'BC5' material shows superhard, superconducting potential

"Our current study reveals a great possibility that BC5 may possess both superhard and superconducting properties that are beneficial to the creation of multifunctional devices under extreme conditions,"says Professor Yanming Ma, who led the research team at Jilin University.

At the heart of their study is the proposal that the synthesized BC5 adopts the diamond-{100} structure with special symmetry. Explains Ma, the BC5 structure has atomic packing of the form ABCABC… along the {100} crystallographic direction of diamond. This makes the deep understanding of this superhard and superconducting species possible. Ma believes that the outstanding mechanical andelectrical propertiesof BC5 can be adapted to design new superconducting nano-electromechanical systems and high-pressure devices.

Quan Li, the study's first author, expects their findings to stimulate further research into other B-C-N compounds with superhard and superconducting properties.

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Scientists Create Nano-Patterned Superconducting Thin Films

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"Such superconducting nanowires and nano-loops might eventually be useful for new electronic devices— that is the long-term vision,"said Brookhaven Lab physicist Ivan Bozovic, who synthesized the superconducting films. He and his collaborators describe the research inNature Nanotechnology, published online June 13, 2010.

It has been a long-standing dream to fabricate superconducting nano-scale wires for faster, more powerful electronics. However, this has turned out to be very difficult if not impossible with conventionalsuperconductorsbecause the minimal size for the sample to be superconducting— known as the coherence length — is large. For example, in the case of niobium, the most widely used superconductor, it is about 40 nanometers. Very thin nano-wires made of such materials wouldn't act as superconductors.

However, in layered copper-oxide superconductors, the coherence length is much smaller— about one or two nanometers within the copper-oxide plane, and as small as a tenth of a nanometer out-of-plane. The fact that these materials operate at warmer temperatures, reducing the need for costly cooling, makes them even more attractive for real-world applications.

To see if they could achieve superconductivity in a thin film material etched to form a pattern of"wires"— much like the circuits etched intocomputer chips— the Brookhaven team started by using a precision technique for making superconductingthin filmsatomic layerby layer. They used molecular beam epitaxy to build a material with alternating layers of copper-oxide and lanthanum and strontium. Bozovic's team had previously used this techniqueto produce thin films that retain superconductivity within a single copper-oxide layer.

Then the team at Bar-Ilan used electron-beam lithography to"etch"a pattern of thousands of loops into the surface of the material. The thickness, or diameter, of the"nanowires"forming the sides of these loops was mere 25 nanometers, while the lengths ranged from 150 to 500 nanometers. Measurements ofelectrical resistanceof the patterned arrays showed that they were indeed superconducting when cooled below about 30 K.

When the scientists applied an externalmagnetic fieldperpendicular to the loops, they found that the loop resistance did not keep increasing steadily with the field strength, but rather changed up and down in an oscillatory manner.

"These oscillations in resistance have a large amplitude, and their frequency corresponds to discrete units (quanta) of magnetic flux— the measure of the strength of the magnetic field piercing the loops,"Bozovic said."A material with such a discrete, switchable form of magneto-resistance— especially from the superconducting to the non-superconducting state — could be extremely useful for engineering new devices."

The observed frequency of the oscillations in resistance may also have implications for understanding the mechanism by which copper-oxide materials become superconductors in the first place. The current findings seem to rule out certain theoretical models that propose that an ordered alignment of charge carriers known as"stripes"is essential to superconductivity in copper-oxide compounds. A better understanding of the mechanism of superconductivity could lead to even more advances in designing new materials for practical applications.

The Brookhaven scientists' role in this research was supported by DOE's Office of Science. The work was also funded by the German Research Foundation through a German-Israeli cooperative agreement, and by a scholarship granted by the Israel Ministry of Science.

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Zeroing in on quantum effects: New materials yield clues about high-temperature superconductors

In new research appearing online today in the journalPhysical Review Letters, the Rice University-led team offers new evidence about the quantum features of the latest class of high-temperature superconductors, a family of iron-based compounds called"pnictides"(pronounced: NICK-tides).

"In correlated electron systems like the pnictides and their parent compounds, the electrons are caught in a competition between forces,"said Rice physicist Qimiao Si, a co-author of the study."On the one hand, they are compelled to move around, and on the other, they are forced to arrange themselves in a particular way because of their desire to repel one another. In this study, we varied the ratio between these competing forces in an effort to find the tipping point where one takes over from the other."

The aim of the research is to better understand the processes that lead to high-temperature superconductivity. If better understood and developed, high-temperature superconductors could revolutionize electric generators, MRI scanners, high-speed trains and other devices. In today's wiring, electricity is lost due to resistance and heating. This happens because electrons bump and ricochet from atom to atom as they pass down wires, and they lose a bit of energy in the form of heat each time they bounce around.

Almost a century ago, physicists discovered materials that could conduct electrons without losing energy to resistance. These"superconductors"had to be very cold, and it took physicists nearly 50 years to come up with an explanation for them: The electron-electron repulsion in these low-temperature superconductors was so weak that with the mediation of lattice vibrations, electrons overcame it, paired up and glided freely without the bumping and heating.

That explanation sufficed until 1986, when physicists discovered new materials that became superconductors at temperatures above 100 kelvins. These"high-temperature superconductors"were made of layers of copper alloys sandwiched between layers of nonconducting material that were laced, or"doped,"with trace amounts of material that could contribute a few extra electrons to the mix.

Physicists quickly realized their existing theories of superconductivity could not explain what was happening in the new materials. For one thing, the undoped versions of the compounds didn't conduct electricity at all. Their electrons -- due to their desire to repel one another -- tended to lock themselves a comfortable distance away from their neighbors. This locked pattern was dubbed"Mott localization,"which gives rise to an insulating state.

In 2008, the search for answers took another turn when a second class of high-temperature superconductors was discovered. Dubbed the pnictides, these new iron-based superconductors were also layered and also needed to be doped. But unlike their copper cousins, undoped pnictides were not Mott insulators.

"Mott localization doesn't occur in the undoped pnictides, but there is considerable evidence that the electrons in these materials are near the point where Mott localization occurs,"Si said."This proximity to Mott localization endows the system with strong quantum magnetic fluctuations, which we believe underlie thehigh-temperature superconductivityin the pnictides."

In all high-temperaturesuperconductors, the iron or copper atoms in the conducting layers form a grid-like, checkerboard pattern.

In work published earlier this year, Si and colleagues replaced arsenic atoms in one of the intervening layers of a pnictide with slightly smaller phosphorous atoms. This subtle change brought the iron atoms in the checkerboard a tad closer together, and that changed the amount of energy that was compellingelectronsto move between the iron atoms. The experiments confirmed a 2008 prediction of Si and, University of California, Los Angeles (UCLA) theorist Elihu Abrahams, who had predicted that boosting the electrons' kinetic energy would drive the pnictides further away from the Mott tipping point.

In the latest tests, Si and colleagues at Rice, China's Zhejiang University, UCLA, Los Alamos National Laboratory and the State University of New York at Buffalo (SUNY-Buffalo) sought to move the system in the other direction, toward Mott localization.

"We wanted to decrease the kinetic energy by expanding the distance between iron atoms in the lattice,"said study co-author Jian-Xin Zhu, a theorist from Los Alamos."Unfortunately, there is no pnictide material with those properties."

So the team's experimentalists, Rice's Emilia Morosan and Zhejiang's Minghu Fang, hit upon the idea of substituting a similarly patterned material called an iron oxychalcogenide (pronounced: OXY-cal-cah-ge-nyde). Like the iron pnictides, iron oxychalcogenides are layered materials. But compared with the pnictides, the distance between iron atoms is expanded in the oxychalcogenides.

Tests on the new materials confirmed the theoretical predictions of the team; a slight expansion of the iron lattice pushed the system into a Mott insulating state.

"Our results provide further evidence that the undoped iron pnictide parent compounds are on the verge of Mott localization,"Abrahams said.

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Superconductivity breakthrough could lead to more cost effective technologies

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Using the ISIS and Diamond Facilities at the Science and Technology Facilities Council's Rutherford Appleton Laboratory (RAL) and the European Synchrotron Radiation Facility (ESRF) in Grenoble, scientists have demonstrated how a new material made from metal atoms and buckyballs (tiny carbon-60 molecules shaped like a football) becomes a high temperature superconductor when it is squashed. The applied pressure shrinks the structure and overcomes the repulsion between the electrons, allowing them to pair up and travel through the material without resistance.

The Liverpool and Durham researchers made the new material supported by funding from the Engineering and Physical Sciences Research Council for a program investigating ways of creating higher temperature superconductors, to reduce some of the costs involved with keeping them at their optimum temperature and broaden their applications. AnMRI scannerfor example, contains person-sized superconductive magnet that needs to be kept inside a bath ofliquid heliumin order to regulate the superconductor's temperature at - 270 degrees Celcius. The ultimate aim is for a superconductor to operate at room temperature to eliminate the need for large and expensive cooling systems.

Dr Peter Baker, Muon instrument scientist at STFC's ISIS Facility6:"This research suggests that there is a universal trend in high temperature superconducting materials, which is a great step forward in understanding the fundamental nature ofsuperconductivity. Once we know how superconductivity works it will be easier to develop high temperature superconducting materials with specific properties, opening the door to new applications and ultra efficient energy transmission."

The advantage of investigating carbon-based superconducting materials is that they can be made with different structures that alter their properties; whereas the active components of other high high-temperature superconductors, such as copper oxide materials, are always arranged in one way. This structural flexibility offers a new way of looking at the mechanisms that drive high-temperature superconductivity, offering more insight into how to make higher temperaturesuperconductors. It has also established a universal pattern in the superconductivity of carbon-based materials which can now be used to help guide future theoretical models of superconductivity.

Matthew Rosseinsky, Professor of Inorganic Chemistry, University of Liverpool said;"We've shown for the first time how controlling the arrangement of molecules in ahigh temperature superconductorcontrols its properties. This is possible because we have found two arrangements of the same basic molecular unit which have both superconducting and magnetic properties."

Kosmas Prassides, Professor of Chemistry, Durham University said;"This is important in the context of high-temperature superconductivity as it allowed us to see at which point superconductivity emerges out of the competing insulating state irrespective of the exact atomic structure - something that has not been possible before for any other known material".

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New material may reveal inner workings of hi-temp superconductors

Copper-based high-temperaturesuperconductorsare created by taking a nonconducting material called a Mott insulator and either adding or removing some electrons from itscrystal structure. As the quantity of electrons is raised or lowered, the material undergoes a gradual transformation to one that, at certain temperatures, conducts electricity utterly without resistance. Until now, all materials that fit the bill could only be pushed towardsuperconductivityeither by adding or removing electrons—but not both.

However, the new material tested at the NIST Center for Neutron Research (NCNR) is the first one ever found that exhibits properties of both of these regimes. A team of researchers from Osaka University, the University of Virginia, the Japanese Central Research Institute of Electric Power Industry, Tohoku University and the NIST NCNR used neutron diffraction to explore the novel material, known only by its chemical formula of YLBLCO.

The material can only be made to superconduct by removing electrons. But ifelectronsare added, it also exhibits some properties only seen in those materials that superconduct with an electron surplus—hinting that scientists may now be able to study the relationship between the two ways of creating superconductors, an opportunity that was unavailable before this"ambipolar"material was found.

The results are described in detail in a"News and Views"article in the August, 2010, issue ofNature Physics,"Doped Mott insulators: Breaking through to the other side."**

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Electron 'pairing': Triplet superconductivity proven experientially for first time

The results achieved by this research team headed by Prof. Kurt Westerholt and Prof. Hartmut Zabel (Department of Physics and Astronomy at RUB) could contribute to new, power saving components in the future. The researchers reported on their findings in the American Physical Society's noted journalThe Physical Review B.

If it were possible to eliminateelectrical resistancewe could reduce our electric bill significantly and make a significant contribution to solving the energy problem, if it were not for a few other problems. Many metals as well asoxidesdemonstrate a superconductive state, however only at low temperatures. The superconductive effect results from Cooper pairs that migrate through the metal together"without resistance". The electrons in each Cooper pair are arranged so that their compositeangular momentumis zero. Each electron has an angular momentum, the so-called spin, with a value of 1/2. When one electron spins counterclockwise (-1/2) and the other clockwise (+1/2), the total of the two spin values is zero. This effect, found only in superconductors, is called the singlet state.

If a superconductor is brought into contact with a ferromagnetic material, the Cooper pairs are broken up along the shortest path and the superconductor becomes a normal conductor. Cooper pairs cannot continue to exist in a singlet state in aferromagnetic material. Researches at RUB (Prof. Konstantin Efetov,Solid State Physics) among others have, however, theoretically predicted a new type of Cooper pair, which has a better chance of survival in ferromagnetic materials. In such Cooper pairs the electrons spin in parallel with one another so that they have a finite spin with a value of 1. Since this angular momentum can have three orientations in space, it is also known as the triplet state."Obviously there can also be only one certain, small fraction of Cooper pairs in a triplet state, which then quickly revert to the singlet state"explained Prof. Kurt Westerholt."The challenge was to verify these triplet Cooper pairs experimentally".

Superconductorsallow us to produce highly sensitive detectors for magnetic fields, which even allow detection of magnetic fields resulting from brain waves. These detectors are called SQUID's (superconducting quantum interference devices)– components which use the superconductive quantum properties. The central feature in these components consists of so-called tunnel barriers with a series of layers made up of a superconductor, insulator and another superconductor. Quantum mechanics allows a Cooper pair to be"tunneled"through a very thin insulating layer. Tunneling of a large number of Cooper pairs results in a tunnel current."Naturally the barrier cannot be too thick, otherwise the tunnel current subsides. A thickness of one to two nanometers is ideal", according to Prof. Hermann Kohlstedt (CAU).

If part of the tunnel barrier is replaced by a ferromagnetic layer, the Cooper pairs are broken up while they are still in the barrier and do not reach the superconductor on the other side. The tunnel current decreases drastically."Triplet Cooper pairs can, however, be tunneled much better through such a ferromagnetic barrier", says Dirk Sprungmann, who was involved as Ph.D. student. If we are successful in converting a portion of the singlet Cooper pairs to triplet Cooper pairs, the tunnel current should be significantly stronger and be able to pass through a thicker ferromagnetic layer. This is precisely what the physicists in Bochum and Kiel tested. They allowed the Cooper pairs to pass through ferromagnetic barriers with thicknesses of up to 10 nanometers. With this attempt the physicists achieved a double success. On the one hand they were able to experimentally verify the existence of triplet Cooper pairs, and, on the other, they demonstrated that the tunnel current is greater than for singlet Cooper pairs in conventional tunnel contacts."These new ferromagnetic tunnel barriers may possibly be used for new types of components", states Dr. Martin Weides (Santa Barbara). With their research findings the scientists confirmed, among other things, the theoretical work of a Norwegian research team published only a few weeks before.

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Stripes offer clues to superconductivity

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In order to identify the stripes that represent regions with dense superconducting currents, a group of Stanford University researchers measured the depth that magnetic fields penetrated into a superconducting sample. When exposed to amagnetic field, currents in a superconductor flow in a way that creates a field inside the material that is the opposite of the applied field. When added together, the applied and internal fields cancel each other out inside the superconductor. Essentially, it's as if the superconductor prevents a magnetic field from penetrating it (this is the source of the Meissner effect, which allows strong magnets to levitate over a superconductor). The better the superconductor, the more completely it can exclude a magnetic field. By scanning an iron-pnictide superconductor with a probe that measures the depth that a magnetic field penetrates the material, the researchers could determine the regions where superconducting currents are strongest.

Unlike metal superconductors, which require temperatures close toabsolute zeroin order to operate, pnictides and many superconductors that function at higher temperatures (typically 10 to 135 degrees above absolute zero) are ceramics that are built of crystal grains. Although the underlying mechanisms are not clear, measurements of magnetic field penetration indicate that superconducting currents flow best along the boundaries between the crystals.

In aViewpointappearing in the current edition ofAPS Physics, John Tranquada of Brookhaven National Laboratory points out that identifying the connection between crystal boundaries andsuperconductivityshould help us to develop better high temperature superconductors. Ultimately, superconductors operating closer to room temperature could help save energy by reducing the inefficiencies that comes with transporting electricity. In addition, high temperaturesuperconductorscould be handy for creating powerful magnets for medical imaging and various industrial applications, as well as potentially leading to high speed computers and other novel electronic devices.

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