Helium, the lightest of the noble gases, liquefies at 4.2 K. But because its interatomic attraction is so weak and its zero-point quantum fluctuation so large, it never solidifies at pressures less than 25 atmospheres. At 2.2 K, a fraction of liquid helium-4, the dominant isotope, becomes a superfluid; it flows without viscosity or frictional dissipation.

Around 1970, a number of theorists suggested that even solid 4He might exhibit superfluid-like behavior. Because lattice vacancies in crystalline 4He were thought to persist and remain mobile down to absolute zero, it was conjectured that they would condense into a coherent quantum state in which vacancies, and therefore the atoms whose absence they mark, would flow unhindered through the solid lattice.

Experimenters soon undertook searches for a small “supersolid” phase in solid 4He. But only in 2004 was there an apparently real sighting (see Physics Today, April 2004, page...

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