Confining superfluid helium to a new state.

Superfluid helium-3 represents one of the more exotic forms of matter created in the laboratory. At temperatures below 2.5 millikelvins, this liquid can flow without friction (SN: 10/19/96, p. 247).

When liquid helium-3 is allowed to soak into a highly porous glass sponge, known as an aerogel (SN: 11/17/90, p. 316), its characteristics change. The cobweblike silica filaments of the aerogel interfere with the coordinated movement of the helium-3 atoms, and the liquid must be cooled to even lower temperatures before it begins to act as a superfluid.

Now, researchers have discovered that a magnetic field further depresses the superfluid transition temperature of helium-3 and facilitates the formation of a new type of superfluid phase. "This was totally unexpected," says physicist William P. Halperin of Northwestern University in Evanston, Ill. Halperin and his coworkers report their findings in the Nov. 25 Physical Review Letters.

The researchers used a cylindrical sample of aerogel 1 centimeter long and 4 millimeters in diameter. Only 1.8 percent of the sample's volume consisted of silica, in the form of thin, randomly interconnected strands. The rest was open space into which a liquid could soak.

Immersed in helium-3, an aerogel's open network of glass threads serves as a means of introducing disorder into the confined liquid. Last year, both Hal- perin's group and the team of Jeevak M. Parpia and James V. Porto at Cornell University reported that such confinement substantially reduces the tempera- ture at which helium-3 becomes a superfluid.

However, researchers had expected the superfluidity to disappear completely. "That doesn't happen," Parpia says. The atomic pairing responsible for super- fluidity is much stronger than anyone had anticipated.

Physicists had also previously found that a magnetic field has little effect on the superfluid transition temperature of pure helium-3. So when Halperin and his colleagues subsequently discovered that a magnetic field strongly influences the superfluidity of helium-3 in an aerogel, they were again sur- prised. "It was a large effect," Halperin says.

"Having so much [aerogel] surface in close proximity to the liquid can clearly do odd things when you apply a magnetic field," Parpia comments. Halperin now suspects that the sensitivity of helium-3 to a magnetic field is due to inter- actions between a solid layer, one atom thick, of helium-3 coating the glass filaments of the aerogel and the remaining helium-3 atoms wandering about in the liquid phase between the coated strands.

The ordered atoms of the surface coating have individual magnetic fields. When atoms in the liquid phase bump into surface atoms, the interaction can reverse the magnetic orientation of the rebounding helium-3 nuclei, disrupting the usual pairing of atoms in the superfluid. An applied magnetic field enhances the effect.

Halperin and his group tested the proposed model by introducing a small amount of helium-4, which displaces helium-3 atoms from filament surfaces. Because helium-4 and helium-3 atoms have different magnetic behavior, collisions between surface helium-4 atoms and wandering helium-3 atoms no longer cause the same spin-flipping effect. The helium-3 superfluid transition temperature goes back to what they expect it would be in the absence of a magnetic field.

"I think the helium-3-aerogel system is very important," Parpia notes. He adds that studies of this system's unusual behavior are bound to provide new insights into the mechanisms underlying both superfluidity and super- conductivity in a variety of materials.