July 30, 1999 EVANSTON, Ill. --- Northwestern University physicists have for the first time shown that superfluid helium-3 -- the lighter isotope of helium, which is a liquid that has lost all internal friction, allowing it to flow without resistance and ooze through tiny spaces that normal liquids cannot penetrate -- actually behaves like a solid in its ability to conduct sound waves.
The finding, reported in the July 29 issue of the journal Nature, is the first demonstration in a liquid of the 'acoustic Faraday effect,' a response of sound waves to a magnetic field that is exactly analogous to the response of light waves to a magnetic field first observed in 1845 by British scientist Michael Faraday. The acoustic effect provides conclusive proof of the existence of transverse sound waves -- which are characteristic of solids but not of liquids -- in superfluid helium-3.
"Faraday's finding was the first indication that light and magnetism were related," says William Halperin, professor of physics and astronomy at Northwestern. "I wouldn't say that our discovery is of that magnitude, but it is significant as the first observation of a previously unknown mode of wave propagation in a liquid -- one that is of the type you would expect to see in a solid."
Light is a transverse wave, meaning that the oscillations are at right angles to the direction of propagation or travel. If the oscillations are all in a single direction, the light is said to be polarized. Faraday showed that polarized light is rotated by a magnetic field, twisting the wave crests into a spiral along the axis of propagation.
In the acoustic realm, liquids are expected to propagate only compressional waves in which the oscillations are along the axis of propagation, like the end-to-end waves of a Slinky.
"Showing a Faraday effect -- a rotation of the wave by a magnet -- means that this acoustic wave is transverse," said James Sauls, a theoretical physicist and the other senior author on the report. The acoustic Faraday effect was proposed by Sauls and former Northwestern graduate student Geneva Moores as a way of detecting transverse sound waves. The waves were predicted nearly 40 years ago by the late Lev Landau, a Nobel Prize-winning physicist.
Sauls, professor of physics and astronomy at Northwestern, marveled at the experiments that were conducted in Halperin's cryogenic laboratory at temperatures the tiniest fraction of a degree above absolute zero, the total absence of heat: minus 273.15 degrees on the Celsius scale, or about minus 460 degrees Fahrenheit. Only at these temperatures does helium become superfluid.
"At sufficiently low temperature, quantum mechanics endows the helium atoms with a collective behavior that is not present at higher temperatures in the classical regime. It is the collective behavior of all of the helium atoms which supports this propagating shear wave," Sauls said.
"The technology that allowed this discovery pushes the frontier in low temperature physics," Sauls said. "These oscillations are just extraordinary. You can see the superfluid transition in the acoustic cavity. Nothing is going on until you get down to very low temperatures. The amplitude gets larger and larger -- this is the development of a growing transverse sound mode in the cavity."
The experiments were performed at one-thousandth of a Celsius degree above absolute zero. This incredibly low temperature was achieved in a series of refrigeration stages, Halperin said.
"There's a vacuum to isolate the system from the outside world, then liquid helium-4 is placed around that to get it to a temperature where it's starting to get cold, and then we have a cascade of refrigeration," Halperin said. Helium-4, a more common isotope of helium, has a natural boiling temperature of 4 degrees Celsius above absolute zero, or 4 degrees Kelvin.
"Cold as it is, that's only about a hundred times colder than room temperature, which is 300 K," Halperin said. "From there, a pumped refrigerator gets us down to 1 K, and then a dilution refrigerator takes us down another factor of up to 100," he said. "In the last stage, the temperature gets lowered another factor of 100 by demagnetization refrigeration."
Halperin estimates there are about a dozen laboratories in the world with demagnetization refrigerators like the one at Northwestern.
Helium-3 was the second isotope of helium discovered to exhibit superfluidity at ultra-low temperatures. It is one of nature's most bizarre liquids, able to flow uphill and even climb the walls of its container and pour out. Its discovery in 1972 led to the awarding of the Nobel Prize in 1996 to American physicists Douglas Osheroff, David Lee and Robert Richardson.
The recent finding, Sauls said, "suggests a new capability for studying the internal structure of superfluid helium-3 and its natural modes of oscillation."
"There are other natural modes of oscillation in quantum liquids that are not easily studied unless you have this transverse current mode. It may be possible to use it as a spectroscopic tool for studying these other modes," Sauls said. Halperin said its uses may be even broader, but remain to be explored.
"Faraday's magneto-optic effect is used in detecting the distance of faraway astrophysical sources," he said. "Sources that are polarized emit light that passes through the magnetic field of interstellar space. By detecting how much the light is rotated, you can tell how far away it was, knowing something about the magnetic fields in stellar space as we do. Since the magneto-optical effect is weak, the degree of rotation is quite small."
In fact, the acoustic Faraday effect, as compared to the optical Faraday effect, is about a million times stronger. "In our case, a full rotation takes place in the width of a human hair, under the influence of a field not much stronger than a refrigerator magnet," Halperin said. "Whether we can use that to some advantage other than understanding the structure of these quantum fluids remains to be seen. In 1845, Michael Faraday would not have been able to foresee the practical application of his magneto-optic effect."
Other authors on the Nature report are Northwestern graduate students Yoonseok Lee and Thomas Haard. The research was funded by the National Science Foundation and the New Energy and Development Organization of Japan.
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