Helium Supersolid Has Superfluid Properties

Physicists may have created the first “supersolid,” a new state of solid matter that, counterintuitively, has superfluid properties. Supersolids belong to the family of Bose-Einstein condensates (BECs), in which bosons (atoms with integer nuclear spin) are forced into an identical quantum state.

For about a decade, scientists have been squeezing and chilling both gases and liquids to make completely frictionless BECs. Solids should also form BECs at low temperatures and high pressures, but they“ve proven elusive.

Now, Pennsylvania State University physics professor Moses H. W. Chan and graduate student Eun-Seong Kim report that they“ve created supersolid helium-4 [Nature, 427, 225 (2004)]. The paradoxical superfluid motion in such a solid stems from holes in the crystal lattice. One ⁴He atom moves without friction to fill the hole, leaving a new hole that is then filled by another atom, and so on.

Chan and Kim doused a porous glass disk with ⁴He and pressurized it to 60 atm. They lowered the temperature below 2 K, and the helium became solid. But as the system approached 0.1 K, the solid helium likely uncoupled from the glass pores, becoming a frictionless supersolid.

Though still considered “probable,” the discovery and its potential to further the understanding of quantum mechanics has generated enthusiasm. It would “open a lot of exciting possibilities if it turns out to be right,” says Mark W. Meisel, physics professor at the University of Florida, Gainesville.