Number 290, 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).
THE THEORY OF BIG BANG NUCLEOSYNTHESIS (BBN) tells the saga of how nuclei,
especially deuterium, helium, and lithium, were made in the early minutes
of the universe (see Update 247). Along with the microwave background and
the mutual recession of galaxies, the observed abundance of primordial
He, D, and Li is one of the chief supports of big bang cosmology. Recent
measurements, however, suggest that BBN estimates for D and He-3 abundance
are too high, and that the theory (or the observations) must be amended.
Physicists at Berkeley (Erich Holtmann, firstname.lastname@example.org) and the
University of Tokyo tackle this problem by invoking a hypothetical exotic
particle. The presence of such a particle (with the right mass and lifetime)
in the early universe might, in the act of decaying, have provided a torrent
of gamma rays which dismembered deuterium (into two hydrogens) and He-3
(into H and D), bringing their numbers into line with modern measurements.
He-4, more tightly bound that He-3, would be relatively immune to the marauding
gammas. If this scenario is correct, the particle in question might well
have been a "gravitino," the fermion cousin of the graviton.
Gravitinos are ordained as part of "supersymmetry," a theory
which holds that all known fermions (such as electrons or quarks) have
hypothetical boson counterparts and vice versa. (Holtmann et al., upcoming
article in Physical Review Letters.)