Tuesday, June 2, 2009

NEW ELEMENT FOUND TO BE A SUPERCONDUCTOR

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Of the 92 naturally occurring elements, add another to the list of those that are superconductors.

James S. Schilling, Ph.D., professor of physics in Arts & Sciences at Washington University in St. Louis, and Mathew Debessai, Ph.D., — his doctoral student at the time — discovered that europium becomes superconducting at 1.8 K (-456 °F) and 80 GPa (790,000 atmospheres) of pressure, making it the 53rd known elemental superconductor and the 23rd at high pressure.

Debessai, who received his doctorate in physics at Washington University's Commencement May 15, 2009, is now a postdoctoral research associate at Washington State University.

"It has been seven years since someone discovered a new elemental superconductor," Schilling said. "It gets harder and harder because there are fewer elements left in the periodic table."

This discovery adds data to help improve scientists' theoretical understanding of superconductivity, which could lead to the design of room-temperature superconductors that could be used for efficient energy transport and storage.

Schilling's research is supported by a four-year $500,000 grant from the National Science Foundation, Division of Materials Research.

Europium belongs to a group of elements called the rare earth elements. These elements are magnetic; therefore, they are not superconductors.

"Superconductivity and magnetism hate each other. To get superconductivity, you have to kill the magnetism," Schilling explained.

Of the rare earths, europium is most likely to lose its magnetism under high pressures due to its electronic structure. In an elemental solid almost all rare earths are trivalent, which means that each atom releases three electrons to conduct electricity.

"However, when europium atoms condense to form a solid, only two electrons per atom are released and europium remains magnetic. Applying sufficient pressure squeezes a third electron out and europium metal becomes trivalent. Trivalent europium is nonmagnetic, thus opening the possibility for it to become superconducting under the right conditions," Schilling said.

Schilling uses a diamond anvil cell to generate such high pressures on a sample. A circular metal gasket separates two opposing 0.17-carat diamond anvils with faces (culets) 0.18 mm in diameter. The sample is placed in a small hole in the gasket, flanked by the faces of the diamond anvils.

Pressure is applied to the sample space by inflating a doughnut-like bellow with helium gas. Much like a woman in stilettos exerts more pressure on the ground than an elephant does because the woman's force is spread over a smaller area, a small amount of helium gas pressure (60 atmospheres) creates a large force (1.5 tons) on the tiny sample space, thus generating extremely high pressures on the sample.

Superconducting materials have unique electrical and magnetic properties. They have no electrical resistance, so current will flow through them forever, and they are diamagnetic, meaning that a magnet held above them will levitate.

These properties can be exploited to create powerful magnets for medical imaging, make power lines that transport electricity efficiently or make efficient power generators.

However, there are no known materials that are superconductors at room temperature and pressure. All known superconducting materials have to be cooled to extreme temperatures and/or compressed at high pressure.

"At ambient pressure, the highest temperature at which a material becomes superconducting is 134 K (-218 °F). This material is complex because it is a mixture of five different elements. We do not understand why it is such a good superconductor," Schilling said.

Scientists do not have enough theoretical understanding to be able to design a combination of elements that will be superconductors at room temperature and pressure. Schilling's result provides more data to help refine current theoretical models of superconductivity.

"Theoretically, the elemental solids are relatively easy to understand because they only contain one kind of atom," Schilling said. "By applying pressure, however, we can bring the elemental solids into new regimes, where theory has difficulty understanding things.

"When we understand the element's behavior in these new regimes, we might be able to duplicate it by combining the elements into different compounds that superconduct at higher temperatures."


RESEARCH: MOCKINGBIRDS, NO BIRD BRAINS, CAN RECOGNIZE A FACE IN A CROWD

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The birds are watching. They know who you are. And they will attack. Nope, not Hitchcock. It’s science.

University of Florida biologists are reporting that mockingbirds recognize and remember people whom the birds perceive as threatening their nests. If the white-and-grey songbirds common in cities and towns throughout the Southeast spot their unwelcome guests, they screech, dive bomb and even sometimes graze the visitors’ heads — while ignoring other passers-by or nearby strangers.

“We tend to view all mockingbirds as equal, but the feeling is not mutual,” said Doug Levey, a UF professor of biology. “Mockingbirds certainly do not view all humans as equal.”

The paper describes the first published research showing that wild animals living in their natural settings recognize individuals of other species, Levey said. It may provide clues as to why mockingbirds and selected other bird and animal species flourish in heavily populated cities and suburbs — while other species either grow rare or disappear entirely.

“The real puzzle in the field of urban ecology is to figure out why certain species thrive around humans,” Levey said. “One of the hypotheses is that they have some innate ability to adapt and innovate in ways that other species don’t.”

Mockingbirds are among the most common birds on the University of Florida campus in Gainesville, where they nest in trees and shrubs close to the ground. For the research, student volunteers walked up to the nests, reached through the foliage and gently touched the nests’ edges, then walked away. The same volunteers repeated the same visits again the next day, and again for two more days. On the fifth day, however, different volunteers approached the nests. All told, 10 volunteers tested 24 nests at least five times last spring and summer, during the mockingbird nesting season.

It didn’t take a bird’s eye view to spot the resulting pattern, Levey said.

On the third and fourth days, the birds flushed from their nests more rapidly each time the increasingly familiar students appeared — even though the students took different paths toward the nests on successive days and wore different clothes. The birds also gave more alarm calls and flew more and aggressively each succeeding day, with some especially defensive birds even grazing intruders’ heads — not exactly deadly, but annoying, because the birds tend to hit the same spot repeatedly, Levey said.

And yet when different students approached the nests on the fifth day, the birds hardly ruffled their feathers, waiting to flush until last moment. They also gave fewer alarm calls and attacked much less than on the previous day with the familiar intruder.

On a campus of 51,000-plus students, paths are filled with students walking back and forth from class all day every weekday — so it’s no stretch to say that thousands of different people come within a few feet of mockingbird nests during the breeding season.

And yet, the mockingbirds in the study were clearly able to recognize and remember a single individual, based on just two brief negative encounters at their nest. Levey said that sharply contrasts with laboratory studies, in which pigeons recognized people only after extensive training. “Sixty seconds of exposure was all it took for mockingbirds to learn to identify different individuals and pick them out of all other students on campus,” Levey said.

For most wild animals, urban development brings less habitat and more predators. Many species flee or die off, but a few persist, and some thrive. It seems obvious that these species do better around people, but why?

Few people bother mockingbird nests, so that is hardly an answer. Rather, Levey said, the birds’ ability to recognize people suggests perceptual powers that give them an edge in dealing with the complexities of urban environments — such as being able to judge which cats may be aware of nests and which are simply passing blithely nearby.

“We don’t believe mockingbirds evolved an ability to distinguish between humans. Mockingbirds and humans haven’t been living in close association long enough for that to occur.” Levey said. “We think instead that our experiments reveal an underlying ability to be incredibly perceptive of everything around them, and to respond appropriately when the stakes are high.”

U. Florida

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