The Science Behind the Prize

The discovery by Lee, Osheroff, and Richardson of superfluidity in liquid helium-3 completed the triad of discoveries that collectively constitute the major achievement of low temperature physics in this century. The phenomenon in helium-3 resembles and yet differs remarkably from its other two manifestations in superconductors and in the more common isotope, liquid helium-4.

The impact of the discovery on subsequent physics has been immense. It has transformed the direction of theoretical and experimental research in low temperature physics, stimulating advances in our understanding of the hydrodynamics of intricately ordered systems, the microscopic theory of electrons in metals, and the range of phenomena accessible to nuclear magnetic resonance probes. Some have even seen in it suggestive implications for phenomena in astrophysics.

Two and a half decades later their achievement shines as the last great conquest on the low-temperature frontier. Surely some day it will be surpassed, but as subsequent explorations at the lowest achievable temperatures fell back from that pinnacle, theirs has now acquired the same quality as a piece of science as the human presence on the moon has acquired as a piece of technology: a standard from a golden age, against which future achievements are to be judged.

Here is the context for the discovery. The Bardeen-Cooper-Schrieffer theory of superconductivity, now so firm a piece of physics that the possibility of a new mechanism for superconductivity at higher temperatures has been hailed as revolutionary, had been in existence for only 15 years in 1972. While by 1972 the theory was no longer controversial, one of its most dramatic predictions had not been confirmed: that the same kind of self-organization responsible for the superconductivity of metallic electrons, should result in a new kind of superfluidity in the rare isotope helium-3, similar only in name to the superfluidity that had been known for decades to exist in the more common helium-4. An international quest for this holy grail had become embarassingly unsuccessful, as each new advance in the technology of reaching lower temperatures failed to reveal the expected superfluid behavior. By the 1970's the search had proceed to temperatures a thousand times lower than the temperature at which helium-4 becomes superfluid, repeatedly defying the predictions of theorists.

Lee, Richardson, and Osheroff found the signature of the transition not in the hydrodynamics of helium-3, but in its magnetic behavior. Their discovery was the culmination of a program in low temperature physics, started single-handedly by Lee a decade earlier, which had already achieved worldwide recognition. They and their competitors immediately recognized the significance of what they had seen, and work in all the great low temperature laboratories of the world was diverted to the further study of both the magnetic and the hydrodynamic properties of what still, 25 years later, is without question the world's most remarkable form of condensed matter. Superfluid helium-3, which the original discovery already revealed to come unexpectedly in two varieties (called the A-phase and the B-phase), combines in one material the quantum phase coherence of a conventional superconductor with the orientational ordering of a liquid crystal, as well as exhibiting a variety of spectacular magnetic, acoustic, and hydrodynamic properties that simply do not exist in any other known materials. The original discoverers continued to play a major role in the subsequent exploration and elucidation of these many diverse properties.

Nobel prizes have been awarded for work in low temperature physics in 1913 (Kamerlingh Onnes), 1962 (Landau), 1972 (Bardeen, Cooper, and Schrieffer), 1973 (Josephson), 1978 (Kapitsa), and 1987 (Bednorz and Muller). They have all been for work on superfluidity or superconductivity. The discovery of Lee, Osheroff, and Richardson ranks with the loftiest peaks in this tradition. It has afforded the pairing theory of Bardeen, Cooper, and Schrieffer a new test of an entirely different kind, it has provided new and spectacular examples of Josephson's phase coherence, it has afforded entirely novel tests of the two-fluid hydrodynamics invented by Landau to account for the very different superfluidity of helium-4, it has enlarged the arena of phenomena from which to search for a theoretical understanding of the discoveries of Bednorz and Muller, and it has provided the first example of a neutral syhstem, consisting of whole atoms, displaying the same strange behavior that Kamerlingh Onnes was so startled to find, more than 80 years ago, in the electronic behavior of metals.

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