Macroscopic quantum properties of helium-4, one of the simplest and oldest elements in the universe, continue to puzzle and amaze scientists. Supertransport in solid helium-4 is the most elusive and controversial conundrum of all.
Among known states of natural matter, supersolids are often regarded as counterintuitive in the extreme. For more than 50 years, a number of authors with different perspectives such as Gross [1], Andreev and Lifshitz [2], Thouless [3], and Chester [4], have contemplated this exotic state. Thought to be the coexistence of superfluid and crystalline orders in the same single-component material, supersolids still attract broad interest. The original predictions were made for perfect quantum crystals in free space, with the most promising candidate being solid helium-4 (4He), which is known to have the largest amplitude of zero-point motion of atoms in the ground state. Such motion was presumed to create a stable (repulsive) gas of zero-point vacancies undergoing Bose-Einstein condensation at low temperature. Thus the perfect crystal would simultaneously be a solid and a superfluid. This simple and attractive idea sparked a wide experimental search for supersolidity in solid 4He, which, however, yielded no positive results. Recent interest in the supersolid state of matter was reignited after observations of a signal consistent with supersolidity in the torsional oscillator experiment performed by Kim and Chan at Penn State [5] in 2004. In what follows, we will survey both the recent experimental findings and the theoretical landscape in supersolid research.
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"I predicted this in 1969. (1969). "Destruction of Superflow in Unsaturated 4He Films and the Prediction of a New Crystalline Phase of 4He with Bose-Einstein Condensation", Physics Letters, Vol. 30, No. 5, November 3, 1969, pp. 300–301." - Jack Sarfatti