Number 290 (Story #1), October 9, 1996 by Phillip F. Schewe and Ben Stein|
THE 1996 PHYSICS NOBEL PRIZE RECOGNIZES THE DISCOVERY OF SUPERFLUIDITY IN HELIUM-3 . David Lee and Robert Richardson of Cornell and Douglas Osheroff of Stanford, working at Cornell in the early 1970's, had to chill their helium-3 sample to a temperature of only about 2 mK before it transformed into a superfluid, a special liquid state of matter which can flow without viscosity. Superfluidity in the two helium isotopes is very different, a fact that stems from the fact that He-4, which consists of two electrons and a nucleus containing two protons and two neutrons, is a boson while He-3, which consists of two electrons and a nucleus containing two protons and only one neutron, is a fermion (Scientific American, December 1976). In He-4, the superfluid state is essentially a Bose-Einstein condensation of He atoms into a single quantum state. In contrast, the He-3 superfluid state consists of a condensation of pairs of atoms, somewhat analogous to the pairing of electrons in low-temperature superconductivity. (The discovery of superfluidity in He-4, at the much warmer of temperature of 2 K, occurred in 1938.) Furthermore, because its constituents (pairs of atoms) are magnetic and possess an internal structure, the He-3 superfluid is more complex than its He-4 counterpart. Indeed, superfluid He-3 exists in three different forms (or phases) related to different magnetic or temperature conditions. In one of these phases, the A phase, the superfluid is highly anisotropic; that is, it is directional, somewhat like a liquid crystal. To put it another way, this phase of He-3 (unlike He-4) has texture. This property was exploited in a recent experiment (Nature, 25 July 1996) in which vortices set in motion within a He-3 sample simulated the formation of topological defects ("cosmic strings") in the early universe. Another notable experiment in recent years was the verification (by Douglas Osheroff) of the "baked Alaska" model. This theory, formulated by Anthony Leggett of the University of Illinois, explains the somewhat piecemeal transition from the A phase of superfluid He-3 into the lower-temperature B phase by supposing that B-phase droplets can be nucleated within the supercooled A-phase by the ionizing energy of passing cosmic rays (Physics Today, June 1992).