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03/21/94

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Second superfluid state of matter proposed

STANFORD -- Begin with a thin film of superfluid, a liquid in which waves move without friction. Now, imagine that the fluid is filled with a regularly spaced array of tiny whirlpools.

That, in essence, is a new state of matter, which Stanford University physicist Shoucheng Zhang will describe on Monday, March 21, at the spring meeting of the American Physical Society in Pittsburgh. He has proposed this unusual state to explain unexpected observations, published two years ago, of liquid helium cooled to within a degree of absolute zero.

Superfluidity, which is defined as the movement of liquid without friction, was first discovered in 1938. It is closely related to superconductivity, the movement of electrical current without resistance. Although a number of different materials are superconducting, only one substance, liquid helium, can be transformed into a superfluid and, then, only by cooling it down to temperatures below -271 degrees Celsius, within three degrees of absolute zero.

Because scientists believed that they understood what caused it, and because superfluidity has no apparent applications, there has been very little scientific attention paid to the phenomenon in the last decade.

Two years ago, however, a team of scientists led by Jack Mochel at the University of Illinois at Urbana-Champaign discovered a new wrinkle. While studying thin films of cryogenically cooled helium, they discovered that when the thickness of the film is reduced to a few atomic layers the liquid no longer changes directly from a normal fluid into a superfluid, but unexpectedly goes through an intermediate phase.

Zhang argues that this intermediate stage is a new state of matter, one in which the fluid is dominated by a series of clockwise and counterclockwise superfluid eddies that form a relatively stable lattice, analogous to that formed by the atoms in an ionic crystallike salt.

In his experiments, Mochel used sound waves to identify the superfluid state. A superfluid transmits sound waves differently than an ordinary liquid. So what he observed was a phase in which sound waves behaved in a manner that differed both from those traveling in an ordinary liquid or in a superfluid. Zhang's theory explains this new mode as arising from the vibrations between the crystal-like array of eddies.

Since Zhang first advanced his theory last September in Physical Review Letters, one of his predictions has been confirmed: adding small amounts of the lighter isotope of helium, helium-3, to the mix increases the temperature range of the new state. Further experiments are under way to test another prediction - how this unusual state of matter responds when jiggled.

If Zhang's theory proves correct, it may further strengthen the similarity between superfluidity and superconductivity. Stanford associate professor of applied physics Aharon Kapitulnik and his French collaborator, Mark Gabay, independently proposed a similar mechanism to explain superconductivity in thin films. Their paper appeared with Zhang's in the same issue of Physical Review Letters.

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