Shining extremely intense light on the two-faced behavior of certain subatomic particles, physicists are attempting to steal a revealing glimpse of the nature of substances that can act both as insulators thwarting electrical flow and conductors promoting it.
This new, first-hand look at the machinations of electrons -- elementary particles with an electrical charge -- at the quantum or subatomic level where classical laws of physics are defied may help scientists gain greater insight into superconductors, materials that permit electricity to flow without resistance or energy loss.
The alluring potential of superconductivity, first observed in 1911, has galvanized a scientific stampede to this frontier. So far, all the efforts have produced a stream of announcements of new metals, alloys and compounds that become superconducting -- but only at temperatures that are stiflingly cold from a marketing perspective. Thus nearly a century of attempts to achieve superconductivity at temperatures high enough to discard the current need for complex and expensive refrigeration remains unfulfilled.
The most valuable application so far has been in magnets for magnetic resonance imaging or MRI. But superconductors continue to give rise to grander, more far-reaching visions: of perpetual engines, trains that magnetically "float" above their tracks, super-fast computers, power lines that relay electricity with no loss of energy, and improved magnetic and electronic data storage.
A better understanding of conducting-insulating materials -- whose dual role confounds traditional views of metallic conductivity -- might provide important clues and bring commercial superconductivity closer to reality.
"We understand the world by analogy with the familiar," Andrew Millis, a physicist at Columbia University in New York who wrote an accompanying commentary, told United Press International. "The enduring interest of quantum mechanics, and of solid state physics, which is really the application of quantum mechanics to the things of everyday life, is its strangeness -- its lack of analogy to the familiar," he said.
"The experiment of (lead study author Tonica) Valla, et al., provides one of the first ... glimpses of the relation between one kind of quantum mechanical strangeness and a relatively intuitive phenomenon, namely electrical conductivity," Millis said.
Valla, a physicist at the U.S. Department of Energy's Brookhaven National Laboratory in Upton, N.Y., and his colleagues looked into why some materials made of stacks of metallic planes behave as conductors when the electrons travel in the direction of the planes, but as insulators when the charged particles flow in a different direction. Such behavior challenges the traditional view of metallic conductivity, in which electrical current is carried by electrons in every direction.
"The ease with which electrons move through some metals depends on which direction they take," Millis said. "Watching electrons move along metal planes gives new insight, curiously, into how they move between them."
Scientists have suspected the dual conductor/insulator property is due to electrons interacting so strongly with one another that they move as a group, not as individuals, to carry electrical current within the planes. Now, there is evidence of such interactions, the authors report in the June 6 issue of the British journal Nature.
"This is an area of physics with many theoretical calculations and speculations, but Valla, et al., provide the first actual evidence of what is really going on," Millis told UPI.
"A material that is both conducting and insulating is quite intriguing," Valla said. "Such a dual behavior has puzzled physicists for several years. And though theoretical explanations have been suggested, we now show for the first time that the strength of the interactions between excited electrons influences their behavior."
Valla and his collaborators from Brookhaven, the University of Connecticut in Storrs, Princeton University in New Jersey and Osaka University in Japan, found in two conducting-insulating materials they studied, electrons confined within the planes at high temperatures can move between the planes at lower temperatures, allowing the entire matrix to behave more like a metal.
The "critical" temperature at which the change occurs ranges between minus 100 degrees and minus 300 degrees Fahrenheit, depending on the material. Cold as this may sound, these temperatures are warm enough to permit cooling by liquid nitrogen instead of the much more expensive liquid helium required by conventional superconductors.
"These planes act like trains, and electrons like passengers in the trains," said study co-author Peter Johnson, a physicist at Brookhaven. "At high temperatures, the electrons are bound together in the planes like passengers inside moving trains. Then, below the critical temperature, the electrons are not bound any more and start moving around in the same way as passengers leave a stopped train."
To examine electron interactions, the scientists used extremely intense ultraviolet light. Generated by the National Synchrotron Light Source at Brookhaven, the UV light excites the electrons in the materials. Then, using a method called angle-resolved photoemission spectroscopy -- ARPES for short -- the scientists attempted to accurately measure the intensity of the light energy carried by the excited electrons.
"In our experiments, we bring in light or photons and use that to eject electrons from the system," Johnson said in a telephone interview.
They determined the ARPES spectrum for various temperatures.
"This technique lets you see whether electrons in a solid are in fact moving as conventional, electron-like particles. If so, you see a peak," Millis said.
Below the critical temperature, a weak signal started to appear in the spectrum, strengthening as the temperature decreased.
"This signal is the telltale evidence of individual electrons," Valla said. "Theorists had predicted the existence of such a signal, but nobody had observed it before."
The results may reveal how superconductors conduct electricity without heat dissipation when they are cooled below a certain temperature. In particular, the researchers seek insights into high-temperature superconductors, with critical temperatures ranging from minus 396 degrees to minus 216 degrees Fahrenheit. They assume strong interactions between electrons are at play.
"One of the biggest questions is trying to understand materials that go superconducting at much higher temperatures," Johnson told UPI. "If you had a superconductor that worked at high temperatures -- and not have to get to those incredibly low temperatures -- you'd have the possibility of enormous savings in energy. Understanding these strongly correlated materials has been one of the Holy Grails of physics."
The research also sheds light on materials with new electrical and magnetic properties that may arise from strong interactions between electrons.
"We expect to see dramatic new results and applications stemming from the study of materials with strongly-interacting electrons," Johnson said.
