Several years ago, now, a group at Penn State announced a weird finding in helium at extremely low temperatures and high pressures (which is what you need to make helium solidify): when they made a pendulum out of a cylindrical container with a thin shell of solid He toward the outside edge, twisting about its axis, they saw a small but dramatic change in the oscillation frequency as they cooled the system below a particular temperature. They interpreted this as a “supersolid” phase of helium, with a quantum phase transition taking place that caused the “supersolid” to stop rotating with the cylinder, instead moving without resistance, as a superfluid does when you try to spin it.
This was a dramatic claim, and has not been without controversy. Experiments since then have been ambiguous, with some people claiming to see the same sort of transition, while others don’t see anything. The counter-theory is, more or less, that the solid helium doesn’t exist as a perfect crystal, but instead has cracks and defects in it, and the observed change in rotation frequency is due to residual (super?)fluid slipping through these gaps, not a solid effect.
Two recent papers might mark steps toward clearing this up.
One, written up in Physics and published in Physical Review Letters, uses nuclear magnetic resonance of a small amount of helium-3 introduced into the helium-4 to look for effects that might be related to the “supersolid” phenomenon. Their results mostly agree with the theory that says there’s no supersolid, but there are a couple of features at the temperature where the transition takes place that they can’t explain, so maybe. The other, published in Science uses the same oscillating-pendulum technique as the original experiments, but doesn’t see the same sort of transition, and claims they can explain their results in terms of other effects.
So, we have one paper that says “almost certainly not,” and another saying “probably not, but there’s a weird thing happening at that temperature.” Which means we can expect this to kick around for another couple of years, at least.
I’d do a more complete write-up of this for ResearchBlogging, but really, the above is about my level of understanding of this phenomenon, and that doesn’t seem to justify the extra citation. People with actual knowledge of the field should feel free to explain further in comments, though.
(A few years back a colleague was pushing what he thought was the grand unified theory of the whole thing, claiming to be able to explain everything. I think this arxiv paper may be it, but I thought I had talked to him about it more recently than 2004, so maybe not…)