Thursday, December 06, 2007

Major Physics Breakthrough In Understanding Supersolidity

I guess one of the major news of this week is the new discovery of the sheer modulus characteristic of a He in the "supersolid" phase[1] (assuming that it did become a supersolid).

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.

A perspective of this work in Nature[2] has a bit more info:

A supersolid can exhibit other anomalies, for instance in the speed at which sound passes through it. Sound speed depends on the shear modulus of the solid, as well as the density of the superfluid component. To assess why the solid behaves in the way it does, it is thus important to measure the shear modulus independently of the superfluid density. This is precisely what Day and Beamish have now done with solid helium.

Again, the authors' experiment3 is conceptually simple. They placed solid 4He between two parallel plates, known as piezoelectric shear transducers. They moved one plate, the driving transducer, in a direction parallel to the second plate. The solid helium transmits the resulting elastic shear stress between the plates, and this is measured by the second transducer. Day and Beamish find3 that the shear modulus of helium rises by up to 10% as the temperature is reduced from 0.2 to 0.02 kelvin. More significantly, the temperature dependence of this large increase in shear modulus closely tracks the changes in period in the torsional-oscillator experiments.

Fascinating stuff coming out of supersolidity lately.


[1] J. Day and J. Beamish, Nature v.450, p.853 (2007).
[2] A.T. Dorsey and D. A. Huse, Nature v.450, p.800 (2007).

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