A superconductor is a material that conducts electricity with almost no resistance. High-temperature superconductors (HTC) are conductors that superconduct at temperatures much “warmer” than conventional superconductors (although still very cold) — for example, -300°F rather than -460°F. This difference, while not huge, is enough to make HTCs more viable for practical applications than materials that must be kept much colder. HTCs are important to creating superconductor-based electric and power-delivery devices, such as power transmission lines, motors, and generators.
HTCs are starting to be introduced into service as reported in this previous post. Much research remains to be done in order to make this a more economical technology. This post reports on the latest efforts to this end being conducted at the Brookhaven National Laboratory.
Unlocking the Secrets of High-temperature Superconductors
Brookhaven National Laboratory, Press release, March, 7, 2007
Although it was discovered more than 20 years ago, a particular type of HTC 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.
Discovered in 1986, the most perplexing of these cuprate superconductors is "LBCO," named for the elements it contains: lanthanum, barium, copper, and oxygen. Because of their unusual properties they are good candidates for research into understanding HTCs.
One of the most perplexing findings involving LBCO is that the HTC 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.
Even though LBCO is not a superconductor at the 1/8 doping, the number of holes still decreases at low temperature. 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," said a BNL researcher. "It has the same magnitude, the same shape and symmetry. Yet, it doesn't have superconductivity." 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.
"Stripes" in High-Temperature 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.
Studies conducted at Brookhaven support the controversial theory that the holes segregate themselves into stripes that alternate with antiferromagnetic regions in the material.
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.
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,they have perfected the process.
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, researchers can make the liquefied material recrystallize as a single uniform crystal.
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. Such 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.
Much was ommited from the press release and you may want to read it, in its entirety if interested.