Like astronomers tweaking images to gain a more detailed glimpse of distant stars, physicists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have found ways to sharpen images of the energy spectra in high-temperature superconductors — materials that carry electrical current effortlessly when cooled below a certain temperature. These new imaging methods confirm that the electron pairs needed to carry current emerge above the transition temperature, before superconductivity sets in, but only in a particular direction.
"Our findings rule out certain explanations for the development of superconductivity in these materials, and lend support to other, competing theories," said Brookhaven physicist Peter Johnson, leader of the group whose work is described in the November 6, 2008, issue of Nature. Honing in on the mechanism for high-temperature (high-Tc) superconductivity may help scientists engineer new materials to make use of the current-carrying phenomenon in transformative applications such as high-efficiency transmission lines in the U.S. power grid.
Scientists already know that electrons in a superconducting material must pair up to carry the current. But whether these pairs form at or above the transition temperature has been a mystery, until now.
To search for pre-formed electron pairs, the Brookhaven team bombarded a copper-oxide material, held at temperatures above and below the transition temperature, with beams of light from the National Synchrotron Light Source, and analyzed the energy spectrum of electrons emitted from the sample. This method, known as angle-resolved photoemission spectroscopy (ARPES), ordinarily gives a clear picture of only half of the energy spectrum — all the levels electrons can occupy below the so-called Fermi level. To glimpse the other half, above the Fermi level, the scientists employed methods of analysis similar to those used by astronomers to increase the resolution of celestial images.
"If you look through a telescope with poor resolution, you'll see the moon, but the stars are lost," Johnson said. "But if you improve your resolution you see the stars and everything else. By improving our resolution we can use ARPES to see the few electrons that occasionally occupy levels above the Fermi level. We have devised ways to sharpen our images so we can look at the weak signals from above the Fermi level in finer and finer detail."
Seeing both sides of the Fermi level is important because, when a material becomes a superconductor, there is an energy gap surrounding the Fermi level. A perfectly symmetrical gap — equally spaced above and below the Fermi level — is a strong indication that electrons are paired up. That superconducting gap exists at and below the transition temperature, as long as a material acts as a superconductor.
But Johnson's team and other scientists had previously observed a second gap, or pseudogap, in some high-Tc materials, well above the transition temperature. If this pseudogap exhibited the same symmetry around the Fermi level, Johnson reasoned, it would be definitive evidence of paired electrons above the transition temperature. Using their new image-enhancing techniques, Johnson's team demonstrated that the pseudogap does indeed exhibit this same symmetry.
"We can now say for certain that electrons are forming pairs above the transition temperature, before the material becomes a superconductor," Johnson said.
The scientists made another interesting observation: The pairing occurs only along certain directions in the crystalline lattice of atoms making up the material — only along the directions in which copper atoms are bonded with oxygen atoms.
Together, the existence of preformed electron pairs and their directional dependence should help clarify the picture of high-Tc superconductivity, Johnson said. For example, the findings rule out some theories to explain the high-Tc phenomenon (e.g. certain "spin density wave" and "charge density wave" derived theories). But the new findings are consistent with theories that consider the pre-superconducting state to be derived from a "Mott insulator," as well as theories in which " [http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=06-57] charge stripes," previously discovered at Brookhaven Lab, might play a role in electron pairing.
"It's still a very complicated picture and one of the great mysteries of modern science," Johnson said. "With something like 150 theorists working in the field, we have 150 theories of how these materials work. But as we develop new techniques, we are making progress narrowing down the mechanism."
Karen McNulty Walsh | EurekAlert!
Multiregional brain on a chip
16.01.2017 | Harvard John A. Paulson School of Engineering and Applied Sciences
Researchers develop environmentally friendly soy air filter
16.01.2017 | Washington State University
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).
Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...
Many pathogens use certain sugar compounds from their host to help conceal themselves against the immune system. Scientists at the University of Bonn have now, in cooperation with researchers at the University of York in the United Kingdom, analyzed the dynamics of a bacterial molecule that is involved in this process. They demonstrate that the protein grabs onto the sugar molecule with a Pac Man-like chewing motion and holds it until it can be used. Their results could help design therapeutics that could make the protein poorer at grabbing and holding and hence compromise the pathogen in the host. The study has now been published in “Biophysical Journal”.
The cells of the mouth, nose and intestinal mucosa produce large quantities of a chemical called sialic acid. Many bacteria possess a special transport system...
UMD, NOAA collaboration demonstrates suitability of in-orbit datasets for weather satellite calibration
"Traffic and weather, together on the hour!" blasts your local radio station, while your smartphone knows the weather halfway across the world. A network of...
Fiber-reinforced plastics (FRP) are frequently used in the aeronautic and automobile industry. However, the repair of workpieces made of these composite materials is often less profitable than exchanging the part. In order to increase the lifetime of FRP parts and to make them more eco-efficient, the Laser Zentrum Hannover e.V. (LZH) and the Apodius GmbH want to combine a new measuring device for fiber layer orientation with an innovative laser-based repair process.
Defects in FRP pieces may be production or operation-related. Whether or not repair is cost-effective depends on the geometry of the defective area, the tools...
10.01.2017 | Event News
09.01.2017 | Event News
05.01.2017 | Event News
16.01.2017 | Power and Electrical Engineering
16.01.2017 | Information Technology
16.01.2017 | Power and Electrical Engineering