Physicists isolate bound states in graphene-superconductor junctions

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Led by University of Illinois physics professor Nadya Mason, the group published its findings in the journalNature Physics.

When a current is applied to a normal conductor, such as metal or graphene, it flows through the material as a stream of single electrons. By contrast, electrons travel in pairs in superconductors. Yet when a normal material is sandwiched between superconductors, the normal metal can carry the supercurrent.

Normal metals can adopt superconducting capacity because the pairedelectronsfrom the superconductor are translated to special electron-hole pairs in the normal metal, called Andreev bound states (ABS).

"If you have two superconductors with a normal metal between, you can actually transport thesuperconductivityacross the normal material via these bound states, even though the normal material doesn't have the electron pairing that thesuperconductorsdo,"Mason said.

ABS are extremely difficult to measure or to observe directly. Researchers can measure conduction and overall magnitude of a current, but have not been able to study individual ABS to understand the fundamental physics contributing to these unique states.

Mason's group developed a method of isolating individual ABS by connecting superconducting probes to tiny, nanoscale flakes of graphene called quantum dots. This confined the ABS to discrete energy levels within the quantum dot, allowing the researchers to measure the superconducting ABS individually and even to manipulate them.

"Before this, it wasn't really possible to understand the fundamentals of what is transporting the current,"Mason said."Watching an individual bound state allows you to change one parameter and see how one mode changes. You can really get at a systematic understanding. It also allows you to manipulate ABS to use them for different things that just couldn't be done before."

Superconductor junctions have been proposed for use as superconducting transistors or bits for quantum computers, called qubits. Greater understanding of ABS may enable other applications as well. In addition, it may be possible to use the superconducting graphenequantum dotsthemselves as solid-state qubits.

"This is a unique case where we found something that we couldn't have discovered without using all of these different elements– without thegraphene, or the superconductor, or the quantum dot, it wouldn't have worked. All of these are really necessary to see this unusual state,"Mason said.

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Plasmonic metamaterials: From microscopes to invisibility cloaks

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In a Perspectives piece in this week's issue of the journalScience, Caltech's Harry Atwater and Purdue University colleague Alexandra Boltasseva describe advances in a particular subtype of these materials—plasmonic metamaterials. They also describe two of the major limitations in the field: the loss of light or, rather, its absorption by metals such as silver and gold, which are contained in the metamaterial; and difficulties in precisely tuning the materials so they bend incoming light to the required index of refraction.

In their article, Atwater and Boltasseva suggest new approaches to overcoming these obstacles by replacing the silver and gold in the metamaterials with semiconductors made more metallic by the addition of metallic impurities, or by adding non-metallic elements to metals, making them less metallic. Examples of these"intermetallic materials"include aluminum oxides and titanium nitride.

Some of the newmetamaterials, the researchers say, are showing promise in uses involving near-infrared light, the range of the spectrum critical for telecommunications and fiber optics. Other materials—such as the negative-index metamaterial developed by Atwater and Caltech graduate student Stanley Burgos anddescribed in an April 2010Nature Materialsarticle—might even work with light in the visible range of the spectrum.

Future photonics technologies will revolve around new types of optical transistors, switches, and data processors, Atwater and Boltasseva note. Indeed, as they point out in the article's abstract,"these materials can be tailored for almost any application because of their extraordinary response to electromagnetic, acoustic, and thermal waves that transcends the properties of natural materials."

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How to tame hammering droplets

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MIT’s Kripa Varanasi, co-author of a report on the new findingpublished this weekin the journalPhysical Review Letters, says the phenomenon could help engineers design more durable condensing surfaces, which are used in desalination plants and steam-based power plants. Other co-authors include MIT mechanical-engineering graduate students Hyuk-Min Kwon and Adam Paxson, and associate professor Neelesh Patankar of Northwestern University.

Varanasi, the d’Arbeloff Assistant Professor of Mechanical Engineering, says the effect explains why blades used in power-plant turbines tend to degrade so rapidly and need to be replaced frequently, and could lead to the design of more durable turbines. Since about half of all electricity generated in the world comes from steam turbines— whether heated by coal, nuclear fuel, natural gas or petroleum— improving their longevity and efficiency could reduce the down time and increase the overall output for these plants, and thus help curb the world emissions of greenhouse gases.

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This video from a high-speed camera shows a droplet being deposited on a superhydrophobic surface, just before it separates from the dropper. At the moment of separation, ripples move down through the droplet, showing the deceleration caused by impact with the surface, which causes a brief burst of high pressure. Credit: Kripa Varanasi

There has been widespread interest in the development of superhydrophobic (water-repelling) surfaces, Varanasi says, which in some cases mimic textured surfaces found in nature, such as lotus leaves and the skin of geckos. But most research conducted so far on how such surfaces behave have been static tests: To see the way droplets of different sizes spread out on such surfaces (called wetting) or how they bead up to form larger droplets, the typical method is to add or subtract water slowly in a stationary droplet. But this is not a realistic simulation of how droplets react on surfaces, Varanasi says.

“In any real application, things are dynamic,” he says. And Varanasi’s research shows the dynamics of moving droplets hitting a surface are quite different from droplets formed in place.

Specifically, such droplets undergo a rapid internal deceleration that produces strong pressures— a small-scale version of the water-hammer effect. It is this tiny but intense burst of pressure that accounts for the pitting and erosion found on power-plant turbine blades, he says, which limits their useful lifetime.

“This is one of the biggest unsolved problems” in power-plant design, he says. In addition to damaging the blades, the formation and growth of water droplets mixed with the flow of steam saps much of the power, accounting for up to 30 percent of the system losses in such plants. Since some steam-based power plants, such as natural-gas combined-cycle plants, can already have efficiencies of up to 85 percent in converting the fuel’s energy to electricity, if these droplet losses could be eliminated it could provide almost a 5-percent boost in power.

“This is a new finding, indeed,” says David Quéré, director of research at the laboratory of physics and mechanics of heterogeneous materials at ESPCI, Paris. He explains that“Superhydrophobic materials, on which water can glide and roll in a unique fashion, have interesting properties, provided water stays at the tops of the decorations we find on them. (I like to call that the fakir effect, since water then sits at the tops of a bed of micro-nails.)”

This research, Quéré says, explains why droplets often fail to stay on top and instead get impaled on the“nails,” and so the new findings are“interesting in the context of superhydrophobic materials, because it helps to design materials able to resist this kind of detrimental effect.”

Small-scale texturing of surfaces can prevent the droplets from wetting the surfaces of turbine blades or other devices, but the spacing and sizes of the surface patterns need to be studied dynamically, using techniques such as those developed by Varanasi and his co-authors, he says. Regularly spaced bumps or pillars on the surface can produce a water-shedding effect, but only if the size and spacing of these features is just right. This research showed that there seems to be a critical scale of texturing that is effective, while sizes either larger or smaller than that fail to produce the water-repelling effect. The analysis developed by this team should make it possible to determine the most effective sizes and shapes of patterning for producing superhydrophic surfaces on turbine blades and other devices.

The work is related to Varanasi’s research on how to prevent ice formation on airplane wings, also using nano-texturing of surfaces, but the potential applications of this latest research are much broader. In addition to power-plant turbines, this could also affect the design of condensers in desalination plants, and even the design of inkjet printers, whose operation is based on depositingdropletsof ink on a surface.

This work was funded by the MIT Energy Initiative, the National Science Foundation, the Dupont-MIT Alliance, and the Initiative for Sustainability and Energy at Northwestern. MIT’s Edgerton Center also provided high-speed video equipment.

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This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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Nanotechnology benefits from volcanoes in the outer solar system

Neutron scattering has discovered that methanol crystals that may be found in outer solar system‘ice lavas’ have unusual expansion properties. The unexpected finding by a British planetary geologist usingneutronsat the Institut Laue-Langevin and the ISIS neutron source will interest developers of‘nano-switches’– single atom thick valves used in‘micro-electronics’ at the nano scale.

Dr Dominic Fortes, UCL (University College London) made the discovery whilst investigating the internal structure of icy moons, such asNeptune’s Triton, to explain the icy eruptions seen by passing space-craft. By studying the behaviour of methanol monohydrate, a known constituent of outer solar system ice, under conditions like those within the moons’ interiors Fortes hoped to understand its role in volcanism.

Fortes measured structural changes in methanolcrystalsover a range of temperatures and pressures. He found that when heated at room pressure they would expand enormously in one direction whilst shrinking in the other two dimensions. However when heated under an even pressure they expanded in two directions, whilst compressing in the third. This unexpected expansion (elongating and thinning) under uniform pressure is known as negative linear compressibility (NLC).

Whilst these results form the next step towards understanding outer solar system volcanic activity, Fortes’ discovery is of significant interest for material scientists developing nanotechnology. The predictable expansion of NLC materials in a particular direction under pressure makes them a good candidate for nano-switches where their shape-shifting properties can be used like a microscopic, pressure-controlled valve directing the flow of electricity.

NLC materials are extremely rare with only around 15 known examples. What causes this property is still relatively unknown. Scientists hope better understanding of the phenomenon can bring forward potential technological application.

“Currently the use of NLC materials in technologies such as nano-switches is purely theoretical and limited by our lack of understanding of the underlying physics”, says Prof. Reinhard Neder chairman of the ILL crystallographic committee who approved Dr Fortes beam-time at the world’s flagship centre for neutron science.“However, the simple structure ofmethanolmonohydrate gives us a good chance to understand the source of this property and how to look for it in other more commercially viable materials.”

“It was certainly unexpected,” explains Dr Fortes.“As a planetary geologist my focus is understanding the mechanisms behind volcanic eruptions in theouter solar system. If my results open doors for more applied science back on Earth, that’s a bonus.”

Professor Richard Wagner, Director at the Institut Laue Langevin added“This research is a good example of how even basic academic studies can have completely unpredictable benefits in other areas of science and technology. It’s because of discoveries like this that the ILL strives to maintain our delivery of world leading neutron science in both‘fundamental’ and‘applied’ fields.”

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Delving into manganite conductivity

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At ambient conditions, manganites have insulating properties, meaning they do not conduct electric charges. When pressure of about 340,000 atmospheres is applied, these compounds change from an insulating state to a metallic state, which easily conducts charges. Scientists have long debated about the trigger for this change inconductivity.

The research team's new evidence, to be published online byPhysical Review Letterson Friday, shows that for the manganite LaMnO₃, this insulator-to-metal transition is strongly linked to a phenomenon called the Jahn-Teller effect. This effect actually causes a unique distortion of the compound's structure. The team's measurements were carried out at the Geophysical Laboratory.

Counter to expectations, the Jahn-Teller distortion is observed until LaMnO3 is in a non-conductive insulating state. Therefore, it is reasonable to believe that the switch from insulator to metal occurs when the distortion is suppressed, settling a longstanding debate about the nature of manganite insulating state. The formation of inhomogeneous domains—some with and some without distortion—was also observed. This evidence suggests that the manganite becomes metallic when the breakdown of undistorted to distorted molecules hits a critical threshold in favor of the undistorted.

"Separation into domains may be a ubiquitous phenomenon at high pressure and opens up the possibility of inducing colossal magnetoresistance by applyingpressure"said Baldini, who was with Stanford at the time the research was conducted, but has now joined Carnegie as a research scientist.

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Discovery of mini 'water hammer' effect could lead to materials that water really hates

In piping systems, the water hammer occurs when fluid is forced to stop abruptly, causing huge pressure spikes that can rupture pipe walls. Now, for the first time, the researchers have observed this force on the scale of microns: such pressure spikes can move through a water droplet, causing it to be impaled on textured superhydrophobic surfaces, even when deposited gently.

This insight of how droplets get stuck on surfaces could lead to the design of more effective superhydrophobic, or highly water-repellant, surfaces for condensers in desalination and steam power plants, de-icing for aircraft engines andwind turbine blades, low-drag surfaces in pipes and even raincoats. In certain cases, improved surfaces could improveenergy efficiencyon many orders of magnitude. (About half of all electricity generated in the world comes from steam turbines.)

The research is published by the journalPhysical Review Letters.

"We want to designsurfacetextures that will cause the water to really hate those surfaces,"said Neelesh A. Patankar, associate professor of mechanical engineering at Northwestern's McCormick School of Engineering and Applied Science."Improving current hydrophobic materials could result in a 60 percent drag reduction in some applications, for example."

Patankar collaborated with Kripa K. Varanasi, the d'Arbeloff Assistant Professor of Mechanical Engineering at MIT. The two are co-corresponding authors of the paper. Patankar initiated this study in which he and Varanasi led the analytical work, and the experiments were conducted at MIT in Varanasi's lab. Other co-authors are MIT mechanical engineering graduate students Hyuk-Min Kwon and Adam Paxson.

In designing superhydrophobic surfaces, one goal is to produce surfaces much like the natural lotus leaf. Water droplets on these leaves bead up and roll off easily, taking any dirt with them. Contrary to what one might think, the surface of the leaves is rough, not smooth. The droplets sit on microscopic bumps, as if resting on a bed of nails.

"If a water droplet impales the grooves of this bumpy texture, it becomes stuck instead of rolling off,"Patankar said."Such transitions are well known for small static droplets. Our study shows that the impalement of water is very sensitive to the dynamic 'water hammer' effect, which was not expected."

To show this, the researchers imaged millimeter-scalewater dropletsgently deposited onto rough superhydrophobic surfaces. (The surfaces were made of silicon posts, with spacing from post edge to post edge ranging from 40 to 100 microns, depending on the experiment.) Since these drops were on the millimeter scale and being deposited gently, prior understanding was to assume that gravitational force is not strong enough to push the water into the roughness grooves. The Northwestern and MIT researchers are the first to show this is not true.

They observed that as a droplet settles down on the surface (due to the drop's own weight) there is a rapid deceleration in the drop that produces a brief burst of high pressure, sending a wave through the droplet. The droplet is consequently pinned on the rough surface. That's the powerful mini water hammer effect at work.

By understanding the underlying physics of this transition, the study reveals that there is actually a window of droplet sizes that avoid impalement. Although focused on drop deposition, this idea is quite general and applies to any scenario where the water velocity is changing on a short (less than a millisecond) time scale. This insight can lead to the design of more robust superhydrophobic surfaces that can resist water impalement even under the dynamic conditions typical in industrial setups.

"One way to reduce impalement is to design a surface texture that results in a surface that sustains extremely high pressures,"Patankar said."It is the length scale of the roughness that is important."To resist impalement, the height of a bump and the distance between bumps need to be just right. Hundreds of nanometer scale roughness can lead to robust surfaces.

"Our ultimate goal,"he added,"is the invention of textured surfaces such that a liquid in contact with it will, at least partially, vaporize next to the surface -- or sustain air pockets -- and self-lubricate. This is similar to how an ice skater glides on ice due to a cushion of thin lubricating liquid film between the skates and the ice. A critical step is to learn how to resist impalement of water on the roughness. Our work on water hammer-induced impalement is a crucial advance toward that goal of ultra-slippery vapor stabilizing surfaces."

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Voiding defects: New technique makes LED lighting more efficient

LED lighting relies on GaNthin filmsto create the diode structure that produces light. The new technique reduces the number of defects in those films by two to three orders of magnitude."This improves the quality of the material that emits light,"says Dr. Salah Bedair, a professor of electrical andcomputer engineeringat NC State and co-author, with NC Statematerials scienceprofessor Nadia El-Masry, of a paper describing the research."So, for a given input of electrical power, the output of light can be increased by a factor of two– which is very big."This is particularly true for lowelectrical powerinput and for LEDs emitting in the ultraviolet range.

The researchers started with a GaN film that was two microns, or two millionths of a meter, thick and embedded half of that thickness with large voids– empty spaces that were one to two microns long and 0.25 microns in diameter. The researchers found that defects in the film were drawn to the voids and became trapped– leaving the portions of the film above the voids with far fewer defects.

Defects are slight dislocations in the crystalline structure of the GaN films. These dislocations run through the material until they reach the surface. By placing voids in the film, the researchers effectively placed a"surface"in the middle of the material, preventing the defects from traveling through the rest of the film.

The voids make an impressive difference.

"Without voids, the GaN films have approximately 1010 defects per square centimeter,"Bedair says."With the voids, they have 107 defects. This technique would add an extra step to the manufacturing process for LEDs, but it would result in higher quality, more efficient LEDs."

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Cerium's unusual behaviour

When a solid is pressurized, the internal energy at some point produces a change in the atomic arrangement, called a phase transformation. Typically when this happens, the atomic arrangement changes from one geometry to another. However, that doesn't happen incerium.

When cerium metal is exposed to 75,000 atmospheres of pressure, theatomic arrangementof the crystal simply shrinks, is transformed into twocrystals. Dan Farber, Kevin Moore, and Chantel Aracne-Ruddle showed that the two crystals are the same structure (one a large cube and the other a small cube) and have the same spatial orientation (the large and small cubes face in the same direction). This means that the two phases have the samecrystal structure, but different volumes. This is entirely unique in phase transformations ofsolid materials.

A phase transformation with two crystals of the same structure, but different volumes is something akin to laying bricks that are the same shape, but where some of the bricks are large while some are small. Making a coherent wall with the two is difficult.

The team (which also included researchers from Université Pierre et Marie Curie, Place Jussieu and CEA, DAM, DIF, all in France) used a diamond anvil cell to create the intense pressures on the cerium crystal. They also applied the X-ray diffraction method to determine the high pressure variation of the cerium structure and volume.

"Our data clearly show that the transformation mechanism can be described on the basis of crystallographic and thermodynamic arguments, showing a fair agreement with an isomorphic scenario and the existence of a critical point,"Moore said."The equation of state of cerium near the critical temperature is determined experimentally and for the first time is shown to be well understood in the framework of the scaling theory of the liquid-gas transition of classical systems. This conclusion represents an important step forward in achieving a reliable and unambiguous picture on the mechanism of phase transformation in cerium, an element archetypical of the localization-delocalization phenomenon encountered in f-electron systems, such as plutonium."

The research is scheduled to appear in a future edition ofPhysical Review Letters.

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Scientists customize a magnet's performance by strategically replacing key atoms

It's a process similar to what bioengineers employ when they add and delete genes to create synthetic organisms, and it was the focus of a group of researchers at the U.S. Department of Energy's Ames Laboratory, when they replaced key atoms in a gadolinium-germaniummagneticcompound with lutetium and lanthanum atoms.

The group was led by Vitalij Pecharsky, Ames Lab senior scientist and Distinguished Professor of Materials Science and Engineering at Iowa State University, and included his Lab colleagues, Karl Gschneidner Jr., Ames Lab senior metallurgist and Distinguished Professor of MS&E at ISU, and Gordon Miller, Ames Lab senior scientist and ISU professor of chemistry, along with assistant scientists Yaroslav Mudryk and Durga Paudyal. Also participating was Sumohan Misra, research associate at the DOE's SLAC National Accelerator in Menlo Park, Calif., formerly a Ph.D. student of Miller's.

Creating materials by design is no easy task, especially in the case of the complex gadolinium-germanium– Gd₅Ge₄– compound. Making things even more difficult, the compound's structure is highly symmetrical, which is common in intermetallics, so predicting which atoms are key to changing the material's characteristics would be difficult if not impossible unless some methodology was available to help in the selection process.

The Gd₅Ge₄compound's uniformity results from the fact that like nearly all metallic solids' atoms are arranged in a highly symmetrical crystal structure called a lattice. The more complex the material, the more intricate its lattice. And while the individual elements making up the lattice influence its characteristics, in some cases the location of specific atoms within the lattice can also have a profound influence on such things as its melting point, mechanical strength or– in the case of magnets– ferromagnetic properties.

"Individuality doesn't happen often among the atoms of metallic crystals,"Pecharsky explained,"But atoms still are able to 'cooperate' with one another in areas such as magnetic ordering and superconductivity."

By discovering these cooperative relationships, scientists can determine what will happen if they replace one or more of the atoms with those of another element, which is precisely what the team accomplished.

"We revealed that a single site occupied by the Gd atoms is much more active than all of the other Gd sites when it comes to bringing the ferromagnetic order in a complex crystal structure of gadolinium germanide,"Pecharsky said.

Pecharsky, Gschneidner and other researchers at the Ames Lab have spent years working with gadolinium alloys, because of the magnetic compound's use in the green, energy-saving field of magnetic refrigeration. However, that was not the main reason the Ames Lab researchers chose Gd₅Ge₄for their work.

As it turns out,"the metal exhibits an impressive combination of intriguing and potentially important properties, the researchers explained in their paper,"Controlling Magnetism of a Complex Metallic System Using Atomic Individualism,"published in the August 10, 2010Physical Review Letters."The extraordinary responsiveness to relatively weak external stimuli makes Gd₅Ge₄and related compounds a phenomenal playground for condensed matter science."

Besides being unusually responsive, Gd₅Ge₄was an ideal alloy for the work, because any changes in its magnetic properties resulting from the group's manipulations could be easily measured.

In 2008, Pecharsky and members of the same research team had already discovered that adding silicon to the alloy resulted in a magnetostructural transition that occurred without the application of a magnetic field. Chemical pressure alone was able to enhance the material's ferromagnetism.

That earlier finding led the team to experiment with other additions to the alloy. To ferret out precisely which atoms in the lattice were the best candidates for manipulation, the researchers called upon density functional theory, which is a means of studying the electronic structure of solids developed by Nobel Prize winning physicist Walter Kohn.

Kohn's methodology enabled the group to model the effects substituting small amounts of gadolinium atoms within the Gd₅Ge₄solid with the elements lutetium and lanthanum. With the modeled results in hand, the group's next step was to create the actual alloys in the lab, in order to test the accuracy of their computer-based predictions.

In fact, the complex fabrication process confirmed the modeling results. The researchers found if they replaced just a few gadolinium atoms with lutetium, the result would be a severe loss in the alloy's ferromagnetism. By contrast, substituting an equal number of lanthanumatomshad no significant effect; though substituting greater amounts of lanthanum might have a more pronounced impact on the resulting alloy's ferromagnetism, the researchers speculated.

Going forward, the lessons learned in this experiment could have important far-reaching implications, as materials scientists search for new exotic substances to be used in this and future generations of high-tech products."Knowing how to identify key atomic positions is similar to understanding the roles specific genes play in an organism's DNA sequence,"Pecharsky said."And that knowledge could ultimately lead to materials by design."

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