Physicists in the US have confirmed that solid helium can behave as a superfluid. Earlier this year Eun-Seong Kim and Moses Chan of Pennsylvania State University observed superfluid behaviour - fluid flow without viscosity - in solid helium that had been confined in porous Vycor glass. However, the effect might have been due to the formation of liquid-like layers in the pores. Now Kim and Chan have repeated the experiment with bulk samples of solid helium and confirmed that it can enter a superfluid state (E Kim and M H W Chan 2004 Sciencexpress 1101501).
Liquid helium-4 becomes superfluid when it is cooled to temperatures below 2.176 Kelvin. However, theory predicts that it should also be possible to observe superfluid behaviour in solid helium-4 under certain conditions.
One way to observe superfluidity in solid helium is to measure the resonant period of a sample of the material in a piece of apparatus called a torsional oscillator. This period depends on the moment of inertia of the sample, and this moment changes when helium enters the superfluid state.
Kim and Chan measured a total of 17 samples of solid helium-4 at pressures between 26 and 66 bars and found that they all became superfluid at temperatures below 230 millikelvin. “Our experiment shows that the superfluid-like behaviour is a general and intrinsic property of solid helium,” they write, “and not the result of confinement in any particular medium.”
However, many of the details of the experiment are not yet understood. In an accompanying article Tony Leggett of the University of Illinois in Urbana writes that the experiment “will force theorists to revise dramatically the generally accepted picture of crystalline solid helium-4.”
