A GHOSTLY material is facing an existential crisis. One of the physicists who first claimed to have glimpsed a "supersolid" says the original experiment was flawed.
Normally the atoms of a solid form a regular lattice, giving them a rigid structure. But quantum theory says this can change: when solids get ultracold, some atoms should pass through the lattice, flowing like a frictionless liquid.
In 2004, Moses Chan of Pennsylvania State University and his erstwhile colleague Eunseong Kim repeatedly spun a chamber containing a chunk of porous glass saturated with helium, first one way, then the other. As they cooled the set-up, the rotation sped up, hinting that less and less helium was travelling with the spinning glass. Instead, it appeared that some helium was standing still inside the glass, a telltale sign of the lack of friction characteristic of a supersolid.
Then in 2007 it emerged that a chunk of solid helium, known as "bulk" helium, gets stiffer than expected at low temperatures. This could explain the faster oscillations without the need to invoke supersolidity. Though bulk helium was not part of the 2004 experiment by design, some may have formed in the gap between the glass and its chamber.
Chan has now repeated the experiment, after sealing the glass to rule out the presence of bulk helium. The changes in oscillation rate vanished, suggesting that the changes seen in 2004 were due to the stiffening of bulk helium, not supersolidity after all (Physical Review Letters, doi.org/jhz).
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