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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