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Scientists at the University of Cambridge have for the first time identified a key component to unravelling the mystery of room temperature superconductivity, according to a paper published in the journal Nature.
"We have successfully unearthed for the first time in a high temperature superconductor the location in the electronic structure where 'pockets' of doped hole carriers aggregate. Our experiments have thus made an important advance toward understanding how superconducting pairs form out of these hole pockets."
Magnetic fields penetrate the superconducting state in an array of vortices where superconductivity is locally destroyed, providing a novel kind of microscopic lens. The vortex cores give us a glimpse into the competing states formed where superconductivity is destroyed, comprising the same copper spins involved in superconductivity. Recent quantum oscillation experiments indicate that this competing order manifests itself as long range order in very strong magnetic fields as vortices become tightly packed. Inter-vortex tunnelling of both electrons (red) and holes (cyan) facilitate their motion in cyclotron orbits, just like in normal metals. While prior diffraction experiments have identified magnetism as a potential contender for competing order in the vortex cores, cyclotron orbit sizes measured by the current experiments indicate a possible modulated form of magnetism, depicted in the figure.
We have successfully unearthed for the first time in a high temperature superconductor the location in the electronic structure where 'pockets' of doped hole carriers aggregate.
Though the “heavy electron superconductors” do not superconduct at as high a temperature as copper and iron based materials do, they have a few properties which may make them desirable as observation materials. First, these heavy electron superconductors have active electrons in higher orbitals than traditional high-temperature materials. Because these crucial electrons are in the f-orbitals of heavy electron superconductors, as opposed to the d-orbitals of copper/iron superconductors, it may be easier to study and understand their interactions.
That's why the discovery earlier this year of an entirely new class of superconducting materials has sent scientists into a frenzy of activity. Although these new superconductors don't break any temperature records just yet, they are breaking all the known rules of superconductivity because they are made from iron. Even better, they offer the tantalising possibility that a little tinkering could raise their operating temperatures, perhaps to as high as room temperature. They could even help us crack one of the deepest mysteries in physics by explaining how all superconductors work.
When the temperature reached 150 K, the researchers noticed that the shape of the lattice became distorted. As the temperature continued to drop, something interesting happened around 134 K. Instead of all pointing in one direction, the electron spins on neighbouring atoms lined up in opposite directions - a phenomenon called antiferromagnetism.
Next, the team repeated the experiment using a sample doped with fluorine atoms. The extra electrons provided by the fluorine changed the profile completely. Gone were both the lattice distortion and the antiferromagnetic order; instead the doped sample lost all its resistance. Astonishingly, this pattern was similar to what happens in high-temperature copper oxide superconductors.
The puzzling phenomena, which the scientists solved, was that in normal superconductors raising the binding energy, to hold these pairs together raises the critical temperature closer to room temperature. However, in cuprate superconductors, which have higher starting temperatures, raising the binding energy actually lowers the Tc, the opposite of the desired result.
Researchers determined that this is due to a "quantum traffic jam" effect. Normally cuprates are stuck in a jammed stated known as the Mott insulating state, named after the late Sir Neville Mott of Cambridge, UK. To create cuprate superconductors, electrons are removed from cuprates, leaving holes. Cooper pairs can then start to flow into these holes, allowing for superconduction, akin to a couple cars exiting the highway during rush hour starting traffic moving.
However, the critical discovery the researchers made was that increasing the binding energy also increased the "Mottness" of cuprate superconductors. Thus, raising the temperature only made the traffic jam worse, lowering the critical temperature. Seamus Davis of Brookhaven National Laboratory and Cornell University, lead author on the paper describes, "It has been a frustrating and embarrassing problem to explain why this is the case."
The super-powerful magnet will be used to "test the properties of newly discovered high-temperature superconductors like iron oxyarsenide, which may improve the performance of MRI machines and high-voltage power lines while lowering their cost."
Via the conventional powder-in-tube method, a research team led by MA Yanwei with the CAS Institute of Electrical Engineering has been successful in fabricating LaO0.9F0.1FeAs wires with the superconducting critical transition temperature (Tc) of about 25K. The wire, first of its kind in the world, is of importance for possible practical applications, according to experts. The work was published in a recent issue of Superconductor Science and Technology after having filed for a patent right.
Since 1911, several families of superconductors have been discovered; each challenged our understanding of the intriguing physics. In 2008, a family of new Fe superconductors of RFeAsO1-xFx, (Ba-K)Fe2As2, and others has been discovered that contain the puckered FeAs planes instead of the hallmark CuO2 planes in the cuprate superconductors. Central to any superconductor is the nature of the superconducting gap, its value, its structure if any, and its temperature dependence. We used Andreev reflection spectroscopy to investigate the gap of these new Fe superconductors, and compared with those measured by other techniques. The relevant physics in the Fe superconductors appears to be not electron-phonon interaction as in conventional low-TC superconductors, nor Mott physics as in high-TC cuprates.
When an electrical current passes through a wire it emanates heat – a principle that's found in toasters and incandescent light bulbs. Some materials, at low temperatures, violate this law and carry current without any heat loss. But this seemingly trivial property, superconductivity, is now at the forefront of our understanding of physics.
In the journal Science, Andrea Bianchi, a professor in the Department of Physics at the Université de Montréal, and his colleagues show that, contrary to previous belief, superconductivity can induce magnetism, which has raised a new quantum conundrum.
In the experiment reported in Science, the scientists cooled a single crystal of CeCoIn5, a metal compound consisting of cerium, cobalt and indium, to a temperature of minus 273.1 degrees, close to absolute zero. To their great surprise, they discovered that magnetism and superconductivity coexist and disappear at the same time when they heat the sample or increase the magnetic field.
This discovery is extraordinary, since magnetic order exists exclusively when this sample is in the superconducting state. In this unique case, magnetism and superconductivity do not compete with each other. Instead, superconductivity generates magnetic order.
Cornell researchers and colleagues have produced the first atomic-scale description of what electrons are doing in the mysterious "pseudogap" in high-temperature superconductors.
Breaking the locked-in pairs required more energy than to break moving pairs. In theory, the more tightly bound the electron pairs are, the more they resist being pulled apart as the temperature rises. But it's a catch-22, Davis said. "When there are few holes, allowing the pairs to be tightly bound, the pairs are not free to move around, and when there are plenty of holes, allowing them to move around, they are too weakly bound to survive higher temperatures."
So to make higher-temperature superconductors, he proposes, requires creating a material where strong pairing occurs, but without the "traffic jam" created by a shortage of holes.
In the October 9, 2008, issue of Nature, scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory report that they have successfully produced two-layer thin films where neither layer is superconducting on its own, but which exhibit a nanometer-thick region of superconductivity at their interface. Furthermore, they demonstrate the ability to elevate the temperature of superconductivity at this interface to temperatures exceeding 50 kelvin (-370°F), a relatively high temperature deemed more practical for real-world devices.
"The preliminary results are amazing," McQueeney said. "I have experience with a similar instrument and ARCS blew it away," adding that it produces better results from smaller samples in a much shorter time frame.
The iron-based material's behavior under pressure may suggest the remarkable possibility of an entirely different mechanism behind superconductivity than with copper oxide materials, NIST Fellow Jeffrey Lynn said. Or it could be that magnetism is simply an ancillary part of HTc superconductivity in general, he said—and that a similar, deeper mechanism underlies the superconductivity in both. Understanding the origin of the superconductivity will help engineers tailor materials to specific applications, guide materials scientists in the search for new materials with improved properties and, scientists hope, usher in higher-temperature superconductors.
Scientists studying a material that appeared to lose its ability to carry current with no resistance say new measurements reveal that the material is indeed a superconductor — but only in two dimensions. Equally surprising, this new form of 2-D superconductivity emerges at a higher temperature than ordinary 3-D superconductivity in other compositions of the same material.
Scientists at U.S. Department of Energy's Argonne National Laboratory used inelastic neutron scattering to show that superconductivity in a new family of iron arsenide superconductors cannot be explained by conventional theories.