Iron-pnictide electron orbital pairing promises higher-temperature superconductors

The paper,Orbital-Independent Superconducting Gaps in Iron Pnictides, was published in the April 29, 2011 issue ofScience. Lead researcher Takahiro Shimojima (affiliated with the University of Tokyo’s Institute for Solid State Physics, and the Japan Science and Technology Agency’s Core Research for Evolutional Science and Technology) notes that while high transition temperature superconductivity of up to 55K (-218C) in ferropnictides was first observed in 2008, this behavior is not predicted from standardelectron pairingbased on lattice vibrations. Therefore, says Shimojima, an alternate explanation was needed.

(Ferropnictides– also known as Fe-pnictides and iron pnictides– are compounds classified as members of the so-called nitrogen group, a periodic table group that includes nitrogen, phosphorus, arsenic, antimony, bismuth, and ununpentium– elements with fiveelectronsin their outermost shell. Pnictides are binary compounds of this group. The new class ofsuperconductorsShimojima and his team investigated has conducting layers of iron and arsenic.)

Shimojima and his team focused on what is know as thesuperconducting gap magnitude– the strength of Cooper pair electron pairing– in various electron orbitals. Using laser-ARPES (laser angle-resolved photoemission spectroscopy), the team determined that electron orbitals, not spin, account for ferropnictide superconductivity. They therefore concluded that electron orbitals– specifically, orbital fluctuations, interorbital pairing induced by magnetism, or a combination of the two– are a third way in which electrons form Cooper pairs.

What prompted the team to consider orbital pairing as a novel binding mechanism in HT superconductivity?“An important insight was Fermi surface orbital polarization in the anti-ferromagnetic metal phase of parent compound BaFe₂As₂observed by laser-ARPES,” notes Shimojima.“It was surprising for us that its ground state is realized through orbital-dependent electronic reconstruction. We also found that this result indirectly supported the orbital ordering proposed by theoretical research. We then became curious about the effect of the orbital degrees of freedom on the material’s superconductivity.”

Laser-ARPES was the key tool in the team’s discovery.“One very important technique we developed was the variable laser polarization for examining orbital characteristics,” says Shimojima.“We also achieved high-energy resolution and bulk sensitivity in order to capture high quality data about the superconducting gaps. As a result, we were able to separate the two peak structure in the (Ba,K)Fe₂As₂spectrum, which previously makes difficult to interpret ARPES data.”

Going forward, Shimojima points out, if other materials having several entangled orbitals near Fermi level are discovered, it may have the potential to show even higher transition temperaturesuperconductivitydue to orbital pairing.“For example,” he adds,“if thetransition temperatureapproaches room temperature, superconducting wires for lossless electronic transportation and storage will quickly be deployed worldwide.”

And so the dream lives on.

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Scientists looking to burst the superconductivity bubble

In a study published today, Monday, 16 may, in IOP Publishing's journalSuperconductor Science and Technology, researchers have examined bismuth strontiumcalciumcopper oxide(Bi₂Sr₂CaCu₂O_x, Bi2212)– one of the most promising superconducting materials capable of creating large magnetic fields way beyond the limit of existing magnets– and found that its capabilities are limited by the formation of bubbles during its fabrication process.

Bi2212 is the only high temperature superconductor capable of being made into round wire, providing the preferred flexibility in magnet construction, and giving it potential uses in medical imaging and particle accelerators, such as the Large Hadron Collider in Switzerland.

For magnet applications, these wires must exhibit a high critical current density - the current density at which electrical resistance develops - and sustain it under large magnetic fields. This remains a stumbling block for utilising the huge potential of Bi2212 in the magnet technology as compellingly high critical current densities have not yet been achieved.

Previous studies have shown that a critical current varies widely between Bi2212 wire lengths– the critical current in wires that were 50 to 200m long was 20 to 50% lower than in 5 to 10cm long samples. This led the researchers, from the Applied Superconductivity Centre and the National High Magnetic Field Laboratory, Florida State University, to conclude that this variability must be caused by the connectivity of Bi2212 grains within the wires.

Bi2212 wires, made up of multiple filaments, are fabricated using the powder-in-tube (PIT) method in which Bi2212 powder is packed inside silver tubes and drawn to the desired size. The filaments of Bi2212 powder must firstly be melted inside their silver sheath and then slowly cooled to allow the Bi2212 to reform, greatly enhancing the critical current density.

As the processes between the critical melt and re-growth step is still largely unknown, the researchers decided to rapidly cool samples at different times in the melting process in order to get a snapshot of what occurs inside Bi2212 wires.

Using ascanning electron microscopeand synchrotron X-ray microtomography, the researchers observed that the small powder pores, inherent to the PIT process, agglomerate into large bubbles on entering the melting stage.

The consequences of this are major as the Bi2212 filaments become divided into discrete segments of excellent connectivity which are then blocked by the residual bubbles, greatly reducing the long-range filament connectivity, and strongly suppressing the flow of current.

The new findings suggest that a key approach to improve the critical current density of the material would be to make it denser before melting.

Lead author Dr Fumitake Kametani, of The Applied Superconductivity Centre, Florida State University, said,"Our study suggested that a large portion ofbubblesoriginates from the 30-40% of empty space, inevitable in any powder-in-tube process, which requires particle rolling to allow deformation of the metal-powder composite wire."

"Densification of the filaments at final size - increasing the powder-packing density from 60-70% to greater than 90% - is an excellent way to reduce or eliminate the bubble formation. Various densification processes are now being tested."

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Study helps explain behavior of latest high-temp superconductors

The research appears online this week in the journalPhysical Review Letters. It describes how themagnetic propertiesof electrons in two dissimilar families of iron-based materials called"pnictides"(pronounced: NICK-tides) could give rise to superconductivity. One of the parent families of pnictides is a metal and was discovered in 2008; the other is aninsulatorand was discovered in late 2010. Experiments have shown that each material, if prepared in a particular way, can become a superconductor at roughly the same temperature. This has left theoretical physicists scrambling to determine what might account for the similar behavior between such different compounds.

Rice physicist Qimiao Si, the lead researcher on the new paper, said the explanation is tied to subtle differences in the way iron atoms are arranged in each material. The pnictides are laminates that contain layers of iron separated by layers of other compounds. In the newest family ofinsulating materials, Chinese scientists found a way to selectively remove iron atoms and leave an orderly pattern of"vacancies"in the iron layers.

Si, who learned about the discovery of the new insulating compounds during a visit to China in late December, suspected that the explanation for the similar behavior between the new and old compounds could lie in the collective way that electrons behave in each as they are cooled to the point of superconductivity. His prior work had shown that the arrangement of the iron atoms in the older materials could give rise tocollective behaviorof the magnetic moments, or"spins,"of electrons. These collective behaviors, or"quasi-localizations,"have been linked to high-temperature superconductivity in both pnictides and other high-temperature superconductors.

"The reason we got there first is we were in a position to really quickly incorporate the effect of vacancies in our model,"Si said."Intuitively, on my flight back (from China last Christmas), I was thinking through the calculations we should begin doing."

Si conducted the calculations and analyses with co-authors Rong Yu, postdoctoral research associate at Rice, and Jian-Xin Zhu, staff scientist at Los Alamos National Laboratory.

"We found that ordered vacancies enhance the tendency of the electrons to lock themselves some distance away from their neighbors in a pattern that physicists call 'Mott localization,' which gives rise to an insulating state,"Yu said."This is an entirely new route toward Mott localization."

By showing that merely creating ordered vacancies can prevent the material from being electrical conductors like their relatives, the researchers concluded that even the metallic parents of the iron pnictides are close to Mott localization.

"What we are learning by comparing the new materials with the older ones is that these quasi-localized spins and the interactions among them are crucial for superconductivity, and that's a lesson that can be potentially applied to tell experimentalists what is good for raising the transition temperature in new families of compounds,"Zhu said.

Superconductivity occurs whenelectronspair up and flow freely through a material without any loss of energy due to resistance. This most often occurs at extremely low temperatures, but compounds like the pnictides and others become superconductors at higher temperatures -- close to or above the temperature of liquid nitrogen -- which creates the possibility that they could be used on an industrial scale. One impediment to their broader use has been the struggle to precisely explain what causes them to become superconductors in the first place. The race to find that has been called the biggest mystery in modern physics.

"The new superconductors are arguably the most important iron-based materials that have been discovered since the initial discovery of iron pnictide high-temperature superconductors in 2008,"Si said."Our theoretical results provide a natural link between the new and old iron-basedsuperconductors, thereby suggesting a universal origin of the superconductivity in these materials."

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Ferromagnetism plus superconductivity

Just in time for the 100th anniversary to commemorate the discovery of superconductivity by the Dutch physicist Heike Kamerlingh Onnes on April 8, 1911, scientists from the Helmholtz-Zentrum Dresden-Rossendorf and the TU Dresden published their research results in the journalPhysical Review B.

Headed by Dr. Thomas Herrmannsdörfer, the team from the HZDR's High Magnetic Field Laboratory (HLD) examined a material consisting of the elementsbismuthand nickel (Bi₃Ni) with a diameter of only a few nanometers– which is unique since it has not been achieved elsewhere so far. This was made possible through a new chemical synthesis procedure at low temperatures which had been developed at the TU Dresden under the leadership of Prof. Michael Ruck. The nano scale size and the special form of the intermetallic compound– namely, tiny fibers– caused the physical properties of the material, which is non-magnetic under normal conditions, to change so dramatically. This is a particularly impressive example of the excellent opportunities modern nanotechnology can provide today, emphasizes Dr. Thomas Herrmannsdörfer."It's really surprising to which extend the properties of a substance can vary if one manages to reduce their size to the nanometer scale."

There are numerous materials which become superconducting at ultralow temperatures. However, this property competes withferromagnetismwhich normally suppresses superconductivity. This does not happen with the analyzed compound: Here, the Dresden researchers discovered with their experiments in high magnetic fields and at ultralow temperatures that the nanostructured material exhibits completely different properties than larger-sized samples of the same material. What's most surprising: The compound is both ferromagnetic and superconducting at the same time. It is, thus, one of those rarely known materials which exhibit this unusual and physically not yet completely understood combination. Perhaps bismuth-3-nickel features a special type ofsuperconductivity, says Dr. Herrmannsdörfer. The physicist and doctoral candidate Richard Skrotzki, who has just turned 25, is making a vital contribution to the research results and describes the phenomenon as"the bundling of contrary properties in a single strand."

The TU Dresden and the HZDR are partners in the research alliance DRESDEN-concept which pursues the objective of making visible the excellence of Dresden research.

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'Giant Proximity Effect' enhances high-temperature superconductivity

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Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory (BNL), collaborating with scientists from the Paul Scherrer Institute (PSI) and the University of Zürich in Switzerland, have found that sandwiching a barrier layer between twosuperconductorscan make it superconducting at significantly higher temperatures. The results will be published online inNature Communicationson April 12, 2011.

Conventional superconducting materials allow electricity to flow without any resistance or energy loss when cooled below a transitiontemperature(T_c) near absolute zero. High-temperature varieties discovered more recently can operate at warmer temperatures, but still require significant cooling, which hampers their use in many large-scale practical applications. Finding ways to raise the temperature further, such as the layering approach described in this paper, could lead to the realization of such applications as low-power consumption, ultra-fast superconducting electronic devices.

“For many years, we have known about a‘Proximity Effect,’ that superconducting electron pairs from one superconducting electrode can drift and penetrate a very thin metallic layer, and then reach the other superconducting electrode without losing their coherence,” said Brookhaven physicist Ivan Bozovic, co-author of the paper.“More recently, we have observed a mysterious‘Giant Proximity Effect’ in copper-oxide materials— cuprates— when supercurrent flows through much thicker barriers.”

Because thicker layers are easier to fabricate and work with, taking advantage of the Giant Proximity Effect could make it much easier to achieve on-chip device uniformity— the requirement that all devices on an electronic chip have similar parameters.“This has been a major technical hurdle for large-scale-integrated superconducting electronics,” Bozovic said.

To explore the Giant Proximity Effect, Bozovic and his team engineered complex cuprates using a process called molecular beam epitaxy. They synthesized samples of thin films containing layers of lanthanum-cuprate superconductors doped with strontium to various levels, to create a series with varying transition temperatures. The Brookhaven samples were studied at PSI using a unique technique called low-energy muon spin rotation to detect superconductivity in each sample’s outer and inner layers.

By mapping the magnetic fields for each structure, the scientists observed the Giant Proximity Effect and found that a thick barrier of superconductor with a T_cof 5 Kelvin could transmit supercurrent at a temperature four times higher, if it is sandwiched between two superconductors with a T_cof 40 Kelvin. Their results also proved that the entire barrier layer is affected by the Giant Proximity Effect.

“In addition to its potential importance for superconducting electronics, the Giant Proximity Effect could be an important hint of what is going on in cuprates— what drives the high-temperature superconductivity,” Bozovic added. Discovering that mechanism could open up a whole new field for engineering superconductors with desired properties.

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Mercury-containing oxides offer new perspective on mechanism of superconductivity

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The Dutch physicist Heike Kamerlingh Onnes discoveredsuperconductivityone hundred years ago, when he noticed that theelectrical resistanceof mercury dropped to zero suddenly at 4.2 K.Superconducting materialsare now used routinely inmagnetic resonance imagingscanners.

In classical superconductors such as mercury, superconductivity arises through the combined vibrations of the atoms in the crystal. This makes the crystal structure a key factor for the superconducting properties of a material. In the case of Hg_xReO₃, theatomic structureconsists of rhenium (Re) and oxygen (O) building blocks. In the empty spaces between them, the mercury atoms arrange in chains (Fig. 1). However, some of the available places along these chains lack mercury atoms, and the team’s work suggests that this leads to an arrangement of paired mercury atoms.

"These pairs move within the channel in an oscillatory motion known as rattling", explains team-member Ayako Yamamoto from the RIKEN Advanced Science Institute in Wako. The rattling vibrations provide a strong feedback for the electrons, and therefore reinforce superconductivity in the material. In comparison to a similar structure lacking mercury pairs, the superconducting temperature of Hg_0.44ReO₃at 7.7 K is almost twice as high."Despite remaining below the present record of 135 K for a superconductor, there is potential for improving operation temperatures", says Yamamoto.“The application of pressure increases the superconducting temperature to 11.1 K, and this could mean that for the right crystal structure further enhancement is possible.”

Yamamoto and her colleagues are now working to optimize the crystal structure further—for example, by replacing rhenium with other elements. A better understanding of the influence of the mercury atoms’ rattling motion may also provide better insight into the mechanism of superconductivity in such structures.“Mercury seems to be a magic element in superconductivity, not only for its role in Kamerlingh Onnes’ discovery, but also for the fact that mercury is part of the material with the highest known superconducting temperature, HgBa₂Ca₂Cu₃O_x,” Yamamoto explains."Once more,mercuryis playing a key role for new superconductors,"she says.

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Power cables light the future path of superconductivity

In honour of this discovery, the April issue ofPhysics Worldis dedicated to the discovery and features a run-down of the top five applications ofsuperconductivity, written by Paul Michael Grant from W2AGZ Technologies, San Jose, California.

Topping Grant’s list are superconducting wires, and in particular a high-temperature tape made from yttrium-barium-copper-oxide (YBCO). YBCO, which superconducts when cooled with liquid nitrogen, has some remarkable properties - despite being hard and brittle, it can be made into batches thousands of metres long.

YBCO could be used for superconducting power cables, carrying electricity without any power loss, which is a big problem for conventional copper cables. The US Department of Energy has in fact just completed a 20-year programme in this field and as Grant writes:“Its fruits are now on the shelf, waiting to be harvested by the utility industry and its suppliers.”

In second place, Grant highlights the application of superconductivity to medical imaging, where superconducting magnets are essential components of Magnetic Resonance Imaging (MRI) scanners. He also highlights a technique that uses superconducting quantum interference devices (SQUIDs) to detect the tiny magnetic fields generated by the very small currents in the heart and brain.

Third place is taken up by the application of superconductivity to high-energy physics, more specifically in the magnets that bend protons around at particle accelerators like the Large Hadron Collider (LHC) at CERN.

Grant’s penultimate application of superconductivity is its ability to produce lighter, smaller and more efficient generators by allowing the required iron core of electromagnets to be removed.

Finally, Grant sheds light on how superconductivity can aid the search for dark matter, which, if discovered, could lead a budding researcher to a Nobel Prize for Physics.

One notable omission from the list is magnetically levitated (maglev) trains. These trains, which are suspended and guided above a track by a large number of magnets, have captured the imagination of the public and could hold the key to faster, quieter, and smoother transport in the future.

As of yet, however, every maglev train that has ever been built, barring a train used on a test line in Japan, has used conventional, albeit powerful, iron-core electromagnets.

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Technique reveals quantum phase transition; could lead to superconducting transistors

"Understanding exactly what happens when a normally insulating copper-oxide material transitions from the insulating to the superconducting state is one of the great mysteries of modern physics,"said Brookhaven physicist Ivan Bozovic, lead author on the study.

One way to explore the transition is to apply an external electric field to increase or decrease the level of"doping"-- that is, the concentration of mobile electrons in the material -- and see how this affects the ability of the material to carry current. But to do this in copper-oxide (cuprate) superconductors, one needs extremely thin films of perfectly uniform composition -- and electric fields measuring more than 10 billion volts per meter. (For comparison, the electric field directly under a power transmission line is 10 thousand volts per meter.)

Bozovic's group has employed a technique called molecular beam epitaxy (MBE) to uniquely create such perfect superconducting thin films oneatomic layerat a time, with precise control of each layer's thickness. Recently, they've shown that in such MBE-created films even a single cuprate layer can exhibit undiminished high-temperature superconductivity.*

Now, they've applied the same technique to build ultrathin superconducting field effect devices that allow them to achieve thecharge separation, and thus electric field strength, for these critical studies.

These devices are similar to the field-effect transistors (FETs) that are the basis of all modern electronics, in which a semiconducting material transports electrical current from the"source"electrode on one end of the device to a"drain"electrode on the other end. FETs are controlled by a third electrode, called a"gate,"positioned above the source-drain channel -- separated by a thin insulator -- which switches the device on or off when a particular gate voltage is applied to it.

But because no known insulator could withstand the high fields required to induce superconductivity in the cuprates, the standard FET scheme doesn't work for high-temperature superconductor FETs. Instead, the scientists used electrolytes, liquids that conduct electricity, to separate the charges.

In this setup, when an external voltage is applied, the electrolyte's positively charged ions travel to the negative electrode and the negatively charged ions travel to the positive electrode. But when the ions reach the electrodes, they abruptly stop, as though they've hit a brick wall. The electrode"walls"carry an equal amount of opposite charge, and the electric field between these two oppositely charged layers can exceed the 10 billion volts per meter goal.

The result is a field effect device in which the critical temperature of a prototype high-temperature superconductor compound (lanthanum-strontium-copper-oxide) can be tuned by as much as 30 degrees Kelvin, which is about 80 percent of its maximal value - almost ten times more than the previous record.

The scientists have now used this enhanced device to study some of the basic physics ofhigh-temperature superconductivity.

One key finding: As the density of mobile charge carriers is increased, their cuprate film transitions from insulating to superconducting behavior when the film sheet resistance reaches 6.45 kilo-ohm. This is exactly equal to the Planck quantum constant divided by twice the electron charge squared - h/(2e)2. Both the Planck constant and electron charge are"atomic"units - the minimum possible quantum of action and of electric charge, respectively, established after the advent of quantum mechanics early in the last century.

"It is striking to see a signature of such clearly quantum-mechanical behavior in a macroscopic sample (up to millimeter scale) and at a relatively high temperature,"Bozovic said. Most people associate quantum mechanics with characteristic behavior of atoms and molecules.

This result also carries another surprising message. While it has been known for many years that electrons are paired in thesuperconducting state, the findings imply that they also form pairs (although localized and immobile) in the insulating state, unlike in any other known material. That sets the scientists on a more focused search for what gets these immobilized pairs moving when the transition to superconductivity occurs.

Superconducting FETs might also have direct practical applications. Semiconductor-based FETs are power-hungry, particularly when packed very densely to increase their speed. In contrast, superconductors operate with no resistance or energy loss. Here, the atomically thin layer construction is in fact advantageous - it enhances the ability to control superconductivity using an external electric field.

"This is just the beginning,"Bozovic said."We still have so much to learn abouthigh-temperature superconductors. But as we continue to explore these mysteries, we are also striving to make ultrafast and power-saving superconducting electronics a reality."

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Fleeting fluctuations in superconductivity disappear close to transition temperature

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"Our findings suggest that in cuprate superconductors, the transition to the non-superconducting state is driven by a loss of coherence among theelectron pairs,"said Brookhaven physicist Ivan Bozovic, a co-author on a paper describing the results inNature Physicsonline, February 13, 2011.

Scientists have been searching for an explanation of high-T_csuperconductivity in cuprates since these materials were discovered some 25 years ago. Because they can operate at temperatures much warmer than conventional superconductors, which must be cooled to nearabsolute zero(0 K or -273 degrees Celsius), high- Tc superconductors have the potential for real world applications. If scientists can unravel the current-carrying mechanism, they may even be able to discover or design versions that operate at room temperature for applications such as zero-loss power transmission lines. For this reason, many researchers believe that understanding how this transition to superconductivity occurs in cuprates is one of the most important open questions in physics today.

In conventional superconductors, electron pairs form at the transition temperature and condense into a collective, coherent state to carry current with no resistance. In high- Tc varieties, which can operate at temperatures as high as 165 K, there are some indications that electron pairs might form at temperatures 100-200 K higher, but only condense to become coherent when cooled to the transition temperature.

To explore the phase transition, the Johns Hopkins-BNL team sought evidence for superconducting fluctuations above T_c.

"These fluctuations are something like small islands or droplets of superconductivity, within which the electron pairs are coherent, which pop up here and there and live for a while and then evaporate to pop up again elsewhere,"Bozovic said."Such fluctuations occur in every superconductor,"he explained,"but in conventional ones only very, very close to T_c— the transition is in fact very sharp."

Some scientists have speculated that in cuprates, on the contrary, superconducting fluctuations might exist in an extremely broad region, all the way up to the temperature at which the electron pairs form. In the present study, the scientists tackle this question head-on, by measuring the conductivity as a function of temperature and frequency up to the terahertz range.

"With this technique, one can see superconducting fluctuations as short-lived as one billionth of one billionth of a second— the shortest possible— and over the entire phase diagram,"Bozovic said.

The scientists studied a superconductor containing variable amounts of lanthanum and strontium layered with copper oxide. The samples were fabricated at Brookhaven, using a unique atomic-layer molecular beam epitaxy system that allows for digital synthesis of atomically smooth and perfect thin films. Terahertz spectroscopy measurements were performed at Johns Hopkins.

The central finding was somewhat surprising: The scientists clearly observed superconducting fluctuations, but these fluctuations faded out relatively quickly, within about 10-15 K above T_c, regardless of the lanthanum/strontium ratio.

This implies that in cuprates at the transition temperature, electron pairs lose their coherence. This is in contrast to what happens in conventional superconductors, where theelectron pairsbreak apart at thetransition temperature.

"So, unlike in conventional superconductors, the transition in cuprates is not driven by electron (de)pairing but rather by loss of coherence between pairs— that is, by phasefluctuations,"Bozovic said."The hope is that understanding this process in full detail may bring us one step closer towards cracking the enigma of high-temperaturesuperconductivity."

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Compact high-temperature superconducting cables demonstrated at NIST

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Described in a paper just published online,* the new method involves winding multiple HTS-coated conductors** around a multi-strand copper“former” or core. The superconducting layers are wound in spirals in alternating directions. One prototype cable is 6.5 millimeters (mm) in outer diameter and carries a current of 1,200 amperes; a second cable is 7.5 mm in diameter and carries a current as high as 2,800 amperes. They are roughly one-tenth the diameter of typical HTS cables used in the power grid. (Standard electrical transmission lines normally operate at currents below 1,000 amperes.)

HTS materials, which conduct electricity without resistance when cooled sufficiently (below 77 K, or minus 196 C/minus 321 F, for the new cables) with liquid nitrogen or helium gas, are used to boost efficiency in some power grids. The main innovation in the compact cables is the tolerance of newer HTS conductors to compressive strain that allows use of the unusually slendercopperformer, says developer Danko van der Laan, a University of Colorado scientist working at NIST.

“The knowledge I gained while working at NIST on electromechanical properties of high-temperature superconductors was very important for inventing the initial cable concept,” van der Laan says.“For instance, my discovery that the conductor surviveslarge compressive strainsmade me realize that wrapping the conductor around a small diameter former would most likely work.”

Van der Laan and NIST colleagues demonstrated the feasibility of the new concept by making several cables and testing their performance. They used an HTS material with a critical current that is less sensitive to strain than some other materials. Although the prototype cables are wound by hand, several manufacturers say mass production is feasible.

NIST researchers are now developing prototype compact HTS cables for the military, which requires small size and light weight as well as flexibility to pull transmission lines through conduits with tight bends. Beside power transmission, the flexible cabling concept could be used for superconducting transformers, generators, and magnetic energy storage devices that require high-current windings. The compact cables also could be used in high-field magnets for fusion and for medical applications such as next-generation magnetic resonance imaging and proton cancer treatment systems.

The work was supported in part by the U.S. Department of Energy.

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Neutron analysis reveals '2 doors down' superconductivity link

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Researchers at the Department of Energy's Oak Ridge National Laboratory and the University of Tennessee, using the Spallation Neutron Source's ARCS Wide Angular Range ChopperSpectrometer, performed spin-wave studies of magnetically ordered iron chalcogenides. They based their conclusions on comparisons with previous spin-wave data on magnetically ordered pnictides, another class of iron-based superconductors.

"As we analyze the spectra, we find that even though the nearest neighbor exchange couplings between chalcogenide and pnictide atoms are different, the next nearest neighbor exchange couplings are closely similar,"said Pengcheng Dai, who has a joint appointment with ORNL's Neutron Sciences Directorate and the University of Tennessee.

Dai referred to theories that have suggested second-nearest-neighbor couplings could be responsible for the widely acclaimed but poorly understood properties ofhigh-temperature superconductors.

"There are theories suggesting that it's the second nearest neighbor that drives the superconductivity,"he said."Our discovery of similar next-nearest-neighbor couplings in these two iron-based systems suggests that superconductivity shares a common magnetic origin."

Oliver Lipscombe of the University of Tennessee, Dai and ORNL's Doug Abernathy used the ARCS time-of-flight instrument on the SNS to study spin waves of the chalcogenide iron-tellurium superconductor and compared these with iron pnictide superconductors.

Scientists have been studying the iron-based superconductors since their discovery in 2008 to see if the dynamics behind their high-temperature superconducting properties -- in which electricity flows without resistance at temperatures well above absolute zero -- could help explain what was until recently thought to be exclusive to copper-oxide-based superconductors.

"Finding commonalities is always a good step when you're looking for a very basic understanding of a phenomenon like high-temperature superconductivity,"said Abernathy, who is lead instrument scientist for the ARCS instrument.

The team's neutron scattering analysis of the materials was made possible by the high intensity of the neutron beams provided by the SNS, which is the world's most powerful pulsedneutronsource. Neutrons, which carry no electric charge but can act as subatomic magnets, are well suited for studying atom-scale spin characteristics.

"Since the interactions in the high-temperature superconductors are so strong, measurement of these materials' spin waves requires beams of energetic neutrons that were unavailable to the research community at this intensity before the SNS,"Abernathy said.

The work, which was funded by the DOE Office of Science, is published inPhysical Review Letters.

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Using complex electron systems to create green materials

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Accordingly, the RIKEN Advanced Science Institute (Japan) established the Green-forefront Materials Department in April 2010, aiming to create‘green-forefront materials’, that is, new materials for helping to solve environmental and energy problems. In the same year, Hidenori Takagi set up and led the Complex Electrons and Functional Materials Research Group in the Green-forefront Materials Department. This group is striving to create high-temperaturesuperconductorsand thermoelectric materials using complex electron systems.

This year will mark the 100th anniversary of the discovery of the superconductor by the Dutch physicist Heike Kamerlingh Onnes. Superconductors are materials in which electrons can flow with no electrical resistance, and the first such materials displayed this behavior when cooled to near absolute zero. This year will also mark the 25th anniversary of the discovery of the first‘high-temperature’ cuprate superconductor—a material that makes the transition to the zero-resistance state at a temperature of 30 K. In that year, 1986, Takagi successfully conducted a follow-up experiment to verify the discovery of the high-temperature cuprate superconductor. A worldwide search for high-temperature superconductors then ensued with the hope of discovering new materials that could significantly contribute to solving environmental and energy problems as a medium for lossless electric power transmission and storage. Today, the highest transition temperature achieved to date for cuprate superconductors is 160 K, or about−113°C, under high pressure.

Cuprates are basically insulators, so it is one of the most intriguing questions in science how such materials can become superconductors. According to Takagi,“Metals contain unbound electrons that can move freely through the material, so when a voltage is applied to these electrons in this‘gaseous’ state, an electrical current flows.” Free electrons are also present in some transition-metal elements such as cuprates (metallic oxides including metals from group III to group II in the periodic table).“Their electron orbits, however, are so narrow that they cannot move to adjacent atom because they are electrically repelled by the existing electrons of the atom. This state can be compared to that of the solid state in which electrons are stuck tightly in place because they repel each other. The material in this state acts as an insulator. Systems composed of such strongly interacting electron populations are known as strongly correlated electron systems or complex electron systems, and they can change their electrical states or electron phases dramatically in response to small changes in conditions.”

For example, as electron vacancies, called holes, are created in such systems, the remaining electrons slowly begin to move, changing the electron state from tightly bound solid state to more fluid mobile state (Fig. 1).“In strongly correlated electron systems, the solid electron state first changes into a sticky liquid and then a gaseous state in which electrons can move freely. These systems change state in response to changes in conditions. That is where the fun is.”

High-temperature superconductivity occurs when complex electron systems change state. Electrons in a superconductor are known to form pairs of electrons called Cooper pairs. The mechanism by which electrons form Cooper pairs in cuprates when they usually repel each other under, however, remains a mystery. Takagi and his group have been conducting world-leading research into achieving direct observations of the electron state in high-temperature cuprate superconductors by scanning tunneling microscopy (STM).“We found that most electrons in high-temperature cuprate superconductors are stuck tightly in place except for a small number of electrons that are left free from atomic bonds. This state is considered to be what creates Cooper pairs and causes superconductivity.

Many unknown electron states lie hidden in complex electron systems.“We aim to discover new electron states and to use them to create new materials.”

The states of complex electron systems can also be changed by taking advantage of the property of electrons known as‘spin’. Spin describes the angular momentum of an electron, similar in nature to the rotation of Earth, and can be either‘up’ or‘down’. In the same was as two bar magnets arranged in the same direction repel each other yet when arranged in opposite directions are pulled together, a square lattice of atoms is most stable when the up and down spins of adjacent are arranged alternately (Fig. 1).“If the directions of electron spins are arranged in an orderly manner, spins are considered to form a spin solid. How are the spins aligned in a triangular atomic lattice? There are three electrons located at the vertices of a triangle, but only two spins can form a pair, leaving one unpaired spin. In such a lattice, the orientations of electron spins are unstable. This state, called‘spin frustration’, is considered to be responsible for s spin liquid state.”

Electron systems tend to become more stable at lower temperature. In 1973, however, it was theoretically predicted that a lattice with extremely strong spin frustration could adopt a special state called a‘quantum spin liquid’, in which the directions of electron spins do not become stable even when the temperature is lower to absolute zero.“This state, of course, is very hard to produce and has long been considered merely a theoretical dream.”

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Figure 2: Na4Ir3O8, a transition-metal oxide that can turn into a quantum spin liquid. The transition-metal oxide Na4Ir3O8 has a triangular lattice that forms spirals. It exhibits strong spin frustration and becomes a quantum spin liquid at absolute zero.

Yet Takagi and his group were eventually successfully in creating a quantum spin liquid using the compound Na4Ir3O8, a transition metal oxide (Fig. 2) consisting of a triangular lattice of atoms in a spiral configuration. Takagi named this structure the‘hyper-kagome lattice’. Another research group also successfully created a quantum spin liquid using a different structure at around the same time, and the field now attracts a great deal of attention.

The possible uses of complex electron systems are actually more practical than might be expected.“One of the best examples is water ice that freezes at 10°C,” says Takagi.“Wine and sushi, for example can be overcooled if placed in normal ice at 0°C. Ice that freezes at 10°C would be very convenient in these types of situations. We have in fact created a coolant in which the electron state changes from solid to liquid at 10°C.”

Takagi and his co-workers have also taken advantage of spin frustration to successfully develop a new compound known as Mn3XN with a thermal expansion coefficient of zero (Fig. 3). Manganese nitride, the base material for the new compound, has a triangular lattice of manganese atoms. The compound adopts a liquid state at high temperature because of its electron state, and becomes a solid when the temperature is lowered to near room temperature. The electrons try to stay at the triangular lattice points, but the directions of electron spins remain unstable because of the strong spin frustration in this system. The surrounding lattice, however, expands when cooled to relax the spin frustration, resulting in a material that expands when cooled and contracts when heated—the opposite behavior to that of most natural compounds. For example, an iron bar increases in length by 0.0012% for every 1°C increase in temperature. Although small, such thermal expansion can sometimes causes problems in processing equipment and high-precision measuring devices, particularly in semiconductor circuits where nanometer-accuracy is required.

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Figure 3: Mn3XN, a manganese nitride with negative thermal expansion coefficient. The manganese nitride Mn3XN has an antiperovskite structure. The symbol X represents copper or zinc. The component X can be replaced with germanium or tin atom, resulting in a material with a zero thermal expansion coefficient.

“Manganese nitride itself contracts discontinuously at a certain temperature. By introducing germanium or tin, we can create a substance that contracts gradually over a temperature range of about 100°C. In 2008, we successfully created a new material with a zero thermal expansion coefficient, meaning it does not change volume, over a temperature range of about 70°C including room temperature.”

Conventional substances with a zero thermal expansion coefficient are compound materials consisting of compounds with different expansion coefficients, which leads to deformation or cracking and low strength. Such materials are also usually expensive because they contain rare elements.“Manganese nitride is resistant to deformation and cracking, and is also less expensive than conventional substances because it is a single compound.”

In 2010, Takagi and his colleagues started the Complex Electrons and Functional Materials Research Group at the RIKEN Advanced Science Institute.“We aim to create‘green-forefront materials’ that help to solve environmental and energy problems by taking advantage of complex electron systems. A typical example of our work is the development of high-efficiency thermal conversion materials.”

Only 30% of the thermal energy produced by the combustion of gasoline in an automobile engine is used to power the vehicle; the other 70% is lost as waste heat. A large proportion of the thermal energy produced by burning fossil fuels in thermal power stations and many factories is also lost as waste heat. The key to reducing fossil fuel usage and carbon dioxide emissions is how effectively that waste heat can be harnessed. There are high hopes for thermal conversion materials, or thermoelectrics, which convert heat energy into electrical energy and vice versa.

“The currently used Bi2Te3-based thermoelectrics cannot be used at high temperatures and are not very efficient at converting heat to electricity. These include the Peltier elements used in some mobile refrigerators, which use electricity to cool beverages, for example, by the reverse thermoelectric effect, but which do not have sufficient conversion efficiency to make ice. We aim to develop efficient thermoelectric materials that could be used to freeze water.”

Spin frustration could be utilized to increase the thermal conversion efficiency of such materials.“The principle of heat-to-electricity conversion is based on temperature difference. A temperature difference causes electrons or holes to move and thus produce a voltage, generating electrical power. The higher the disorder in the electron state, the higher the conversion efficiency, and the more electricity is generated. For example, entangled electrons with strong spin frustration are unstable and move in a more random manner, increasing the amount of disorder, or entropy.” Takagi and his team are working to create new high-efficiency thermoelectrics based on a new principle such as spin frustration.

In order to efficiently use the waste heat generated by vehicles and factories, the thermoelectric materials must be able to withstand temperatures of more than several hundred degrees celsius.“We should be able to develop heat-resistant, high-efficiency thermoelectrics because the transition metal oxides we are dealing with are strong and heat-resistant.”

In 2008, Hideo Hosono and his research group at the Tokyo Institute of Technology in Japan discovered an iron-based superconductor that attracted worldwide attention.“This superconductor has a transition temperature of 55 K, which is currently the highest transition temperature for non-cuprates. It was previously considered impossible to produce magnetic superconducting materials such as those containing iron, cobalt or nickel, so these iron-based superconductors probably have a different superconducting mechanism to that in cuprates,” says Takagi.

In April 2010, Tetsuro Hanaguri, a senior research scientist in RIKEN’s Magnetic Materials Laboratory, successfully observed the structure of a Cooper pair ofelectronsin an iron-based superconductor by STM. Metallic superconductors are known to form a type of Cooper pair called an s wave, whereas cuprate superconductors form a Cooper pair called a d wave. Hanaguri demonstrated for the first time that iron-based superconductors formed a new type of Cooper pair called an s± wave. Spin fluctuation is considered to be the mechanism responsible for creating Cooper pairs in iron-based superconductors.

Recent advances in research on superconductivity have raised the expectations for creating room-temperature superconductors, which would be the ultimate green forefront material.“The highest-temperature superconductor produced so far was discovered in 1994, a record that has remained unbroken for 16 years. This is the longest period since 1911, when superconductivity was first discovered, that there has been no increase the highest recorded transition temperature. We are adrift in research on higher-temperature superconductors. To challenge the status quo, I would like to break the long-standing record of 160 K by discovering a new high-temperature superconductor based on a new principle, which would again trigger a worldwide race for higher-temperature superconductors.”

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Physicists unveil unexpected properties in superconducting material

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But a key characteristic that explains the material's unusual properties remained tantalizingly out of reach in spite of the scientists' rigorous battery of experiments and exacting measurements. So members of that team from the University of Tokyo reached out to theoretical physicists at Rutgers University to help uncover the material's secrets.

In a paper published Jan. 21 in thejournal Science, the Tokyo and Rutgers researchers now report that the material can reach a point where seemingly contradictory electrical andmagnetic propertiescoexist, without being subject to massive changes in pressure, magnetic fields, or chemical impurities.

This point, which physicists call"quantum critical,"often defines whether and how a material can become superconducting– a valued property where all resistance to electrical flow vanishes. Superconductivity, discovered 100 years ago, has since been put to work in a variety of applications, from physics research to medical MRI scanners.

Scientists have long been able to"tune"materials toward quantum criticality by altering the materials' properties. This is done by exposing them to high magnetic fields and pressures, or by adding certain atomic impurities to the materials. The material studied by the Tokyo and Rutgers researchers, however, appears to be the first to exhibit quantum criticality in its natural state, without tuning.

"This is a completely unexpected result,"said Piers Coleman, professor of physics and astronomy, School of Arts and Sciences, at Rutgers."It could be the first example of what physicists describe as a 'strange' metallic phase of matter, manifesting itself intrinsically, without any tuning of the material's properties."

The material synthesized and studied by the Japanese experimental physicists is an exotic crystal made up of the elements ytterbium, boron, and aluminum. It has the chemical formula YbAlB4 but the physicists gave it the nickname"YBAL"(pronounced"why-ball"). Superconductivity had earlier been observed in YBAL, in a particular crystalline form called the"beta"structure. The Tokyo physicists suspected they could find a quantum critical point in the material; however, its superconducting behavior that kicks in slightly above absolute zero masked their ability to pinpoint it.

Coleman and postdoctoral researcher Andriy Nevidomskyy examined the data from the Tokyo experiments at a wide range of temperatures and magnetic field strengths. All the data, they found, collapsed onto a curve that pointed to the unobservable quantum critical point (QCP) hidden by the superconducting phase. The QCP was within hair's breadth of zeromagnetic field, with no externally applied tuning of pressure or other parameters.

"It's kind of a dream system,"said Coleman, also a member of the Rutgers Center for Materials Theory."We've found a material that is intrinsically quantum critical with very simple behavior. It's puzzling, because there's nothing simple about the material's structure. We're not sure why this happens."

Nevidomskyy, now an assistant professor of physics and astronomy at Rice University, likened the discovery of the QCP to finding a black hole in outer space.

"You can't see a black hole because light can't escape from its grip; however, you can observe the gravitational pull that a black hole has on nearby stars,"he said."Similarly, we couldn't see the quantum critical point directly, but we could see evidence of it in the material's magnetic properties and thereby deduce its position underneath the veil of superconductivity."

The discovery that most intrigues the physicists is that beta-YBAL could be revealing an exotic new phase of matter known as the"critical strange metal"phase. At the quantum critical point, the material can shift between conventional electrical behavior, which physicists call a Fermi liquid, to superconducting behavior, and to a condition that resembles neither, called"strange metal"behavior. This behavior has been observed in superconducting materials, but it's not known whether it occurs only in the vicinity of a QCP or whether it can exist over an extended range of physical conditions, which would essentially make it a phase of matter.

Proposed by Nobel laureate Philip Anderson, the idea of strange metal phases has been long debated by physicists."It is extremely controversial,"said Coleman."The experiments our Tokyo colleagues are doing right now might provide more evidence. It could change our basic understanding of materials going forward."

"We are very excited,"said Satoru Nakatsuji, professor and leader of the Tokyo research team."If true, this would be an amazing discovery, opening new horizons in our understanding of quantum criticality."

Coleman praised the working relationship that he and Nevidomskyy have with Nakatsuji's team, including the paper's primary author, Yosuke Matsumoto, and five other researchers: K. Kuga, Y. Karaki, N. Horie, Y. Shimura, and T. Sakakibara. The physicists are affiliated with the University of Tokyo's Institute for Solid State Physics in Kashiwa, Japan.

"In modern science, this interplay between theory and experiment is extremely important,"Coleman said."If you can get a powerful current of ideas going, you can take physics much further. A lot of our work has been done by video conference. Unfortunately with the time difference, it means one of our groups had to get up early while the other had to stay late at night."

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One-dimensional window on superconductivity, magnetism: Atoms are proxies for electrons in ultracold optical emulator

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The research appears this week in the journalNature.Using lithiumatomscooled to within a few billionths of a degree of absolute zero and loaded into optical tubes, the researchers created a precise analog of a one-dimensionalsuperconducting wire.

Because the atoms in the experiment are so cold, they behave according to the same quantum mechanical rules that dictate howelectronsbehave. That means the lithium atoms can serve as stand-ins for electrons, and by trapping and holding thelithiumatoms in beams of light, researchers can observe how electrons would behave in particular types ofsuperconductorsand other materials.

"We can tune the spacing and interactions among theseultracold atomswith great precision, so much so that using the atoms to emulate exotic materials like superconductors can teach us some things we couldn't learn by studying the superconductors themselves,"said study co-author Randy Hulet, a Rice physicist who's leading a team of physicists at Rice and six other universities under the Defense Advanced Research Projects Agency's (DARPA)Optical LatticeEmulator (OLE) program.

In the Nature study, Hulet, Cornell University physicist Erich Mueller, Rice graduate students and postdoctoral researchers Yean-an Liao, Sophie Rittner, Tobias Paprotta, Wenhui Li and Gutherie Partridge and Cornell graduate student Stefan Baur created an emulator that allowed them to simultaneously examine superconductivity and magnetism -- phenomena that do not generally coexist.

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Schematic showing an array of tubes containing lithium atoms. The system is probed by imaging the shadow cast by this ensemble. Image credit: Nature Supplementary Information

Superconductivity occurs when electrons flow in a material without the friction that causeselectrical resistance. Superconductivity usually happens at very low temperatures when pairs of electrons join together in a dance that lets them avoid the subatomic bumps that cause friction.

Magnetism derives from one of the basic properties of all electrons -- the fact that they rotate around their own axis. This property, which is called"spin,"is inherent; like the color of someone's eyes, it never changes. Electron spin also comes in only two orientations, up or down, and magnetic materials are those where the number of electrons with up spins differs from the number with down spins, leaving a"net magnetic moment."

"Generally, magnetism destroys superconductivity because changing the relative number of up and down spins disrupts the basic mechanism of superconductivity,"Hulet said."But in 1964, a group of physicists predicted that a magnetic superconductor could be formed under an exotic set of circumstances where a net magnetic moment arose out of a periodic pattern of excess spins and pairs."

Dubbed the"FFLO"state in honor of the theorists who proposed it -- Fulde, Ferrell, Larkin and Ovchinnikov -- this state of matter has defied conclusive experimental observation for 46 years. Hulet said the new study paves the way for direct observation of the FFLO state.

"The evidence that we've gathered meets the criteria of the FFLO state, but we can't say for certain that we have observed it. To do that, we need to precisely measure the distribution of velocities of the pairs to confirm that they follow the FFLO relationship. We're working on that now."

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Neighbor lends a hand: Spallation Neutron Source's tool to probe ITER's superconducting cable

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ITER is the international research facility in southeastern France whose mission is to demonstrate the feasibility of fusion as a practical long-term energy source. SNS, located a few miles down the road from the U.S. ITER Project Office, is the world's most powerful pulsed neutron source.
Ned Sauthoff, U.S. ITER project manager, said the VULCAN measurements have provided useful data and that VULCAN will have a role in the continuing investigation."Having seen the initial results, I think we have sufficient evidence to state that initial measurements have demonstrated the capability of VULCAN to measure important material properties, that meaningful results were achieved, and that the team is now focused on using the unique capability to gain important understanding aimed at solving the problem,"Sauthoff said.

The central solenoid, a joint Japan-U.S. ITER responsibility, is on a tight schedule. The superconducting cables, supplied by Japan, cost more than $3,000 per meter. Improving the cable performance by reducing the degradation of the superconducting strands is important to staying on schedule and on budget."We are working on an important problem that will have an immediate impact on science and technology on an international scale,"said Ian Anderson, head of ORNL's Neutron Sciences directorate, which operates SNS.

The team of Japanese, U.S. and ITER Organization engineers discovered in late 2010 in a sample test that the superconducting cables making up the central solenoid magnet at the core of the ITER design were losing their current-carrying capacity over time to an extent well beyond that experienced in an earlier ITER model coil test. The cables can generate a magnetic field as strong as 13 tesla, and the electromagnetic (Lorentz) force exerted on the wires by the high magnetic field and powerful current is known to cause some degradation over a period of constant magnetic cycling. The exact cause of the degradation in the conductor sample is unknown.

In addition to the Lorentz force, it may also be attributable to the sample manufacture or the particular sensitivity of the wires to the loads. The magnet team at the U.S. ITER Project Office in Oak Ridge consulted with scientists at SNS about using neutron scattering to examine the states of materials inside the cables. The samples examined at SNS are sections 1.65 inches in diameter and several inches in length cut from the much longer cables. The cables have a complex structure— copper wires interspersed with superconducting wires of a niobium-tin alloy— all contained in a stainless steel tube.

Neutrons are highly penetrating and nondestructive, so neutron scattering can return detailed data about the structure of the cable sections without destroying or altering them. SNS is the ideal facility for studying the thick cables because it has the most intense neutron beams of any pulsed neutron source in the world, said VULCAN instrument scientist Xun-Li Wang. And VULCAN is the ideal instrument because it is designed to handle large industrial-sized specimens rather than small lab samples.

The multi-phase material making up the cable is perfect for characterization by a time-of-flight diffractometer such as VULCAN, Wang noted.
"Neutron diffraction is a well-known technique for mapping strain or stress in engineering materials,"Wang said."With VULCAN we will be able to determine the deformation induced by the Lorentz force."On a fundamental level, we can also study in detail how the critical current in a superconducting wire responds to applied stress and develop a predictive model for the wires."

A plan for future study is being developed with the Japan Atomic Energy Agency, which is responsible for central solenoid conductor fabrication and theITEROrganization.

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Light touch transforms material into a superconductor

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One hundred years aftersuperconductivitywas first observed in 1911, the team from Oxford, Germany and Japan observed conclusive signatures of superconductivity after hitting a non-superconductor with a strong burst oflaser light.

‘We have used light to turn a normal insulator into a superconductor,’ says Professor Andrea Cavalleri of the Department of Physics at Oxford University and the Max Planck Department for Structural Dynamics, Hamburg.‘That’s already exciting in terms of what it tells us about this class of materials. But the question now is can we take a material to a much higher temperature and make it a superconductor?’

The material the researchers used is closely related to high-temperature copper oxidesuperconductors, but the arrangement of electrons and atoms normally act to frustrate any electronic current.

In the journalScience, they describe how a strong infrared laser pulse was used to perturb the positions of some of the atoms in the material. The compound, held at a temperature just 20 degrees above absolute zero, almost instantaneously became a superconductor for a fraction of a second, before relaxing back to its normal state.

Superconductivity describes the phenomenon where an electric current is able to travel through a material without any resistance– the material is a perfect electrical conductor without any energy loss.

High-temperature superconductors can be found among a class of materials made up of layers of copper oxide, and typically superconduct up to a temperature of around–170°C. They are complex materials where the right interplay of the atoms and electrons is thought to‘line up’ the electrons in a state where they collectively move through the material with no resistance.

‘We have shown that the non-superconducting state and the superconducting one are not that different in these materials, in that it takes only a millionth of a millionth of a second to make the electrons“synch up” and superconduct,’ says Professor Cavalleri.‘This must mean that they were essentially already synched in the non-superconductor, but something was preventing them from sliding around with zero resistance. The precisely tuned laser light removes the frustration, unlocking the superconductivity.’

The advance immediately offers a new way to probe with great control how superconductivity arises in this class of materials, a puzzle ever since high-temperature superconductors were first discovered in 1986.

But the researchers are hopeful it could also offer a new route to obtaining superconductivity at higher temperatures. If superconductors that work at room temperature could be achieved, it would open up many more technological applications.

‘There is a school of thought that it should be possible to achieve superconductivity at much higher temperatures, but that some competing type of order in the material gets in the way,’ says Professor Cavalleri.‘We should be able to explore this idea and see if we can disrupt the competing order to reveal superconductivity at higher temperatures. It’s certainly worth trying!’

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Physicists observe exotic state in an unconventional superconductor

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"We've been on the trail of astate of mattercalled a half-quantumvortexfor more than three years,"said Budakian."First proposed in the 1970s to exist in superfluid helium-3, a half-quantum vortex can be thought of as a 'texture' that arises from the spin phase of the superconducting order parameter."

Budukian's group investigated strontium ruthenium oxide (SRO), an unconventional superconductor that has been proposed as the solid-state analog of the A-phase of superfluid helium-3. Using state-of-the-art nanofabrication methods and exquisitely sensitive cantilever-based magnetometry techniques developed by the group, the researchers observed minute fluctuations in the magnetism of tiny rings of SRO.

"Strontium ruthenium oxide is a unique and fascinating material, and the half-quantum vortices that have been conjectured to exist in it are particularly interesting,"said Anthony J. Leggett, the John D. and Catherine T. MacArthur Professor and Center for Advanced Study Professor of Physics, who shared the 2003 Nobel Prize in Physics for his work on superfluid helium-3."It is believed that these half-quantum vortices in SRO may provide the basis for topologicalquantum computing. If this novel form of computing is eventually realized, this experiment will certainly be seen as a major milestone along the road there."

Budakian is an assistant professor of physics and a principal investigator in the Frederick Seitz Materials Research Laboratory at Illinois. Five years ago, he was instrumental in pioneering a technique, magnetic resonance force microscopy, to measure the force exerted on a micrometer-scale silicon cantilever by the spin of a single electron in a bulk material. He and his group have now adapted their ultrasensitive cantilever measurements to observe the magnetic behavior of SRO.

In the experiment, the researchers first fabricated a micron-sized ring of SRO and glued it to the tip of the silicon cantilever. How small are these rings? Fifty of them would fit across the width of a human hair. And the tips of the cantilevers are less than 2μm wide.

"We take the high-energy physics approach to making these rings. First we smash the SRO, and then we sift through what's left,"said Budakian.

The researchers first pulverize the large crystals of SRO into fragments, choose a likely micron-sized flake, and drill a hole in it using a focused beam of gallium ions. The resulting structure, which looks like a microscopic donut, is glued onto the sensitive silicon cantilever and then cooled to 0.4 degrees above absolute zero.

"Positioning the SRO ring on the cantilever is a bit like dropping one grain of sand precisely atop a slightly larger grain of sand,"said Budakian,"only our 'grains of sand' are much smaller."

Budakian added that this technique is the first time such tiny superconducting rings have been fabricated in SRO.

Being able to make these rings is crucial to the experiment, according to Budakian, because the half-quantum vortex state is not expected to be stable in larger structures.

"Once we have the ring attached to the cantilever, we can apply static magnetic fields to change the 'fluxoid' state of the ring and detect the corresponding changes in the circulating current. In addition, we apply time-dependent magnetic fields to generate a dynamic torque on the cantilever. By measuring the frequency change of the cantilever, we can determine themagneticmoment produced by the currents circulating the ring,"said Budakian.

"We've observed transitions between integer fluxoid states, as well as a regime characterized by 'half-integer' transitions,"Budakian noted,"which could be explained by the existence of half-quantum vortices in SRO."

In addition to the advance in fundamental scientific understanding that Budakian's work provides, the experiment may be an important step toward the realization of a so-called"topological"quantum computer, as Leggett alluded.

Unlike a classical computer, which encodes information as bits whose values are either 0 or 1, a quantum computer would rely on the interaction among two-level quantum systems (e.g., the spins of electrons, trapped ions, or currents in superconducting circuits) to encode and process information. The massive parallelism inherent in quantal time evolution would provide rapid solutions to problems that are currently intractable, requiring vast amounts of time in conventional, classical machines.

For a functional quantum computer, the quantum bits or"qubits"must be strongly coupled to each other but remain sufficiently isolated from random environmental fluctuations, which cause the information stored in the quantum computer to decay—a phenomenon known as decoherence. Currently, large-scale, international projects are underway to construct quantum computers, but decoherence remains the central problem for real-world quantum computation.

According to Leggett,"A rather radical solution to the decoherence problem is to encode thequantum informationnonlocally; that is, in the global topological properties of the states in question. Only a very restricted class of physical systems is appropriate for such topologicalquantum computing, and SRO may be one of them, provided that certain conditions are fulfilled in it. One very important such condition is precisely the existence of half-quantum vortices, as suggested by the Budakian experiment."

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Hot booze turns material into a superconductor

The scientist, Dr. Yoshihiko Takano of the National Institute for Materials Science (NIMS) in Tsukuba, Japan, made the discovery after a party, soaking samples of a potential superconductor in hot alcoholic drinks before testing them next day for superconductivity. The commercialalcoholic beverages, especially wine, were much more effective than either water or pure alcohol.

Superconductorsare metallic substances that allow electricity to flow through them with zero resistance below a certain temperature. Those found so far only work at very low temperatures (often as low as nearabsolute zero), and so finding one that works at room temperature could have important applications, such as power lines with superconducting cables, and perhaps inlevitationof large objects like trains, since superconductors can repel magnetic fields. The phenomenon is still not completely understood even though superconductors have been known since their discovery in 1911 by a Dutch scientist Heike Kamerlingh Onnes.

The researchers created the samples of FeTe_0.8S_0.2by sealingiron(Fe), tellurium (Te) and tellurium sulfide (TeS) powders into an evacuate quartz tube and heating the mixture at 600°C for 10 hours. This material is not normally a superconductor but can become one if exposed to oxygen or if soaked in water.

After a party for a visiting researcher Takano wondered if the drinks they were consuming would work as well as pure water. To find out, they tested the FeTe_0.8S_0.2samples with beer, red and white wine, Japanese sake, Shochu (a clear distilled liquor) and whisky, and with various concentrations of ethanol and water. The samples were all heated and kept at 70°C for 24 hours.

The results were that the ethanol-water samples showed increased superconductivity that was not dependant on the ethanol concentration. The samples heated inalcoholic drinksall showed greater superconductivity, but again not dependant on the alcohol content. Red wine was the most effective. The research team calculated the superconducting volume fraction of the samples and found they ranged from 23.1% for Sochu up to 62.4% for red wine, but none of the ethanol samples were over 15%.

The authors speculate that because wine and beer oxidize easily and since oxygen induces superconductivity in the material, the beverages could be playing an important role in supplying oxygen into the sample as a catalyst. Further research is needed to confirm the exact mechanism.

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Superconductors face the future

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But superconductors, especially superconducting electromagnets, have been around for a long time. Indeed the first large-scale application of superconductivity was in particle-physics accelerators, where strong magnetic fields steer beams of charged particles toward high-energy collision points.

Accelerators created the superconductor industry, andsuperconducting magnetshave become the natural choice for any application where strong magnetic fields are needed - for magnetic resonance imaging (MRI) in hospitals, for example, or for magnetic separation of minerals in industry. Other scientific uses are numerous, from nuclear magnetic resonance to ion sources for cyclotrons.

Some of the strongest and most complex superconducting magnets are still built for particle accelerators like CERN’sLarge Hadron Collider(LHC). The LHC uses over 1,200 dipole magnets, whose two adjacent coils of superconducting cable create magnetic fields that bend proton beams traveling in opposite directions around a tunnel 27 kilometers in circumference; the LHC also has almost 400 quadrupole magnets, whose coils create a field with fourmagnetic polesto focus the proton beams within thevacuum chamberand guide them into the experiments.

These LHC magnets use cables made of superconducting niobium titanium (NbTi), and for five years during its construction the LHC contracted for more than 28 percent of the world’s niobium titanium wire production, with significant quantities of NbTi also used in the magnets for the LHC’s giant experiments.

What’s more, although the LHC is still working to reach the energy for which it was designed, the program to improve its future performance is already well underway.

Designing the future

“Enabling the accelerators of the future depends on developing magnets with much greater field strengths than are now possible,” says GianLuca Sabbi of Berkeley Lab’s Accelerator and Fusion Research Division (AFRD). “To do that, we’ll have to use different materials.”

Field strength is limited by the amount of current a magnet coil can carry, which in turn depends on physical properties of the superconducting material such as its critical temperature and critical field. Most superconducting magnets built to date are based on NbTi, which is a ductile alloy; the LHC dipoles are designed to operate at magnetic fields of about eight tesla, or 8 T. (Earth’s puny magnetic field is measured in mere millionths of a tesla.)

The LHC Accelerator Research Program (LARP) is a collaboration among DOE laboratories that’s an important part of U.S. participation in the LHC. Sabbi heads both the Magnet Systems component of LARP and Berkeley Lab’s Superconducting Magnet Program. These programs are currently developing accelerator magnets built with niobium tin (Nb3Sn), a brittle material requiring special fabrication processes but able to generate about twice the field of niobium titanium. Yet the goal for magnets of the future is already set much higher.

“Among the most promising new materials for future magnets are some of the high-temperature superconductors,” says Sabbi. “Unfortunately they’re very difficult to work with.” One of the most promising of all is the high-temperature superconductor Bi 2212 (bismuth strontium calcium copper oxide).

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In the process called“wind and react,” Bi-2212 wire - shown in cross section, upper right, with the powdered superconductor in a matrix of silver - is woven into flat cables, the cables are wrapped into coils, and the coils are gradually heated in a special oven (bottom).

“High temperature” is a relative term. It commonly refers to materials that become superconducting above the boiling point of liquid nitrogen, a toasty 77 kelvin (95 K, or -321 degrees Fahrenheit). But in high-field magnets even high-temperature superconductors will be used at low temperatures. Bi-2212 shows why: although it becomes superconducting at 95 K, its ability to carry high currents and thus generate a high magnetic field increases as the temperature is lowered, typically down to 4.2 K, the boiling point of liquid helium at atmospheric pressure.

In experimental situations Bi-2212 has generated fields of 25 T and could go much higher. But like many high-temperature superconductors Bi 2212 is not a metal alloy but a ceramic, virtually as brittle as a china plate.

As part of the Very High Field Superconducting Magnet Collaboration, which brings together several national laboratories, universities, and industry partners, Berkeley Lab’s program to develop new superconducting materials for high-field magnets recently gained support from the American Recovery and Reinvestment Act (ARRA).

Under the direction of Daniel Dietderich and Arno Godeke, AFRD’s Superconducting Magnet Program is investigating Bi-2212 and other candidate materials. One of the things that makes Bi-2212 promising is that it is now available in the form of round wires.

“The wires are essentially tubes filled with tiny particles of ground-up B-2212 in a silver matrix,” Godeke explains. “While the individual particles are superconducting, the wires aren’t - and can’t be, until they’ve been heat treated so the individual particles melt and grow new textured crystals upon cooling - thus welding all of the material together in the right orientation.”

Orientation is important because Bi-2212 has a layered crystalline structure in which current flows only through two-dimensional planes of copper and oxygen atoms. Out of the plane, current can’t penetrate the intervening layers of other atoms, so the copper-oxygen planes must line up if current is to move without resistance from one Bi-2212 particle to the next.

In a coil fabrication process called“wind and react,” the wires are first assembled into flat cables and the cables are wound into coils. The entire coil is then heated to 888 degrees Celsius (888 C) in a pure oxygen environment. During the “partial melt” stage of the reaction, the temperature of the coil has to be controlled to within a single degree. It’s held at 888 C for one hour and then slowly cooled.

Silver is the only practical matrix material that allows the wires to“breathe” oxygen during the reaction and align their Bi-2212 grains. Unfortunately 888 C is near the melting point of silver, and during the process the silver may become too soft to resist high stress, which will come from the high magnetic fields themselves: the tremendous forces they generate will do their best to blow the coils apart. So far, attempts to process coils have often resulted in damage to the wires, with resultant Bi 2212 current leakage, local hot spots, and other problems.

“The goal of the program to develop Bi-2212 for high-field magnets is to improve the entire suite of wire, cable, coil making, and magnet construction technologies,” says Dietderich. “The magnet technologies are getting close, but the wires are still a challenge. For example, we need to improve current density by a factor of three or four.”

Once the processing steps have been optimized, the results will have to be tested under the most extreme conditions.“Instead of trying to predict coil performance from testing a few strands of wire and extrapolating the results, we need to test the whole cable at operating field strengths,” Dietderich says. “To do this we employ subscale technology: what we can learn from testing a one-third scale structure isreliable at full scale as well.”

Testing the results

Enter the second part of ARRA’s support for future magnets, directed at the Large Dipole Testing Facility.

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The LD1 test magnet design in cross section. The 100 by 150 millimeter rectangular aperture, center, is enclosed by the coils, then by iron pressure pads, and then by the iron yoke segments. The outer diameter of the magnet is 1.36 meters.

“The key element is a test magnet with a large bore, 100 millimeters high by 150 millimeters wide - enough to insert pieces of cable and even miniature coils, so that we can test wires and components without having to build an entire magnet every time,” says AFRD’s Paolo Ferracin, who heads the design of the Large Dipole test magnet.

Called LD1, the test magnet will be based on niobium-tin technology and will exert a field of up to 15 T across the height of the aperture. Inside the aperture, two cable samples will be arranged back to back, running current in opposite directions to minimize the forces generated by interaction between the sample and the external field applied by LD1.

The magnet itself will be about two meters long, mounted vertically in a cryostat underground. LD1’s coils will be cooled to 4.5 K, but a separate cryostat in the bore will allow samples to be tested at temperatures of 10 to 20 K.

“There are two aspects to the design of LD1,” says Ferracin. “The magnetic design deals with how to put the conductors around the aperture to get the field you want. Then you need a support structure to deal with the tremendous forces you create, which is a matter of mechanical design.” LD1 willgenerate horizontal forces equivalent to the weight of 10 fully loaded 747s; imagine hanging them all from a two-meter beam and requiring that the beam not move more than a tenth of a millimeter.

What’s more, Ferracin says, since one of the most important aspects of cables and model coils is their behavior under stress, “we need to add mechanical pressure up to 200 megapascals” - 30,000 pounds per square inch. “We have developed clamping structures that can provide the required force, but devising a mechanism that can apply the pressure during a test will be another major challenge.”

The cable samples and miniature coils will incorporate built-in voltage taps, strain gauges, and thermocouples so their behavior can be checked under a range of conditions, including quenches - sudden losses of superconductivity and the resultant rapid heating, as dense electric currents are dumped into conventional conductors like aluminum or copper.

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At top, two superconducting coils enclose a beam pipe. Field strength is indicated by color, with greatest strength in deep red. To test components of such an arrangement, subscale coils (bottom) will be assessed, starting with only half a dozen cable winds generating a modest two or three tesla, increasing to hybrid assemblies capable of generating up to 10 T.

The design of the LD1 is based on Berkeley Lab’s prior success building high-field dipole magnets, which hold the world’s record for high-energy physics uses. The new test facility will allow testing the advanced designs for conductors and magnets needed for future accelerators like the High-Energy LHC and the proposed Muon Collider.

“These magnets are being developed to make the highest-energy colliders possible,” says Sabbi. “But as we have seen in the past, the new technology will benefit many other fields as well, from undulators for next-generation light sources to more compact medical devices. ARRA’s support for LD1 isan investment in the nation’s science and energy future.”

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