observed a small bright spot between the electrodes in 6 of 20 CNT-molecule-CNT devices. They calculated that, on average, one photon was emitted per 1 billion electrons.

"This is the first time that electroluminescence has been observed from CNT-molecule-CNT junctions," coauthor Ralph Krupke from the Karlsruhe of Technology and DFG Center for Functional Nanostructures told PhysOrg.com. He noted that, in 2004, Dong, et al., observed electroluminescence from a molecule in a scanning tunneling microscope setup.

“In our view, the greatest significance is that we succeeded in forming a rigid solid-state device by integrating a bottom-up structure, the molecule, into a top-down structure, the CNT gap,” he said. “Thereby we had to control the critical dimensions and the molecule had to be tailored to enable light emission under voltage bias. Furthermore, from a molecular electronics point of view, it is the first time that the presence of the molecule in the gap is confirmed by its optical signature.”

Currently, the scientists are fabricating variations of this device by using different molecules that emit light at different wavelengths. The results of the study show that carbon nanotubes could have a variety of applications in molecular electronics.

"Molecular electronics aims at the fundamental understanding of charge transport through molecules and is motivated by the vision of molecular circuits to enable miniscule, powerful and energy-efficient computers,” Krupke said. “Our result is important for fundamental science but it also adds to the molecular electronics vision an optoelectronic component, i.e., the development of optoelectronic components on the basis of single molecules."