Solid helium is an odd and elusive material. At pressures less than 25 atmospheres, it doesn’t even exist. The interaction of the helium atoms is so weak, and their quantum zero-point motion so great, that helium can remain liquid down to absolute zero. Decades ago it was predicted that atomic vacancies, inherent in the solid due to zero-point energy, could condense into a single quantum state and allow the solid to flow without friction. The predicted phase was termed “supersolid,” in parallel with superfluidity and superconductivity.
In 2004, Moses Chan, of the Pennsylvania State University, and his then-student Eun-Seong Kim published results strongly suggesting that they had observed supersolidity. 1 (See Physics Today, Physics Today 0031-9228 57 4 2004 21 https://doi.org/10.1063/1.1752411 April 2004, page 21 , and Physics Today 0031-9228 57 11 2004 23 https://doi.org/10.1063/1.1839368 November 2004, page 23 .) The researchers confined solid helium in a cylindrical cell,...
