Superplasticity in solid Helium!?

Recently, in the Nature News and Views section, Henry R Glyde wrote about the connection between extended defects (like grain boundaries) and flow without resistance in solid Helium. The last paragraph in his article sums up the experimental findings rather nicely:

Given the recently discovered dependence of rhos on sample annealing, and the correlation of superflow with grain boundaries, supersolidity seems to hinge on macroscopic, long-range defects such as grain boundaries or amorphous channels in the helium crystal. But at the same time, a superfluid density of the same magnitude is observed in helium confined in a nanoporous medium such as Vycor. It is difficult to imagine that extended defects could be the same in helium thus confined and in bulk helium. The situation is far from clear: revealing the secrets of this latest superfluid is very much a work in progress.

Now, consider the phenomenon of superplasiticity in materials science. Very crudely put, during superplastic deformation, due to applied stresses, the material flows plastically; such a flow can give rise to defects in the material (like voids and cracks) which will lead to failure. However, if there is an “accomodation” mechanism in the material that helps maintain the material integrity, the material can undergo plastic deformation of several hundred percentages (typically). Here is a (rather ancient) review of superplasticity, if you are so inclined.

So now, here is a thought: if solid Helium undergoes superplastic deformation with an accomodation mechanism that is infinitely fast, would that explain most of the observed features? I think so! (which explains the interrobang in the title).

Finally, there is an interesting aside; in the materials science literature, people speak of the so called constititutional vacancies (Yes; we wrote that paper–me and Abi). These vacancies are sometimes (wrongly) defined as the vacancies that exist at absolute zero temperature (ground state vacancies in the quote below). And, here is the experimental observation as summarised by Glyde:

Unfortunately, however, ground-state vacancies have not been observed in solid 4He. Thermally activated vacancies are found — but at temperatures above TC that are irrelevant to the establishment of superfluidity. Recent calculations also find that individual vacancies are not stable in the ground state of crystalline 4He. The vacancies instead coalesce or migrate to a surface; this is the mechanism by which thermal vacancies leave a classical crystal when it is cooled.

Have fun!


3 Responses to “Superplasticity in solid Helium!?”

  1. Defect induced "super" flow in Helium « Materials, Science of Says:

    […] this news report in Science. Here is one of my earlier posts on this issue. Posted by Guru Filed in Solid Helium, Grain boundaries, […]

  2. Superfluid Helium: follow-up « Entertaining Research Says:

    […] Helium: follow-up I have mentioned grain boundaries and resistance less flow in solid Helium a couple of times here and elsewhere. Chad Orzel at Uncertain principles comments […]

  3. Supersolidity and disorder « Entertaining Research Says:

    […] blog. I have also written about resistance free flow in solid helium a couple of times: here and here for example. Today, I got to hear Prof. Balibar on the role of disorder (defects) in promoting such […]

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