Liquid helium-3 and helium-4 are remarkable substances. They are quantum liquids, meaning that their behavior is governed by the laws of quantum mechanics. Because of their small atomic mass, each isotope exists in a liquid state down to the temperature of absolute zero. And at sufficiently low temperature, each becomes a superfluid. However, the two isotopes have very different properties because 3He is a fermion and 4He is a boson. As a result of their different statistics, superfluidity in 3He appears at a temperature one-thousandth of that at which superfluid 4He forms. A second difference is that 3He has multiple thermodynamic phases.
The discovery of superfluid 3He was recognized with two Nobel Prizes in Physics, one for experimental work and one for theory. The 1996 prize was awarded to David Lee, Douglas Osheroff, and Robert Richardson for their pioneering 3He NMR measurements, carried out in 1971 at Cornell University.1 (See Physics Today, December 1996, page 17.) As the Cornell experiments were being conducted, Anthony Leggett, then at the University of Sussex, showed that superfluid 3He is the realization of the Bardeen-Cooper-Schrieffer (BCS) pairing theory of superconductivity in a quantum liquid; he thus made an important connection that is still relevant to research on superconductivity.2 Leggett’s theory also accounted for the two observed phases of pure superfluid 3He, and for it, he received a share of the 2003 Nobel Prize in Physics. (See Physics Today, December 2003, page 21.)
In this article we highlight the discovery of new superfluid phases in 3He infused into amorphous, extremely light, high-porosity solids known as aerogels. Before 1995 it was not clear that the superfluid could actually survive in such a messy environment. (See the article by Moses Chan, Norbert Mulders, and John Reppy, Physics Today, August 1996, page 30.) And if it could survive, what would be the properties of the disordered superfluid?

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