An eight-year debate over the existence of a ghostly material known as a supersolid is drawing to a close with the first suggestion from one of the original discoverers that their experiment was flawed.
"If we had known then what we do now, we would have designed the experiment differently," says Moses Chan of Pennsylvania State University in University Park.
Fans of exotic materials needn't despair, though. Supersolids are still expected to show up at some point. Furthermore, the intense debate over supersolids has itself been fruitful, sparking the discovery of a surprising new type of material called a quantum plastic.
Normally the atoms of a solid form a regular lattice, giving them a rigid structure, but quantum theory predicts that this can change in some ultra-cold solids. Under these conditions, quantum effects should start to dominate, causing some of the atoms to pass through the lattice, flowing like a frictionless liquid.
Solid helium
Detecting this strange state isn't easy, though. In 2004, Chan and his colleague Eunseong Kim cooled helium that they had collected inside a kind of porous glass called Vycor.
They placed the cold helium-saturated glass in a suspended chamber designed to alternate the direction of its spin, between clockwise and anticlockwise.
As they cooled the set-up to close to absolute zero, the oscillations became faster, suggesting that more and more of the helium was no longer travelling with the moving glass, but instead standing still inside the chamber. This was a tell-tale sign of the lack of friction that is characteristic of a supersolid.
The pair later repeated the experiment ? but this time started with a chunk of solid helium, known as bulk helium, removing the need for the glass. Again, they found again that as the set-up got really cold, the oscillations got faster. "Soon after that the results had been replicated by a number of groups," says Chan, though the size of the effect varied inconsistently.
Quantum plasticity
Then cracks in the supersolidity claims started to emerge. In 2007, John Beamish at the University of Alberta in Edmonton, Canada, suggested bulk helium may become much stiffer than expected at low temperatures and that this alone could account for the faster oscillations, without the need for supersolidity.
This effect was later dubbed quantum plasticity. However, it did not seem like quantum plasticity could explain the original 2004 result as non-bulk helium inside porous glass would not stiffen in the same way.
There was, however, a possibility that a gap between the aluminium chamber and the porous Vycor could have allowed a thin layer of bulk helium to form there, even in the original experiment. Now to rule this out, Chan and his current colleague Duk Kim have redesigned the original Vycor experiment.
This time, they sealed the glass with a thin layer of epoxy resin and inserted the helium through a very thin tube. This meant only a tiny fraction of the helium could become a bulk solid ? and so any speeding up due to quantum plasticity would be negligible.
Plasticity wins
Chan and Duk Kim found that this set-up completely eliminated the changes in oscillation rate that they had originally observed. "We didn't see anything at all," says Chan. That suggests that all of the speeding up in the original experiment must have been due to bulk helium forming a quantum plastic, not supersolidity as originally claimed in 2004.
To be absolutely sure, Chan says he still has to design an experiment that suppresses the stiffening effect in bulk helium, just to check if there are any residual signs of supersolidity, but he strongly expects there won't be. "Ultimately, we now seem to understand what's going on."
Meanwhile, Beamish continues to hunt for a true supersolid ? in helium and other materials. "There may be supersolidity in other contexts," he says. He adds that the work on helium is interesting in its own right. "All the work has shown that solid helium is a very unusual solid."
Chan is also not discouraged by his latest result, pointing out that the discovery of quantum plasticity wouldn't have come about without the intense scrutiny sparked by the supersolid controversy. "Without this I don't think anyone would have gone to such trouble."
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