Imagine a fluid that flows both perfectly and not at all. A Zen master might pose such a riddle, but in a new theoretical proof, physicists have shown that such a paradoxical state of matter can indeed exist.
It’s called a superglass — “super” in the same sense as superconductors and superfluids, which is to say that quantum weirdness lurks.
A superglass would look and feel just like a normal glassy solid. As in regular glass, the atoms in a superglass would be arranged randomly — instead of in a crystalline lattice — because a glass is essentially a liquid that has ceased to flow.
But pick up a piece of superglass and rotate it, and some portion of its atoms won’t rotate. Instead, these atoms flow through the rotating solid with zero friction, as in a superfluid. And because there’s no friction, the rest of the atoms in the solid can’t drag those slippery atoms along — just as a superfluid in a spinning cup will, from an observer’s point of view, hold perfectly still instead of swirling with the cup.
While the existence of superglasses has not been conclusively demonstrated in the lab, research reported in the December 8 Physical Review B shows that the laws of physics do permit this exotic state of matter to exist at temperatures close to absolute zero, or -273º Celsius.
“It’s actually a bit of a paradox that you usually think of a glass as like a fluid that doesn’t flow, but a superglass also behaves like a superfluid,” says Claudio Chamon, a condensed matter theorist at Boston University and coauthor of the study. The phenomenon is “only quantum mechanical — it doesn’t happen in the classical system,” in which atoms are imagined to act like tiny billiard balls.
Physicists still don’t fully understand how some of the atoms in a superglass would be able to move with zero friction. Chamon says that the phenomenon could be related to the fact that, as atoms are cooled to near absolute zero, they begin acting less like particles and more like quantum-mechanical waves. As the wavelike atoms get colder, they “smear out” and begin to overlap with each other. Eventually, some of the atoms in the superglass are smeared out so far that they can easily swap places with neighboring atoms, allowing them to move through the solid unimpeded.
“It depends on the fact that they’re all identical, so they can all occupy the same quantum mechanical state,” Chamon says. For this reason, only atoms containing an even number of subatomic particles can form a superglass. Such atoms are called bosons, and unlike atoms with an odd number of subatomic particles, neighboring bosons can all assume the exact same quantum mechanical state, thus becoming completely indistinguishable from each other.
“It’s really important to show theoretically that a glassy superfluid state can be stable,” comments Boris Svistunov, a condensed matter theorist at the University of Massachusetts in Amherst. In 2006, Svistunov and his colleagues discovered a superglass phase in computer simulations of solid helium-4. But those simulations could not show whether the new state of matter was stable, or that it would last for more than a fraction of a second.
In the new proof, Chamon and his colleagues “basically show for the first time that there is a model in which the superglass state is a guaranteed stable state,” Svistunov says.
