Figure 3: Super rotation
The most convincing evidence that solid helium-4 can
become a supersolid comes from torsional-oscillator experiments. As
discovered by Eun-Seong Kim and Moses Chan in 2004, the rotational
inertia and hence oscillation period of solid helium-4 suddenly drops at
a temperature of about 175 mK (red), indicating that some of the crystal
has undergone a phase transition to a superfluid state that remained at
rest as the surrounding "normal" solid helium-4 rotated around it. The
researchers saw no such behaviour for a dummy cell (black) or when the
experiment was repeated with helium-4's fermionic cousin helium-3
(blue). Moreover, by increasing the concentration of helium-3 impurities
in a helium-4 sample (coloured lines, shifted vertically down for clarity),
the supersolid transition temperature increased and the effect
eventually disappeared above a helium-3 contamination of 0.1%.