Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Unlocking the secrets of high-temperature superconductors

Although it was discovered more than 20 years ago, a particular type of high-temperature (Tc) superconductor – material that conducts electricity with almost zero resistance – is regaining the attention of scientists at the U.S. Department of Energy's Brookhaven National Laboratory. Copper-oxide compounds, called cuprates, operate at temperatures warmer than traditional superconductors but still far below freezing.

Understanding the mechanism for these superconductors may one day help scientists design superconductors able to function closer to room temperature for applications such as more-efficient power transmission.

Discovered in 1986, the most perplexing of these cuprate superconductors is "LBCO," named for the elements it contains: lanthanum, barium, copper, and oxygen. After years of research on similar materials, Brookhaven researchers have learned how to "grow" better samples of LBCO, which has allowed for extensive studies on its intriguing properties. Three Brookhaven physicists will discuss their most recent findings about LBCO at the March meeting of the American Physical Society. The details of their research are highlighted below.

A Superconductor with Insulating Properties

One of the most perplexing findings involving LBCO is that the high-temperature superconductor actually has distinct insulating-like properties. Each barium atom has one fewer electron than lanthanum, so increasing barium adds electron "holes," or the absence of electrons, to the system. The more barium that is "doped" into the material, the more holes, and the greater the superconductivity – until the composition reaches a point where there is exactly one barium atom for every eight copper atoms, a state known as the 1/8 doping. Then, oddly, the superconductivity disappears. Above this point, as more holes (barium atoms) are added, the superconductivity reappears.

Brookhaven physicist Christopher Homes will discuss this odd phenomenon at 9:12 a.m. Mountain Time on March 5, 2007, in the Four Seasons Ballroom 2-3 at the Colorado Convention Center. At Brookhaven's National Synchrotron Light Source and other facilities on site, Homes investigates LBCO's electronic properties by shining various types of light onto an LBCO crystal and measuring the intensity that is reflected back. This optical picture tells scientists about the behavior of the charge carriers – or holes – in LBCO. Most materials have a set number of carriers that scientists can count using these methods. As a material becomes a superconductor, some of the holes lower their energy by falling into a superconducting state that allows them to flow without resistance. As these carriers condense, there is a characteristic change in the optical conductivity. However, even though LBCO is not a superconductor at the 1/8 doping, the number of holes still decreases at low temperature. Homes and other researchers attribute this feature to the formation of the so-called "energy gap." In semiconductors, the charge gap blocks the flow of current because of its isotropic nature (the gap spreads evenly in all directions). Superconductors also have energy gaps, but in the cuprates these gaps have different energies in different directions with respect to the copper-oxygen chemical bonds.

"The more we look at this charge gap, the more it looks like a superconducting gap," Homes said. "It has the same magnitude, the same shape and symmetry. Yet, it doesn't have superconductivity." Homes and other BNL researchers continue to tackle this mysterious problem in order to understand why a material that wants to be a superconductor is behaving like an insulator.

Looking for "Stripes" in High-Tc superconductors

In LBCO, as in all materials, negatively charged electrons repel one another. But by trying to stay as far apart as possible, each individual electron is confined to a limited space, which costs energy. To achieve a lower-energy state, the electrons arrange themselves with their spins aligned in alternating directions on adjacent atoms, a configuration known as antiferromagnetic order. As mentioned above, scientists can dope the material with electron "holes," or the absence of electrons, to allow the electrons/holes to move more freely and carry current as a superconductor. The question is: How do these holes arrange themselves?

Brookhaven physicist John Tranquada will answer that question during his talk about superconducting "stripes" at 9:48 a.m. Mountain Time on March 5, 2007, in the Four Seasons Ballroom 2-3 at the Colorado Convention Center. Studies conducted by Tranquada and other Brookhaven researchers support the controversial theory that the holes segregate themselves into stripes that alternate with antiferromagnetic regions in the material.

"There's a lot of excitement in trying to understand why these materials are superconducting, and there's plenty of controversy surrounding it," Tranquada said.

Most recently, Tranquada's research group examined the effect of the stripes on vibrations in the crystal lattice. Lattice vibrations play a role in pairing up the electrons that carry current in conventional superconductors. At the Laboratorie Leon Brillouin, Saclay, in France, researchers bombarded samples of superconducting materials and the same stripe-ordered non-superconductor with beams of neutrons and measured how the beams scattered. Comparing the energy and momentum of the incoming beams with those scattered by the samples gives the scientists a measure of how much energy and momentum is transferred to the lattice vibrations. Each of these vibrations normally has a particular, well-defined frequency for a given wavelength. But in the superconductor experiment, at a particular wavelength, the scientists observed an anomaly: a wider range of frequencies in the lattice vibrations.

The scientists observed this anomalous signature most clearly in samples with observable stripe order, but they also saw it in samples of good superconductors without static stripes. This indicates the presence of dynamic stripes – meaning that the stripes can wiggle through the crystal lattice – and suggests that stripes might be important in the mechanism for high-Tc superconductivity, Tranquada said.

Paving the Way for Crystal Growth

In order to study the properties of LBCO superconductors, scientists need to produce large, single crystals of the material – a difficult task that wasn't possible until recently. At the state-of-the-art crystal growth facility in Brookhaven's physics building, physicist Genda Gu and his colleagues have perfected the process. Gu will discuss his crystal growth method at 11:51 a.m. Mountain Time on March 7, 2007, in Korbel Ballroom 1D at the Colorado Convention Center.

The crystals are grown in an infrared image furnace, a machine with two mirrors that focuses infrared light onto a feed rod, heating it to about 2,200 degrees Celsius (3,992 degrees Fahrenheit) and causing it to melt. Under just the right conditions, Gu and his colleagues can make the liquefied material recrystallize as a single uniform crystal. At present, the most interesting form of LBCO has one barium atom for every eight copper atoms, or a 1/8 "doping," at which point the material loses its superconductivity. Achieving this high barium concentration is extremely difficult and is the reason many scientists previously opted to use different but related materials for their research on superconducting stripes and other properties, Gu said.

"LBCO was the first high-temperature superconductor discovered, but everyone switched over to studying other materials for a while because they weren't able to grow single crystals with a concentration of barium greater than 11 percent," Gu said. "Now, we can study the whole class of high-Tc materials."

Each crystal takes about a month to make, with precise control over growth temperature, atmosphere, and other factors. Brookhaven is currently capable of making crystals with barium concentrations up to 16.5 percent, a world record, Gu said.

Karen McNulty Walsh | EurekAlert!
Further information:

More articles from Physics and Astronomy:

nachricht Hubble captures massive dead disk galaxy that challenges theories of galaxy evolution
22.06.2017 | NASA/Goddard Space Flight Center

nachricht New femto-camera with quadrillion fractions of a second resolution
22.06.2017 | ITMO University

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Climate satellite: Tracking methane with robust laser technology

Heatwaves in the Arctic, longer periods of vegetation in Europe, severe floods in West Africa – starting in 2021, scientists want to explore the emissions of the greenhouse gas methane with the German-French satellite MERLIN. This is made possible by a new robust laser system of the Fraunhofer Institute for Laser Technology ILT in Aachen, which achieves unprecedented measurement accuracy.

Methane is primarily the result of the decomposition of organic matter. The gas has a 25 times greater warming potential than carbon dioxide, but is not as...

Im Focus: How protons move through a fuel cell

Hydrogen is regarded as the energy source of the future: It is produced with solar power and can be used to generate heat and electricity in fuel cells. Empa researchers have now succeeded in decoding the movement of hydrogen ions in crystals – a key step towards more efficient energy conversion in the hydrogen industry of tomorrow.

As charge carriers, electrons and ions play the leading role in electrochemical energy storage devices and converters such as batteries and fuel cells. Proton...

Im Focus: A unique data centre for cosmological simulations

Scientists from the Excellence Cluster Universe at the Ludwig-Maximilians-Universität Munich have establised "Cosmowebportal", a unique data centre for cosmological simulations located at the Leibniz Supercomputing Centre (LRZ) of the Bavarian Academy of Sciences. The complete results of a series of large hydrodynamical cosmological simulations are available, with data volumes typically exceeding several hundred terabytes. Scientists worldwide can interactively explore these complex simulations via a web interface and directly access the results.

With current telescopes, scientists can observe our Universe’s galaxies and galaxy clusters and their distribution along an invisible cosmic web. From the...

Im Focus: Scientists develop molecular thermometer for contactless measurement using infrared light

Temperature measurements possible even on the smallest scale / Molecular ruby for use in material sciences, biology, and medicine

Chemists at Johannes Gutenberg University Mainz (JGU) in cooperation with researchers of the German Federal Institute for Materials Research and Testing (BAM)...

Im Focus: Optoelectronic Inline Measurement – Accurate to the Nanometer

Germany counts high-precision manufacturing processes among its advantages as a location. It’s not just the aerospace and automotive industries that require almost waste-free, high-precision manufacturing to provide an efficient way of testing the shape and orientation tolerances of products. Since current inline measurement technology not yet provides the required accuracy, the Fraunhofer Institute for Laser Technology ILT is collaborating with four renowned industry partners in the INSPIRE project to develop inline sensors with a new accuracy class. Funded by the German Federal Ministry of Education and Research (BMBF), the project is scheduled to run until the end of 2019.

New Manufacturing Technologies for New Products

All Focus news of the innovation-report >>>



Event News

Plants are networkers

19.06.2017 | Event News

Digital Survival Training for Executives

13.06.2017 | Event News

Global Learning Council Summit 2017

13.06.2017 | Event News

Latest News

Hubble captures massive dead disk galaxy that challenges theories of galaxy evolution

22.06.2017 | Physics and Astronomy

New femto-camera with quadrillion fractions of a second resolution

22.06.2017 | Physics and Astronomy

Rice U. chemists create 3-D printed graphene foam

22.06.2017 | Materials Sciences

More VideoLinks >>>