Researchers at Penn State recently discovered the existence of a new phase of matter called a "supersolid," a form of helium-4 that has unique properties.

When frozen to a temperature two-tenths of a degree above absolute zero, a small percentage of this type of helium behaves like a superfluid, which means it flows without resistance, said Moses Chan, Evan Pugh professor of physics.

"In the particular state, helium-4 acts like a superfluid that can flow through its surroundings without friction," Chan said. "It is analogous to super-conductivity, where electrons flow without resistance."

Chan said that although Newton's Law generally governs motion, the conditions that exist when helium-4 is frozen to such a low temperature must be explained by quantum physics.

"When things are very small and get to be extremely cold, then quantum physics becomes important," he said. "People generally think that solids do not move, but now we have found a fraction that not only flow, but flow without friction. Therefore, physicists may have to rethink the definition of a solid."

Because this material behaves differently than a normal solid, the researchers dubbed the low-temperature helium-4 a "supersolid," with superfluid properties.

In their experiment, the researchers -- Chan and his former student and current postdoctoral associate Eunseong Kim -- used a device called a torsional oscillator that rapidly rotates back and forth. This device is about 2 to 3 centimeters in length, and can be compared to a pendulum, said Kim.

"When we cooled the helium to a low enough temperature, suddenly a fraction of the helium-4 turned into the supersolid," Kim said. "At that point, it had no friction with the rotating cell and didn't follow its rotation."

Kim said that the total mass appeared to have decreased because the supersolid mass can no longer be detected, even though it is still present. By using simple subtraction, they were able to determine that the "missing" mass accounted for 1.5 percent of the total amount of helium in the oscillator.

"We used a very sensitive scale for measuring the mass," he said. "It can measure as little as one tenth of a microgram [0.1 millionth of a gram]."

Chan said that in order to understand the properties of a supersolid, it is important to know that there are two types of particles of matter, bosons and fermions.

Bosons include atoms with an even mass number, such as helium-4, whereas fermions are particles with an odd mass number.

"Bosons like to congregate with one another in the same energy state," Chan said.

Kim said that he thinks it will take long time to find a useful application of a supersolid because researchers are just beginning to study the basic principles. However, this experiment may have important implications in quantum mechanics.

"These results indicate that quantum physics is truly the most important part of understanding what goes on in nature," Kim said. "In terms of supersolids, this is the first study that has been done. No other properties have been examined, and there are still a lot of things we can do [with this research]."