In recent years, no topic in condensed matter physics has been hotter than the study of ultracold solid helium. Subtle experiments suggest that as temperatures dip toward absolute zero, crystalline helium can bizarrely flow like a liquid with no viscosity, a phenomenon known as supersolidity. Now, a new experiment lends credence to that controversial claim by revealing a possible second sign of the transition.
The first evidence for supersolidity emerged in 2003 and 2004. Moses Chan and Eun-Seong Kim of Pennsylvania State University in State College filled a tiny can with liquid helium and squeezed the ultracold stuff to greater than 25 times atmospheric pressure to make it solidify. They then set the can twisting back and forth on top of a thin shaft. Below a temperature of about 0.2 kelvin--2/10 of a degree above absolute zero--the frequency of the twisting shot up as though the can had become less massive. That implied that some of the helium had let go of the can and was standing stock-still while the rest of it continued to twist back and forth. That in turn suggested that the solid helium was flowing through itself without any resistance, a phenomenon known as supersolidity that had been hypothesized in the 1960s (Science, 1 July 2005, p. 38).
But some believed that Kim and Chan had observed less-mysterious "superfluid" liquid helium wending its way through cracklike defects in the crystal. That alternative interpretation got a shot in the arm in 2006, when Ann Sophie Rittner and John Reppy of Cornell University claimed that they, too, had seen the phenomenon but could make it go away by gently heating and cooling the solid helium to smooth over defects in the crystal (ScienceNOW, 15 March 2006).
Now, Chan and colleagues Xi Lin and Anthony Clark have new results that suggest supersolidity may be a property of the solid crystal after all. If that were true, then the onset of supersolidity ought to be a "thermodynamic phase transition," much like the freezing of water into ice or the emergence of magnetism in hot iron as it cools. During such transitions, the heat capacity of a material--the amount of heat required to raise the temperature of the stuff--increases dramatically. And that's what Chan and colleagues see in solid helium, they report this week in Nature.
Technically, this was no small feat. The heat capacity of the helium is much smaller than that of the metal container holding it. So to spot the peak, the researchers had to use a special container fashioned out of silicon and take precautions to prevent any unaccounted heat leaks. Chan cautions that the team hasn't yet proved that the heat-capacity signal, which actually occurs at a slightly lower temperature than the onset of flow, is tied to supersolidity, however. "That's why we use the word 'probable,' " he says, "in case there is another explanation."
But if the peak is really there, it bolsters the case that supersolidity involves a real phase transition, says experimenter Norbert Mulders of the University of Delaware, Newark. However, he adds, "I have a pretty good idea of how difficult these measurements are, so it would probably take [confirmation by] some competing experiment to completely convince me."
