Another twist in the saga of the mysterious supersolid, a quantum-mechanical crystal that can flow, comes from new data challenging the interpretation of experiments taken to be proof of the supersolid’s existence.
In 2004 Moses Chan and Eunseong Kim at Pennsylvania State University showed that the oscillations of a wobbling vessel (called a torsional oscillator) filled with solid helium-4 increased in frequency when its temperature was cooled, just as might be expected if part of the helium-4 was becoming supersolid, and flowing through itself rather than being dragged around with the rest of the vessel. The results appeared to fit the theoretical prediction of a supersolid proposed by Russian theorists in the 1960s.
While nobody questions that Chan and Kim have discovered some strange and novel exotic behavior in helium-4, the supersolid interpretation quickly came under pressure. In 2006, Sophie Rittner and John Reppy at Cornell University in Ithaca, New York, showed that the putative supersolid signal got weaker if a helium-4 crystal was heated to remove imperfections before being cooled down again for the experiments; the opposite of what the classic theory would predict. Then in 2007, James Day and John Beamish at the University of Alberta in Edmonton found by shearing helium-4 samples that the material’s stiffness increased at low temperatures in a way that might mimick the signal attributed to the supersolid (because a stiffer crystal would oscillate more quickly).
Now Seamus Davis, also at Cornell, and his colleagues, show in Science that the microscopic defects in the crystalline structure that produce the shearing effect Day and Beamish measured are indeed the ones that cause the torsional oscillator frequency to increase. They discovered this by measuring the motion of a torsional oscillator using an extremely sensitive position sensor that enabled them to gauge exactly how fast the material inside was responding as it was shaken. They found the response time varies gradually with temperature, as would be expected for effects due to the freezing of defects in the crystalline structure, rather than suddenly, as would be expected for a transition into a supersolid state.
Davis suggests that the material, rather than being a supersolid, is actually closer to a glass, a frozen sea of defects, that may also be able to flow. “If there is a superfluid component then it’s a superglass,” Davis says.
Chan says he finds the paper interesting but that any claim there is no supersolid must also explain a control experiment he and Kim did in 2004, in which they blocked the ability of the putative supersolid to flow and showed the signal went away – as expected based on the supersolid interpretation – and a more recent paper in which Kim, now at KAIST in South Korea, found evidence for apparent vortices forming in solid helium 4 in a torsional oscillator, another strong sign of a supersolid’s existence. Robert Hallock of the University of Massachusetts, Amherst, sounds a similar note, pointing to work from his lab showing that fluid helium-4 can apparently flow through a crystal of solid helium 4. “It may be that more than one physical process is at work in the solid, he says, “time will tell.”
Davis acknowledges these other results and says he simply doesn’t know if the supersolid is the correct explanation of them or not.
Reppy says Davis’ has done a beautiful experiment that’s a real advance. The problem faced by everyone in the field, he says, is that there might be a very small supersolid effect buried under another effect — that caused by changes in defects in the crystalline structure — with the same experimental signature. “The similarity of the phenomena creates a real problem for the experimentalist,” he says.
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