One step closer to superconductors
Call it the 135 K wall. What might sound like the end of a torturous foot race is a temperature barrier (135 degrees Kelvin) that scientists have yet to breach in their quest for new materials that, when properly chilled, carry electricity without any resistance.
Known as superconductors, these materials hold the promise of smaller, faster computers, smaller and more powerful electric motors, and a more reliable and energy-efficient electrical-utility grid. The goal is to push superconductor operating temperatures higher, so they can work without expensive equipment that keeps them chilled at more than 200 degrees below 0 F.
Now, scientists in Colorado report that they have created a new state of matter that could provide insights to help researchers punch through the 135 Kelvin (minus 216.7 degrees F) wall. The team speculates that such insights could lead to materials that conduct electricity without losses at room temperatures. Their work appears in a recent online issue of the journal Physical Review Letters.
"We've opened a door, and where it goes is unclear," says physicist Deborah Jin, who led researchers at JILA, a research institute run by the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.
Still, she adds, the discovery "provides a model system for understanding superconductivity," especially in a class of materials discovered in the late 1980s that breached an earlier, colder temperature barrier.
Even without a potential superconductor connection, "this is an outstanding scientific accomplishment," says Martin Maley, former science director at the Los Alamos National Laboratory's Superconductor Science Center.
The new form of matter is known as a fermionic condensate. It joins a growing list of states of matter that form in the most extreme cold and act in bizarre ways compared with their counterparts at more torrid temperatures. The condensate draws its name from the particles that form it.
Physicists have placed fundamental particles of matter into two broad categories, explains Eric Cornell, an NIST researcher who shared a Nobel prize in 2001 for creating an extremely low-temperature form of matter known as a Bose-Einstein condensate. Fermions - such as electrons, protons, neutrons, and quarks - form the building blocks of matter. Particles called bosons carry forces acting on fermions. Moreover, he adds, bosons can readily be coaxed to congregate, while fermions are loners.
These same "social" characteristics can hold true for atoms. In 1995, Dr. Cornell and colleague Carl Weiman of the University of Colorado in Boulder and, separately, Wolfgang Ketterle of the Massachusetts Institute of Technology, formed a condensate of bosons from tenuous clouds of rubidium and sodium atoms. The clouds were chilled to 20 billionths of a degree above absolute zero. At those temperatures, the atoms - numbering only several thousand - appeared to merge and behave as one collective super atom - a Bose-Einstein condensate.
Once that was achieved, physicists asked, "What's the next big thing?" Cornell says. The answer: "You'd like to see a condensate in fermions."
It's a much tougher condensate to produce, and the race to achieve it has been intense. Several "elite research groups are trying to realize a condensate in fermions," he says.
Dr. Jin and her two colleagues used an isotope of potassium for their experiments. Using lasers and a special trap, they chilled a 500,000-atom cloud of isotopes to 50 billionths of a degree above absolute zero - the point at which movement in an atom halts.
By their nature, these fermion-like atoms did not yield the collective super-atomlike behavior that Cornell and others received when they chilled their bosons. But when subjected to a specially tuned magnetic field, Jin's atoms formed pairs. And while the partners in the pairs didn't bond to form a molecule, they did move together, like partners on a dance floor. Once in pairs, the ensemble of atoms formed a condensate.
This pairing tantalizes Jin and her colleagues, because in superconductors, fermions - in this case, electrons - move in pairs as a current flows. Three years ago, another team at JILA theorized that fermionic condensates are the bridge between Bose-Einstein condensates and superconductivity. Thus, some researchers say, the study of fermionic condensates could help point theorists in promising directions as they struggle to understand mechanisms behind superconductivity in materials discovered 20 years ago.
These so-called high-temperature superconductors generated an enormous amount of excitement among physicists when they were unveiled.
The leap in operating temperatures, while still chilly by any standard, raised the hope that further research could eventually lead to room-temperature superconductivity. The superconductors were made from ceramic materials and could be cooled with liquid nitrogen, rather than pricier liquid helium required to cool earlier superconductors.
Thus, wider applications were predicted for the "HT superconductors," compared with their older, colder counterparts. Indeed, a number of companies are manufacturing components and prototypes from new materials for use in fields ranging from microelectronics to electric-power distribution.
Since the late '80s, "our knowledge of the materials involved has grown tremendously," says Los Alamos's Dr. Maley. "But it's still pretty murky. There are two or three competing theories, but none match the whole array of experimental data."
A successful theory would give scientists the guidance needed to look for new superconducting materials, push operating temperatures even higher, and perhaps help settle the question of whether room-temperature superconductivity is even possible.
