Major Physics Breakthrough In Understanding Supersolidity
Physicists at the University of Alberta, in Edmonton, Alberta, Canada, have made a major advance in the understanding of what appears to be a new state of matter.
Doing something ‘easy’ on superfluid solids: Solid helium [S] comes to a higher level inside the tube than outside. Liquid helium [L] fills the rest of the apparatus. The height difference [h] can be eliminated only if liquid helium can pass through the solid helium blocking the tube. (Image: Frederic Caupin)
Prof. John Beamish, chair of the Department of Physics, and PhD student James Day work in the highly specialized field of quantum fluids and solids. At very low temperatures, helium gas turns into a liquid. Put under extreme pressure the liquid turns into a solid.
Physicists have been manipulating solid helium so they can study its unusual behaviour.
In 2004, a research team at Penn State university in the United States, led by Dr. Moses Chan, electrified the physics world when it announced that it may have discovered an entirely new state of matter — supersolidity. The team made the discovery by cooling solid helium to an extremely low temperature and oscillating the material at different speeds. They found that the particles behaved in a way not seen before, which suggested it might show the “perpetual flow” seen in superfluids like liquid helium.
Day and Dr. Beamish have taken this research a different direction. In an experiment not done before, they cooled the solid helium and manipulated the material another way — by shearing it elastically. In doing so, they found that the solid behaved in an entirely new and unexpected way — it became much stiffer at the lowest temperatures.
“The experimental results from the University of Alberta are remarkable,” Dr. Chan said. “Namely, Professor Beamish and his student James Day found that the shear modulus of solid helium increases by 20% when it is cooled below 0.25K.
“Furthermore, the temperature dependence of the shear modulus seems to track the period change seen in torsional oscillator. It seems the two phenomena are related and probably have the same mechanical origin.
“This is an important breakthrough since the original discovery,” Chan said.
Other physicists around the world are also studying the implications. Through this discovery, Beamish and Day have significantly added to the body of knowledge about the fundamental states of matter allowed by nature.
Nature 450, 853-856 (6 December 2007) | doi:10.1038/nature06383; Received 21 August 2007; Accepted 2 October 2007
- Department of Physics, University of Alberta, Edmonton, Alberta, T6G 2G7 Canada
Superfluidity—liquid flow without friction—is familiar in helium. The first evidence for ‘supersolidity’, its analogue in quantum solids, came from torsional oscillator measurements1, 2 involving 4He. At temperatures below 200 mK, the torsional oscillator frequencies increased, suggesting that some of the solid decoupled from the oscillator. This behaviour has been replicated by several groups3, 4, 5, 6, 7, but solid 4He does not respond to pressure differences8, and persistent currents and other signatures of superflow have not been seen. Both experiments and theory9, 10, 11, 12, 13, 14 indicate that defects are involved; these should also affect the solid’s mechanical behaviour. Here we report a measurement of the shear modulus of solid 4He at low frequencies and strains. We observe large increases below 200 mK, with the same dependence on measurement amplitude, 3He impurity concentration and annealing as the decoupling seen in the torsional oscillator experiments. We explain this unusual elastic behaviour in terms of a dislocation network that is pinned by 3He at the lowest temperatures but becomes mobile above 100 mK. The frequency changes in the torsional oscillator experiments appear to be related to the motion of these dislocations, perhaps by disrupting a possible supersolid state.
see also: Nature