In Cornell University's Laboratory of Atomic and Solid State Physics, scientists are artificially inducing disorder where none occurs naturally, in one of the most unusual states of matter ever created – superfluid helium-3. This fluid is in a unique state that allows it to flow without resistance. Understanding its properties in this disordered state could help understand the basic mechanism of high-temperature superconductivity.

In work led by Jeevak Parpia, Cornell professor of physics, researchers are using aerogels – weblike structures of glass almost as light as air – to change the properties of the liquid helium-3. "This is a unique opportunity to alter the fluid's properties," Parpia said. "In the past, confinement in a variety of powdered materials or even between closely spaced slabs has led to a rapid breakdown of the superfluidity of the helium-3."

But, in work at Cornell, Northwestern University and Penn State University, small cubes of aerogels are creating new possibilities. Among them: that superfluids can be tailored by the addition of a few percent of a glassy material. Parpia and a doctoral student in his lab, James Porto, described some of their work at the American Physical Society annual meeting March 19 in St. Louis.

"The aerogel may be a means of introducing controlled amounts of disorder. If we are correct, aerogel and helium-3 represent a unique opportunity to understand the role of disorder in condensed matter, with implications spanning magnetism and high-temperature superconductors," Parpia said.

Superconductors are materials in which electricity, or charge, flows without resistance. Similarly, superfluids are fluids that flow without resistance. Cornell scientists in the 1970s, led by Robert Richardson and David Lee, both still professors of physics at Cornell, discovered this more exotic form of superfluidity in the helium-3 isotope when cooled to a temperature of only 2 1/2-thousandths of a degree above absolute zero.

The problem with studying this lab-created material is that it is self-purifying – nothing added to it seems to perturb its properties, thus limiting its utility as a model system. But former Cornell graduate student Moses Chan, now at Penn State, found in recent experiments that aerogels could be a useful material with which to study helium-3 and its more common isotope, helium-4. Parpia and Porto first observed that helium-3 in aerogel was "strikingly different" from the same material in bulk form, Parpia said. Northwestern researchers, led by William Halperin, a former Cornell graduate student, also found similar results and identified the state of the superfluid. The superfluidity of helium-3 in aerogel was "a great surprise," which the researchers from both groups reported in Physical Review Letters (Vol. 74, p. 4667, 1995; Vol. 75, p. 661, 1995) last year.

The most recent Cornell findings, which are preliminary, show some unexpected features in the flow of helium-3 in a magnetic field. But these are different from results recently seen at Northwestern, where Halperin found the magnetic field seemed to inhibit superfluidity. Experimental and theoretical physicists now are working to sort out this discrepancy. "However, these observations, taken together with new theoretical work, all point to new possibilities for the superfluid," Parpia said. "The implications are still being sorted out by the scientific community , but it is true that aerogels, seemingly an insignificant class of materials, can alter the properties of one of the most exotic and unusual states of matter created in the laboratory."