Third sound - surface waves on an ocean of superfluid - has been found in superfluid helium-3, say UC Berkeley researchers

By Robert Sanders, Public Affairs

BERKELEY -- Cold, stormy waves on a liquid helium sea could open a window onto the quantum phenomena that underlie the strange behavior of superfluids and superconductors.

Physicists at the University of California, Berkeley, have succeeded in generating surface waves on a superfluid helium-3 "sea" the size of a quarter and only 200 atoms deep - dimensions too small for waves in any normal fluid.

In superfluids, however, surface waves (which are known as "third sound") are possible. In third sound, while the normal fluid component remains stationary - too thick or viscous to move in the shallow layer - the superfluid component of the liquid helium sloshes back and forth unhindered.

UC Berkeley physicists Séamus Davis and Richard Packard and graduate students Andrew Schechter and Raymond Simmonds report the first demonstration of "third sound" in superfluid helium-3 in the Dec. 10 issue of the British journal Nature.

Third sound had been produced previously in superfluid helium-4, but it has been difficult to demonstrate in superfluid helium-3, which is more than 1,000 times colder. Physicists have nevertheless been trying since the discovery of superfluid helium-3 in 1972.

"We now know that this wave-like phenomenon does indeed exist in superfluid helium-3 and that we can measure its speed to confirm our other expectations," Davis said.

The technique Davis and Packard developed to create a shallow helium-3 sea could also be useful in tackling unanswered questions about superfluids. That is because it is easier to search for quantum phenomena in a very thin, two-dimensional film than it is to look for the same phenomena in the bulk superfluid.

"We have opened a new window on the physics of this exotic material, superfluid helium-3, in two dimensions," said Davis, an associate professor of physics.

"Materials in two dimensions are very good tests for physics theory," he said. "We can't test these theories in higher dimensions, but we do know how to work with smaller systems to test our calculations. So it is very important to be able to make experiments on two-dimensional systems."

Packard, a professor of physics, points out that this year's Nobel Prize in Physics went to scientists who discovered a novel phenomenon - the fractional quantum Hall effect - in an electron gas in two dimensions.

Two dimensional physics is a rich and fascinating area, displaying many unusual and counter-intuitive phenomena," Packard said. "It is also of great practical significance because industries - especially computers, communications, biotechnology and other high-tech areas - utilize extremely thin layers of materials in their fabrication processes."

Among the phenomena Packard and Davis plan to look for are the superfluid equivalent of the Hall effect in superconductors, and an analogous quantum Hall effect.

"The Hall effect is unknown in liquids, but it is predicted in superfluid helium-3," Packard said.

Although liquid helium-3 is the purest known material, its superfluid phase is one of the most complex and intriguing liquids in nature, Packard said. A lighter isotope of the helium found in balloons - helium-4 - helium-3 becomes superfluid when chilled to a thousandth of a degree above absolute zero, that is, 459.67 degrees below zero Fahrenheit and about one millionth of room temperature.

Davis and Packard created the waves by immersing a copper cylinder, about the diameter of a quarter and a half-inch tall, into a vat of liquid helium-3 until the fluid rose up the side and flowed over the top. Part of the uncertainty was how thin a layer they could get and still retain the superfluid properties of the helium. They found that above 90 nanometers - the width of about 200 atoms - the helium-3 remained superfluid, allowing them to conduct experiments in the thin layer.

Superfluid helium-3 can be thought of as a combination of normal fluid and superfluid occupying the same space at the same time. In this model, first sound occurs when the two components move in phase with each other, second sound when they are out of phase, and fourth sound when only the superfluid fraction can move because the normal fraction is clamped in a porous medium. Third sound, which is a surface wave, takes place in a thin film when the normal component is stationary, essentially clamped to the substrate.

To create third sound in the superfluid they suspended a washer over the top of the liquid and sent a pulsing current through it, which pulled and pushed the surface to create waves only a few atoms high. The waves, like the standing ripples you get when you jiggle a cup of coffee, were detected by a capacitor sitting in the middle of the washer.

In addition to the concentric standing waves, which most closely resemble the vibrational modes of a drumhead, they also generated waves that traveled around the perimeter of the surface - so-called azimuthal modes. The vibration frequency of these surface waves was relatively low - about 1-3 vibrations per second, or 1-3 Hertz.

Superfluid helium-3 is described as a fluid of Fermions - atomic particles with an odd number of either protons, neutrons or electrons - in which, below some critical temperature, pairs of particles correlate their motions over distances hundreds of time larger than the separation between neighboring atoms. The 1996 Nobel Prize in Physics was awarded for the discovery of this astonishing material. The paired atoms, which are analogous to pairs of electrons in a superconductor, form an anisotropic quantum liquid.

Theorists have already predicted several fascinating new physical phenomena that two-dimensional helium-3 should exhibit. For example one theory predicts that if the film flows in one direction a pressure difference will appear across the perpendicular direction. This is an analog of an effect in electricity known as the Hall effect, in which an electric current flowing in the presence of a magnetic field produces a voltage difference perpendicular to the current direction.

The superfluid is believed to have a sort of self-generated field, rather analogous to that in a ferromagnet, that leads to the predicted effect, Packard said. For sufficiently thin films the superfluid Hall effect should become quantized like the electric quantum Hall effect - a phenomenon whose discoverers received the Noble prize several years ago.

A second theory suggests that the two-dimensional superfluid state will break down because of the spontaneous generation of an infinite number of tiny whirlpools - called quantized vortices - as the temperature is raised.

This theory is now used to explain the physics of high-temperature as well as low-temperature superconductors, superfluid helium-4 and several other systems in two dimensions. A test of its predictive power using third sound in superfluid helium-3 will be an important development, Packard said.

The work of Davis and Packard was supported by the National Science Foundation, the Office of Naval Research, the National Aeronautics and Space Administration, and the David & Lucile Packard Foundation.

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