No Supersolid State For Helium

Solid helium cooled to millikelvin temperatures does not form a friction-free supersolid, report Duk Y. Kim and Moses H. W. Chan of Pennsylvania State University (Phys. Rev. Lett., DOI: 10.1103/physrevlett.109.155301). In 2004, Chan and then-graduate student Eunseong Kim reported what seemed to be evidence of a supersolid state when they filled a porous glass with helium and then cooled it to less than 0.2 K (C&EN, Jan. 19, 2004, page 14; Nature, DOI: 10.1038/nature02220). When they oscillated the glass, some of the helium appeared not to move with it, a sign that they had produced a frictionless state. Research by Chan’s group and others since then produced inconsistent results. In the new work, Kim and Chan redesigned the experimental apparatus to eliminate gaps where helium could exist outside the glass pores. Using the improved system, they saw no evidence for super­solidity. The effects observed in the earlier experiment were likely the result of an abrupt increase in the shear modulus—a measure of material stiffness—of bulk solid helium at very low temperatures, Chan says.