One of the other ScienceBloggers is prone to complaining in the back-channel forums that we don’t have enough bloggers who work in some subfield of biology or another– we need more left-handed shrew ecologists, or some such. This is, of course, patently ridiculous. What we need is a physics blogger from the condensed matter world, so we’d have somebody to explain what’s up with “supersolid” helium:
Superfluidity was discovered in the liquid phase in 1938, when Pjotr Kapitsa – who shared the 1978 Nobel prize for the work – found that liquid helium-4 suddenly behaves as if it has zero viscosity when cooled below a temperature of about 2 K. With no resistance to flow, a superfluid can do bizarre things such as creep up the sides of a vessel containing the material or pass through holes just a few atoms wide. Superconductivity, a similarly dramatic low-temperature phenomenon in which electrical current flows without resistance, is due to the superfluidity of electron pairs. However, in 2004 Moses Chan of Penn State University in the US and his then graduate student Eun-Seong Kim reported evidence for superfluidity in a much more unlikely setting: the atomic lattice of bulk-solid helium-4.
Such a “supersolid” phase of matter would flow through a classical solid as if it were not there. Like superfluidity in a liquid, this weird behaviour is predicted to be a consequence of Bose-Einstein condensation – a phase transition in which all the particles in a system collapse to the same ground state and can therefore no longer be treated as individual entities moving at random. Such quantum degeneracy is possible because helium-4 atoms are bosons, i.e. particles that have integer multiples of spin angular momentum.
I recognize all the words in those paragraphs, but I’m not entirely sure what they mean. Of course, my impression from the rest of the article is that nobody else is entirely sure what’s going on, either, so I guess I’m in good company.
The chief evidence for “supersolid” helium is a torsion pendulum experiment done by Moses Chan and his group. Their apparatus consists of a small cylinder suspended from a thin wires, that they can cause to twist back and forth. The cylinder is mostly solid, with a narrow channel near the rim that they can fill with helium. They fill this up with helium, and by varying the temeprature and pressure in the system, they can force that helium into a solid phase, and then continue lowering the temperature until they see the “supersolid” transition.
What they measure is the rotational frequency of the torsion pendulum. The cylinder twists back and forth at a characteristic frequency that depends on both the mass of the cylinder and how it’s distributed. The frequency of oscillation when the cylinder is full of He is higher than the frequency when it’s empty, for example, because there’s more mass toward the outside in the full cylinder.
There’s also a difference between the oscillation frequencies for the solid and “supersolid” phases. When the system goes through the “supersolid” transition, some fraction of the atoms in the system begin moving freely through the solid lattice, without friction. This allows them to essentially stop rotating– while the regular solid atoms turn with the cylinder, the “supersolid” atoms just stay put and let the rest of the solid flow past them.
The effect is a little like the hard-boiled egg trick. If you want to know whether an egg is hard-boiled or not without breaking it open, you can set it on its side on a table, and spin it. A hard-boiled egg will spin happily for a long time, while an uncooked egg will stop spinning relatively quickly because the liquid yolk doesn’t rotate with the shell. In the case of the egg, the non-rotating liquid creates drag on the shell that stops the spin quickly. In the case of “supersolid” He, the non-rotating solid is frictionless, so it just smoothly decouples from the motion. The effect looks like a small reduction in the mass of the twisting cylinder.
That’s the story, anyway. The problem is, all sorts of quirky results are cropping up. Chan’s original experiments showed something like 2% of the atoms moving into the “supersolid” state. Subsequent experiments changing the amount of disorder in the system– freezing the helium very slowly, to produce nearly perfect crystals, or freezing it very quickly to produce lots of cracks and fissures– have found that the amount of disorder changes the supersolid fraction dramatically. Increasing the amount of disorder pushes the supersolid fraction as high as 20%, while decreasing it pushes the fraction as low as 0.5%. Existing theories really don’t explain this, so something really weird is going on.
(It pains me to have to add that nobody is suggesting that Chan and his colleagues did anything inappropriate. The basic effect that they see has been reproduced by other groups, but the magnitude of that effect varies in an unexpected way with some other parameters. The original interpretation of the results is in question, but the results themselves are solid. Pardon the pun.)
Unfortunately, right here, just where the story starts to get interestingly twisty, is where I decouple from the motion. My condensed matter/ solid state background is not that good, and I really can’t follow the intricacies of the debate about what’s really going on. the Physics Wold article includes a bunch of speculations from theorists about what’s going on, but for all I understand of it, they might as well be saying “Mwah wah mwah mwah WAH wanh,” like the adults in a Charlie Brown cartoon.
Which is why we need some good condensed matter physics bloggers here. Somebody needs to explain what’s going on here in terms that even an idiot atomic physicist can understand.