Helium-4 is a remarkable element. At a pressure of 1 atmosphere (1.01 bar), it remains a gas until cooled to 4.2 K, at which point the 4He vapor pressure and atmospheric pressure are equal and the 4He liquefies. Upon further cooling in equilibrium with its vapor, at about 2.2 K the liquid undergoes a spectacular phase transition to a superfluid state. And due to quantum mechanical zero-point motion, it remains a liquid upon cooling to as close to absolute zero as physicists have been able to reach. It solidifies only with the application of some 25 atm pressure at very low temperature.
Theorists, notably Alexander Andreev and Ilya Lifshitz at the Institute for Physical Problems in Moscow (now the P. L. Kapitza Institute for Physical Problems) and Geoffrey Chester at Cornell University, investigated solid 4He from several angles. By around 1970 they had concluded that it might be possible for the solid to undergo a rather strange transition to a state of matter in which crystalline order and Bose condensation coexist. At the time, some theorists thought that the solid 4He possesses mobile ground-state vacancies. Since the motion of vacancies comes with motion of atoms, Chester and other theorists speculated that the solid could demonstrate superfluid-like properties: Atoms of 4He might be able to move coherently through an otherwise well-ordered solid.
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