There’s some interesting new work out on supersolid helium, a subject of great interest and controversy. The work was performed by John Beamish and James Day at the University of Alberta and is reported in this weeks Nature (Day, J. & Beamish, J. Nature 450, 853-856 (2007). Article here, Perspective here.)
As a refresher for what supersolid helium is, there is an old post I wrote on the subject back in 2005:
Yesterday I went to a condensed matter seminar on “super solid Helium” by Greg Dash. What, you ask, is super solid Helium? Well certainly you may have heard of superfluids. When you take Helium 4, and cool it down, somewhere around 2 Kelivin the liquid He4 (assuming the pressure is not too high so that it does not solidify) makes a transition to a state of matter, the superfluid, where it is a liquid which doesn’t have any viscosity. Well actually I think what happens is that you get a state of matter which has one part which is superfluid and the other part which is normal. The superfluid part of the liquid, along with having no viscosity, also has infinite thermal conductivity so it’s impossible to set up a temperature gradient with a superfluid. He3 also forms a superfluid at cold enough temperatures. But He3 does this at a much lower temperature, I believe around a few micro Kelvin. The mechanisms for superfluidity in these systems is different: in He4 it is Bose condensation of the He4′s themselves (which are bosons) and in He3 it is a Bose condensation of the He3 which act as composite particles with Bose statistics (the mechanism is similar to the role Cooper pairs play in superconductivity.)
So what is super solid Helium? Well the theoretical conjecture is that if you take a solid, this solid has vacancies (i.e. it’s not perfect and there are places where the solid is missing atoms in the lattice where it should have atoms in the lattice.) and it is these vacancies which can form a Bose condensate at low enough temperature. So the idea behind super solid Helium is that you have a highly pressurized chunk of cold Helium and below a certain temperature vacancies in the solid will all condense into the same state. Thus in such a substance the vacancies should flow without resistance in the solid. (I say Helium here, but it could possibly occur in other substances as well.)
But the question is, does such a mechanism occur? Over the years, various experiments have been performed looking for super solids and no one has seen any evidence of this strange phase of matter. Well in 2004, Eun-Seong Kim and Moses Chan of Pennsylvania State University performed experiments in which they claimed to have observed super solid Helium.
The basic idea behind the experiment is pretty simple: if you take a superfluid and try to spin it, it will be much easier to spin because of the lack of viscosity of the fluid. Thus if you take a torsional pendulum (a pendulum which instead of swinging back and forth like a normal pendulum is a disk attached to a rod and the disk is rotated by an angle and then this rotation angle oscillates like a pendulum) and start it oscillating, and then cool the system from above the superfluid transition temperature to below the superfluid transition temperature, then the system will all of a sudden become easier to spin, i.e. it’s moment of inertia will decrease. This will result in an increase in the oscillation frequency of the torsion pendulum. So what Kim and Chan did was they got highly pressurized Helium, so that it was solid, and put it on such a torsional pendulum. And at around a one tenth of a Kelvin, Kim and Chan observed exactly the effect of a decrease in the moment of inertia!
Now of course, science isn’t just about one team observing something and then everybody sitting back and saying “yes that must be super solid Helium.” Instead what happens is that (1) theorists get all up in arms trying to figure out if there are alternative explanations for this experiment and begin thinking about how to test these explanations and (2) experimentalists design experiments to duplicate or to make complementary confirmations of different properties super solid Helium would exhibit. The talk I went to yesterday was about some of the theoretical ideas for alternative explanations of the results of Kim and Chan as well as some discussion of more recent results reported by Kim and Chan. Interesting, I’d say that right now there is a stalemate: the alternative explanations now have problems with explaining the experimental results, but new, more recent experiments also exhibit effects which are harder to fit with what the theory of super solid Helium would predict (in particular an experiment I did not understand very well which attempted to verify in a different manner the existence of the super solid phase ( i.e. one of those complementary confirmations seemed to fail.)) What was nice to hear was that a different experimental group was gearing up to repeat the experiment of Kim and Chan. So maybe soon we will have either a confirmation of the effect seen by Kim and Chan, or no confirmation and then trying to figure out what the heck is causing the effect seen by Kim and Chan.
Science in action. Ain’t it beautiful?
Since that post, there have been a lot of interesting developments. One development is that experiments have reproduced the original work by Kim and Chan. However no evidence of persistent supercurrents or other evidence of superflows has been found. The experiment described in Nature by Beamish and Day adds some new interesting data to the story. In particular these authors observe an increase in the shear modulus of the solid as the solid gets lower and lower in temperature. The shear modulus is the ratio of the pressure applied parallel to the face of the solid and the shear strain (the distance the solid shears divided by the initial length.)
One possible explanation for the results of Kim and Chan could thus be that this increase in shear modulus is responsible for the reported supersolid results in torsional pendulums. However, Beamish and Day note that the size of this change in shear modulus doesn’t appear to be big enough to explain the Kim and Chan results. Another possible explanation, and this is the one that seems to be gaining ground from these results, is that dislocations in the solid become mobile and thus contribute both to the observed effects in the torsional pendulums and the changes in shear modulus. In fact other experiments seem to show that the more imperfect the helium crystal, the stronger the supersolid effect. So it seems that things are beginning to condense (sorry for the bad pun) on an answer to the puzzling experiments on supersolid Helium: dislocations in the crystal are the culprit. However, understanding how these dislocations exactly produce the observed effect, seems to be a yet unsolved mystery.