Neutron scattering results have thrown doubt over the existence of a ghostly state of matter called a supersolid. The research, carried out by scientists in the US and the UK, found that samples of ultracold helium do not exhibit the trademark of a supersolid -- mass occupancy of the ground state. This will dampen physicists' hopes that solids -- like liquids and gases -- can indeed become a quantum "condensate" (Phys. Rev. Lett. 98 205301).
Supersolidity was first predicted in 1969 by Russian theorists Alexander Andreev and Ilya Liftshitz. They said that lattice vacancies, which usually only occur at finite temperature, could still exist at temperatures close to absolute zero in weakly-bound elements such as helium due to quantum “zero point” energy. By cooling solid helium to low temperatures, these vacancies could all collapse into the same ground state, becoming what is known as a Bose-Einstein condensate (BEC). In this supersolid state, vacancies would behave as a coherent entity, moving throughout the rest of the solid effortlessly like a superfluid.
Physicists Moses Chan and Eun-Seong Kim of Pennsylvania State University in the US found the first evidence for supersolidity when they noted a small change in the rotational inertia of a sample of pure helium-4 supported inside a torsion oscillator below a temperature of 230 mK. This, they concluded, meant 1% of the sample had condensed into a supersolid and so had remained at rest in the lab frame. Since then, however, several groups have shown that the supersolid state can be removed by “annealing” a helium sample beforehand to remove any impurities. This has led physicists to assume that supersolidity must rely on a finite amount of disorder (see related story: “Supersolids reliant on disorder, say physicists”).
However, new results obtained using the ISIS neutron source at the Rutherford Appleton Laboratory in the UK appear to contradict the findings of Chan and Kim. Oleg Kirichek of Rutherford and colleagues from the UK and US investigated how neutrons scatter off the atoms and vacant lattice sites inside unannealed helium-4 that had been cooled to 80 mK. By using a computer model to convert their data into a momentum distribution, they have found that there is no mass occupation of the ground (zero-momentum) state by either atoms or vacant lattice sites.
According to Kirichek, these results show that the theory by Andreev and Liftshitz does not apply to “high quality” solid helium-4. However, they do not reveal why Kim and Chan observed the superfluid-like effects of supersolidity in their torsion experiment, which used samples of the same quality helium. “I don’t think there is an unambiguous answer,” John Goodkind, who was the first to start looking for the supersolid state in the 1980s, told Physics Web. “We are all hoping that more experiments will clarify the situation.”
