Writing in the 15 January 2004 issue of the journal Nature, Penn State physicist Moses H. W. Chan and his graduate student, Eun-Seong Kim, explain that their material is a solid in the sense that all its helium-4 atoms are frozen into a rigid crystal lattice, much like the atoms and molecules in a normal solid such as ice. The difference is that "frozen," in this case, doesn't mean "stationary." Because helium-4 lattice is so very cold, less than one tenth of a degree above absolute zero, the laws of quantum uncertainty take over. In effect, the helium atoms start to behave as if they were both solid and fluid--at the same time. Under the right circumstances, in fact, some fraction of the helium atoms can begin to move through the lattice like a substance known as a "superfluid": a liquid that moves with no friction whatsoever. Thus the name "supersolid."
Chan and Kim's work, which was funded by the National Science Foundation (NSF), is described in a Penn State press release posted on the EurekAlert site. That site has an embargo of 1 pm Eastern time, 14 January 2004. After that time, the release will also be available at http://www.science.psu.edu/alert/Chan1-2004.htm.
In addition, NSF has prepared an animation that illustrates the basics of Chan and Kim's experimental setup, and the supersolid behavior they believe they have detected.