How to identify chiral superconductivity in new materials

Beenakker is a scientist at the Instituut-Lorentz of Leiden University in The Netherlands. Along with Serban, Béri and Akhmerov, Beenakker is in a group that says it has produced a test for determining whether or not a material meets the criteria for chiral p-wavesuperconductivity. The work of the group is described inPhysical Review Letters:“Domain Wall in a Chiral p-Wave Superconductor: A Pathway for Electrical Current.”

“Efforts are going into creating a chiral superconductor, in which there is transport in one direction, instead of two. This superconductor would have electrons moving in only one direction,” Beenakker explains. “This has been seen in the quantum Hall effects, and scientists are interested in other systems that would show similar characteristics of electrons moving in one direction without resistance.”

Right now, chiral transport in a superconductor is difficult to detect. While many labs and scientific groups are laboring with different materials to create chiral transport in a superconductor, there are challenges to actually knowing when this is accomplished. This is where Beenakker and his colleagues, along with their test, come in.“We propose what should work as a test to verify that a chiral p-wave superconductor has, in fact, been created,” he says.

The test would be administered by first hooking up a wire to the opposite ends of a domain wall of the material.“Next, we would apply a voltage to see if you can send current from one side to the other,” Beenakker says. “Then, you could invert the voltage, to see if the current can flow in the opposite direction. In this way you could find out whether it is going in only one direction.”

Such a test is a step in efforts to developsuperconductorsthat could be used for a variety of applications in the future.“We are theorists, coming up with ideas that could be useful in experiments,” Beenakker explains. “This test could be used in the development of future superconductor technology. A group would say that they think they have developed a chiral superconductor, and then they could use this test to determine whether or not it truly is such a superconductor. This test provides a way to observe chirality in a way that has not been available up to this point.”

Beenakker says that the work of the group in The Netherlands is especially exciting since it could lead to different ways of building quantum computers.“Chiral p-wave superconductors are among the candidates for supercomputer platforms,” he points out. “Being able to find these materials, would be very helpful in moving forward with quantum computing. If p-wave superconductors really exist, and we can make them accessible and robust in labs, itcould be a significant step forward in terms of making the building blocks of quantum computing.”

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New mechanism for superconductivity discovered in iron-based superconductors

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In classical theory,superconductivityoccurs when two electrons are bound together to form a pair, known as a Cooper pair, by lattice vibrations. This pairing mechanism, however, has never been confirmed for high-temperature superconductors, whose transition temperatures well above the theoretical limit of about 40 K pose an enigma for condensed matter physics.

The iron-based superconductors investigated by the research team, first discovered in 2008 by Japanese researchers, offer the greatest chance of solving this enigma. With a maximumtransition temperatureof 55K, these superconductors are governed by an electron pairing mechanism that is different from earliersuperconductorsmediated by lattice vibrations, one based on two types of electrons with different momenta.

Magnetic-field induced change in the intensity of electronic standing waves evidencing the“s±-wave” structure. When magnetic field is applied there appear two types of spots; one is enhanced by the field (blue) and the other is suppressed by the field (red). This behavior is evidence of the “s±-wave” structure of Cooper pairs, strongly suggesting a magnetism-related pairing mechanism.

To analyze this complex pairing mechanism, the researchers applied scanning tunnelling microscopy to electron pairing in Fe(Se, Te), the iron-based superconductor with the simplest crystal structure. Imaging electronic standing waves produced by scattering interference under a powerful 10-Tesla magnetic field, they found that Cooper pairs adopted a characteristic“s±-wave” structure that is unique to a material with two types of electrons.

The discovery of s±-wave structure breaks new ground by supporting a mechanism for electron pairing based not on lattice vibrations, as in other forms of superconductivity, but on magnetism. In providing a powerful constraint ontheoretical models, the finding thus marks a major advance toward unraveling the mystery of high-temperature superconductivity.

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Watching the Tug of War between Structure and Superconductivity

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Relatively new to the field of superconductivity, iron-based materials are now world-famous for their frictionless transport of electrons at"high"temperatures— above about 50 Kelvin, or -190 degrees Celsius.

"Iron-based superconductors have second highesttransition temperaturethat anyone knows about, a characteristic that's very important if we want to use them for practical applications,"said Princeton researcher Robert Cava."All aspects of research on these materials are now underway. Our part of the picture is to try and understand why they're superconducting in the first place."

Cava and a group of fellow researchers from Princeton, Stony Brook University, Brookhaven, and Johannes Gutenberg University, in Germany, focused their investigations on a particular material made from conducting layers of iron and selenium, called ironselenide. In recent years, scientists have explored numerous aspects of the underlying physics of iron-based superconductors, often making connections to the material's structure or innate magnetism. But the exact relationships between these properties were unclear.

"In order for superconductivity to exist, it must arise as the winner in a tug of war between different physical properties,"Cava said."The research community has known that magnetism competes with superconductivity in the ironsuperconductors, but no one had a good idea of how crystal structure competed."

Cava's group compared the structures of superconducting and non-superconducting iron selenide using two different types of tools: powerful beams of x-rays at the NSLS and a suite of advanced microscopes at the CFN. At NSLS beamline X16C, the researchers used synchrotron x-ray powder diffraction to provide"snapshots"of the materials on the order of hundreds of nanometers. They combined that data with images taken with transmission electron microscopy and electron diffraction, resulting in resolution an order of magnitude higher than the x-ray technique.

"We needed both of these sophisticated techniques to really understand what was going on here,"Cava said, adding that the findings weren't as straightforward as expected.

Among the results, which were published in the July 31, 2009 edition ofPhysical Review Letters, the group showed that the superconducting form of iron selenide can be distinguished from the non-superconducting form by a change in its structure. The superconductor gains its power by giving way to a slight structural"distortion,"and the non-superconductor holds strong.

The researchers also showed that this unique structural change is unrelated to magnetism, as hypothesized in the past. The material's fundamental bonding— the bonding between atoms — stays the same while the angles between the bonds change, similar to the expansion of a baby gate. Unlike a gate, the angles only change by a few degrees — still enough, though, to give birth to superconductivity.

But the real surprise is that this structural change actually exists in both the superconducting and non-superconducting forms of the material, just on different scales and with different effects.

"This distortion can still be found in the non-superconducting material, but it's only present over tens of atoms,"Cava said."In the superconducting material, it's present over very long distances. This detail was hidden until we were able to take a really close look at the materials. The challenge now is to figure out over what distance acrystal structurehas to be distorted in order to affect its properties."

Next, Cava's group will try to resolve the structure-superconductivity relationships within other iron-basedmaterials.

"By showing the scientific community this clear example, we hope to inspire them to think about manipulating this phase transition,"he said.

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New property in warm superconductors discovered

For 25 years, they’ve speculated that magnetism could be a problem.

TheProceedings of the National Academy of Scienceshas published the finding that there is a weak magnetism in a certain type of lanthanum-based copper oxide material, which is the closest known warm-temperature superconductor.

Sonier, the lead scientist in this research, says:“The search for room-temperaturesuperconductivityis big news. The cover story of the June 2010 issue ofScientific Americanpredicted the discovery would be one of the‘12 events that will change everything.’”

Superconductors, materials that have zero electrical resistance, could potentially drive ever day devices in electronics, medicine and transportation, but are super expensive because they only operate at extremely low temperatures. Ifsuperconductorswere operational at room temperature they wouldn’t need to be driven by expensive cooling systems using liquid helium.

When charge carriers are added to copper oxide materials, known as cuprates, they are capable of superconductivity. Some cuprates function at -140 degrees Celsius, a temperature markedly above -240 degrees Celsius, which is the normal operational temperature of all other kinds of superconducting materials.

Adding charge carriers (electric charge carrying particle) is known as chemical doping. With increased chemical doping the operational temperature of a cuprate superconductor rises to a certain point and then collapses.

Until this latest research, scientists could only speculate on whether a competing magnetic phase might exist during high chemical doping and ultimately destroy their superconductivity.

Sonier and his colleagues used a subatomic particle, called a muon, to microscopically probe the magnetic nature of a cuprate. This led them to discover that a strange kind of magnetism appears to accompany the destruction of superconductivity during high chemical doping.

The scientists are now trying to figure out the origin of the magnetism and whether it actually competes with superconductivity.

Sonier says,“Understanding what destroys superconductivity during high chemical doping could provide a vital clue about the microscopic mechanism responsible for high-temperature superconductivity. Knowledge of this would be a monumental step toward making a room-temperature superconductor.”

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Roller coaster superconductivity discovered

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Until now, copper-laden materials called cuprates have been the only superconductors whose transition temperatures are higher than the liquid nitrogen boiling point at -321F (77 K). Whether researchers can make transition temperatures higher in such materials remains a challenge.

Now, researchers at the Carnegie Institution's Geophysical Laboratory, with colleagues, have unexpectedly found that the transition temperature can be induced under two different intense pressures in a three-layered bismuth oxide crystal referred to as"Bi2223."The higherpressureproduces the higher transition temperature. They believe this unusual two-step phenomena comes from competition of electronic behavior in different kinds of copper-oxygen layers in the crystal. The work is published in the August 19, 2010, issue ofNature.

"Bi2223 is like a layered cake,"explained lead author Xiao-Jia Chen at Carnegie."On the top and bottom there are insulating bismuth-oxide layers. On the inside of those, come layers of strontium oxide. Next, are layers ofcopper oxide, then calcium, and finally the middle is another copper-oxide layer. Interestingly, the outermost and inner layers of copper oxide have different physical properties resulting in an imbalance of electric charge between the layers."

One way scientists have found to increase the transition temperature ofsuperconducting materialsis to"dope"them by adding charged particles.

Under normal pressure, the optimally doped Bi2223's transition temperature is -265F (108K). The scientists subjected doped crystals of the material to a range of pressures up to 359,000 times the atmospheric pressure at sea level (36.4 Giga Pascal), the highest pressure yet for magnetic measurements in cuprate superconductors. The first higher transition temperature happened at 100,666 atmospheres (10.2 GPa).

"After that, increasing pressures ended up with lower transition temperatures,"remarked Chen."Then to our complete surprise at about 237,000 atmospheres (24 GPa) thesuperconducting statereappeared. Under even more pressure, 359,000 atmospheres, the transition temperature rose to -215F (136K). That was the highest pressure our measuring system could detect."

Other research has shown that some multilayered superconducting materials like this one exhibit different electronic and vibrational behaviors in different layers. The researchers think that 237,000 atmospheres might be a critical point where pressure suppresses one behavior and enhances superconductivity.

"The finding gives new perspectives on making highertransition temperaturein multilayer cuprate superconductors. The research may offer a promising way of designing and engineeringsuperconductorswith much higher transition temperatures at ambient conditions,"concluded coauthor Viktor Struzhkin also of Carnegie.

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New superconductor research may solve key problem in physics

The data Lawler analyzed have been available for several years, but have not been well understood until now."The pattern looked so mysterious and interesting,"he said."It's so different from any other material we've ever looked at. Trying to understand what this data is really trying to tell us has been one of our big ambitions, and we think we have captured one of its essential ingredients."

Lawler, a theoretical physicist, worked with physicists at Cornell University, Brookhaven National Laboratory and laboratories in Japan and Korea on this research. They found what may be the key to unlocking the secrets of the so-called"pseudogap phenomenon"in superconductors.

The"pseudogap phenomenon"is the remarkable vanishing of the low-energy electronic excitations in high-temperature superconductors. A material experiencing this rare phenomenon becomes mostly insulating but otherwise behaves like a superconductor. And because this can happen at room temperature, scientists believe it may be possible forsuperconductivityto exist at these temperatures.

Superconductors are materials - often but not always metals - thatconduct electricitywithout resistance below a certain temperature. For decades, it was thought that these materials could conduct electricity only at temperatures far below freezing. In the last 20 years, however, scientists have discovered several compounds that superconduct at much higher temperatures.

In principle, a room-temperature superconductor could allow:

• Electricity to travel with zeroenergy lossfrompower plantsto houses.
  
• High-speed trains to float on top of the superconductor.
  
• Cell phone towers that could handle many cell phone carriers in high-population areas.

"It's one of the most interesting problems that we have in physics,"Lawler said."I believe that having a challenge at that level can help produce breakthroughs in science."

He and his colleagues found that the electronic states of two neighboring oxygen atoms in these superconductors are different from each other. Looking at the electronic structure, then, the physicists were able to observe a broken symmetry."It is like the electronic states were stretched along the X-direction compared to the Y-direction,"Lawler said."That the pseudogap phase has this order allows us to make the bold claim that it is actually a distinct phase of electronic matter."

To understand this observation better, consider the phases of rod-like objects. Rod-like polymers have many more phases than the solid, liquid and gas phases of more ordinary atoms. At high temperatures, they are in a gas phase like such atoms. However, at lower temperatures, all the rods can point in one direction while still moving around freely like a gas or liquid. Physicists call this a"nematic phase."The organization of the rods in this phase is similar to what the researchers observed in the electronic states associated with the pseudogap phenomena.

More phases of rod-like objects exist at lower temperatures until eventually the rods freeze into a crystal. Physicists call these intermediate phases"liquid-crystal phases."They are responsible for the liquid crystal displays commonly used in watches and televisions.

Lawler, who joined Binghamton's faculty in 2008, earned his PhD at the University of Illinois at Urbana-Champaign and was a postdoctoral scholar at the University of Toronto. A self-described"pencil-and-paper theorist,"he is open to discovery in unexpected places. That was certainly the case with this project, as the inspiration for the data analysis came to him while he was shopping at Home Depot.

The researchers' success, Lawler said, is owed to both the unusual data analysis—which is derived from radio technology - and the unique capabilities of his Cornell colleagues, who have a scanning tunneling microscope that enables them to look at single atoms while maintaining a large field of view.

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