The heart of Chan and Kim's experimental apparatus is a disk filled with solid helium-4. To run the experiment, they hang the disk from a stiff rod and oscillate the disk back and forth. By measuring the frequency of oscillation, the scientists detect whether the solid helium-4 behaves like a supersolid.
An oscillating disk of normal matter, for example, behaves as expected: Because the atoms are rigidly linked, they rotate together.
In an oscillating disk of supersolid matter, many of the atoms rotate, but some do not. Instead, those atoms slip through the lattice like a superfluid, with no friction whatsoever, and sit motionless. That reduces the mass of the disk, which allows it to oscillate faster.
This animation has been exaggerated. In fact, the fraction of helium-4 atoms that refuse to rotate is closer to only 1 percent. And the oscillation frequency Chan and Kim measuredhow many times the disk changes direction over a period of timeis actually closer to 1000 times per second. The amplitude of the oscillationthe distance the disk moves in either directionis not much bigger than the width of a single atom.
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Credit: Trent L. Schindler / National Science Foundation
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