The mysterious saga of "supersolid" helium continues this week. If you recall, there were some new results a little while back showing that the effect depends on disorder in the samples, followed by neutron scattering studies that didn't show the expected distribution of states in the sample. These results suggest that something else is going on in these samples, and the explanation of the observed effects isn't all that simple.
Now, Moses Chan and co-workers at Penn State, who made the initial discovery, have returned with a new paper in which they see "supersolid" behavior in single crystals. This would appear to directly contradict the finding that annealing the sample destroyed the effect.
In a certain sense, this is a wonderfully exciting picture of science in action. There are strange experimental results in need of explanation, and we get to watch different groups trying to replicate the findings, and come up with an explanation that fits all the data. This is how it's done, people.
In another sense, though, it's intensely frustrating. Is it a supersolid or not, dammit? Pick one, and stick with it...
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The Incoherent Ponderer and I had a discussion about this. I think it all comes down to whether this latest setup really produces only single crystals, and that's extremely tough to assess. It's a clever design similar to that used in studies of solid 3He, with a single spot set up to nucleate crystal growth. With 3He you can use NMR imaging to figure out whether you've got a single crystal, but since 4He has no nuclear moment, that's out, and something like neutron scattering probably wouldn't be able to see whether you have tiny bits of uncrystallized stuff (and would also be a real pain to do).
Yeah, that's pretty much what I thought. Determining whether you really have a single-crystal sample has got to be damn near impossible.
Single crystal is not the same thing as defect-free. Besides grain boundaries, there can be dislocations, interstitials, vacancies, impurities (say 3He), amorphous regions, etc. -- single crystal only implies the absence of grain boundaries.
Admittedly, it is not completely clear what is going on; nonetheless if Chan et al. are right about there being supersolidity in single crystals, it doesn't necessarily go against, say, the annealing results or the conclusion (and theoretical evidence) that the supersolidity is defect dependent.