The existence of a new state of matter called a "supersolid" has been called into question by the reports of two groups with apparently conflicting results. Physicists from Cornell University in NY, USA claim to have reproduced previous findings of a supersolid transition in helium below 250 mK (Phys. Rev. Lett. 97 165301). On the other hand, a group from Helsinki University of Technology in Finland has found no transition when helium is cooled down to 10 mK (Phys. Rev. Lett. 97 165302).
The supersolid state of matter is predicted to occur at very low temperatures when the vacant lattice sites in a regular solid condense into the lowest quantum state and exhibit fluid-like properties. The result is a material that appears mostly like a solid, but with a small proportion (about 1%) flowing like a liquid. This bizarre behaviour was first glimpsed by Pennsylvania State physicists Eunseong Kim and Moses Chan in 2004, who noted a change in the rotational inertia of solid helium at temperatures below 230 mK.
The Cornell physicists, led by John Reppy, confirmed these findings using a similar technique. By sealing a sample of solid helium in a torsion chamber – a device that supports the sample as it twists to and fro on a springy length of rod – they could monitor the rate of oscillations as it was cooled. At around 200 mK, the chamber began to twist faster. This, they concluded, was due to the vacancies decoupling from the oscillation and moving throughout the helium like a fluid.
The physicists then reheated the helium and found that it did not turn back into a supersolid when they cooled it down again half a day later. According to Chan, the Cornell group is the first to have witnessed these controversial annealing effects. “Three other groups including us have failed to show the destruction of a supersolid state by annealing. One possibility is that [annealing] is reducing the defects in the crystal, but there is no widespread agreement.”
Over in Europe, however, the Helsinki team failed to find any evidence for helium becoming a supersolid at temperatures as low as 10 mK. Opting for a thermodynamic approach, they measured the melting temperature of helium at different pressures, discovering an “excess” of entropy (disorder) below 80 mK. But rather than increasing with temperature, as would have been the signature of a supersolid transition, the entropy actually decreased. Igor Todoshchenko, leader of the Helsinki group, was unsure what this odd behaviour was due to. “It might be that only crystals of bad quality show [the transition]. This is a hot topic at the moment and many physicists are trying to understand it.”
