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The San Diego Union-Tribune

Ultracold may create a new state


September 29, 2004

Quantum mechanics has done it again. In an experiment with supercooled helium, researchers at Penn State say they have found that as a solid ring spins around, part of it can remain perfectly still.

At ultracold temperatures, matter often behaves far differently from the way it does in everyday experience. For instance, many materials turn into superconductors, able to conduct electricity with no resistance, because electrons coalesce and move in synchrony without hitting the surrounding lattice of atoms.

In a similar way, other materials at low temperatures become superfluids, which flow without viscosity to slow them down.

The new experiment suggests a new state of matter: a supersolid.

In the 1970s, physicists played with the notion that a solid – a material whose atoms generally stack together in a neat crystal pattern – might also undergo a quantum transformation. Some suggested that when sufficiently chilled, some of the atoms in a solid would "melt" into a superfluid and effortlessly flow through the surrounding solid. They called this state a supersolid.

Most physicists, including some who had suggested the idea, concluded that although this state was possible, it was unlikely to be observed.

"I have written at least one paper in the remote past about the possibility of supersolid behavior," said Anthony J. Leggett, a professor of physics at the University of Illinois who was awarded the Nobel Prize in Physics last year for theoretical work on superfluids. "I would have bet at least 100-to-1 against it."

Change of place

In an article published this month on the Web site of the journal Science, Moses H.W. Chan, a professor of physics at Penn State, and Eunseong Kim, a postdoctoral researcher, reported that they had turned helium into a supersolid.

"The theorists will be surprised," Chan said, "but they do not exclude such a possibility."

Helium, the same gas that levitates blimps and gives a person a Donald-Duck voice, turns into a liquid at minus-452 degrees. Chilled slightly further and squeezed under immense pressure, helium atoms are pushed together into a solid.

In the experiment, Chan and Kim placed solid helium in a small ring, cooled it to almost absolute zero and exerted pressures ranging from 380 pounds per square inch to 970 pounds per square inch. The usual atmospheric pressure on the earth's surface is 14.7 pounds per square inch.

The ring, attached to a rod, was then twisted and released. The twisting force in the rod caused the ring to oscillate – spinning clockwise, slowing, stopping, reversing direction to counterclockwise, and so on – at a steady frequency determined by the mass and size of the ring.

As the temperature was cooled below minus-459.22 degrees – half a degree above absolute zero – the frequency of oscillations sped up, as if the ring were losing mass, until it appeared that 1.5 percent of the helium had vanished.

The Penn State scientists believe that 1.5 percent of the solid helium had turned into a superfluid, and according to the rules of the quantum mechanics, fell into the laziest possible energy state: motionless. As the solid ring oscillated back and forth, the superfluid flowed in the opposite direction in such a way that it appeared to remain exactly still.

To check that some of the helium had not actually escaped, the scientists warmed the ring back up, and the oscillations slowed again. They repeated the experiment with a lighter version of helium, helium-3, which is known not to change into a superfluid, and they found no shift in the oscillation frequency.

Major puzzle

So far, other physicists agree that the supersolid explanation makes the most sense, but they would like to see more data. "I think everybody agrees we need more," said Wayne Saslow, a professor of physics at Texas A&M University. "It's calling out for more experiments."

How a superfluid could flow through a solid defies ready explanation. "I think it's a major puzzle," Leggett said. "If this stands up, then it's going to make us totally rethink our concept of what a quantum solid is."

One possibility, Leggett said, is that the helium solid is not perfect, that a few locations in the helium crystal lattice are empty, and the superfluid is flowing from one empty location to another. But higher pressures, which presumably would push atoms to fill the empty locations, did not diminish the supersolid effect.

Another possibility is that helium atoms momentarily double up at a lattice site before one of the occupants is pushed out to another site.

Three decades ago, Saslow read Leggett's paper describing how a supersolid might behave and wrote computer programs to explore the idea in more depth, before moving on to other research.

"I thought I would never see it again," Saslow said. "It's like it came from the dead."

When he heard of the Penn State work several months ago, Saslow searched for his old computer programs. He thought they were lost until he opened a file folder containing printouts of the programs.

In quantum mechanics, a helium atom is not a discrete object, but a fuzzy blur of possibilities. Saslow said the superfluid could be thought of as the flow of fuzziness between the atoms. "Nothing here makes sense," he admitted.

The programs, which produce much more precise answers today running on his laptop than was possible on a 1970s-era mainframe computer, predict a superfluid fraction of 2 percent, roughly in agreement with the Penn State results.

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