Number 173, April 12, 1994 by Phillip F. Schewe and Ben Stein
COSMIC STRINGS are hypothetical defects in space/time resulting from
the failure of the early expanding (and cooling) universe to pass over
entirely from a symmetric phase, in which all the physical forces are comparable
in strength, to an asymmetric phase in which the forces are unequal. Some
cosmologists believe that cosmic strings might have acted as the seeds
for the development of early galaxies. Now, scientists at the University
of Lancaster in Britain have simulated this process in a container of superfluid
helium-4. Their effort at "experimental cosmology" consisted
of rapidly decreasing the pressure of the helium while maintaining constant
temperature. The ensuing phase transition set up swirling vortices, with
non-superfluid trapped inside and superfluid helium on the outside. Previously
cosmic-string-like defects were observed in liquid crystals but, as W.H.
Zurek of Los Alamos puts it in a commentary on the Lancaster results, liquid
crystals are a "messy" system and do not exhibit the interesting
rotation effects seen in the superfluid. (P.C. Hendry et al., Nature, 24
March 1994.)
IS HIGH-TEMPERATURE SUPERCONDUCTIVITY d-WAVE in nature? Low-temperature
superconductivity is characterized by "s-wave" Cooper pairs.
That is, supercurrent consists of pairs of electrons in composite states
with zero angular momentum. By contrast, in high- temperature superconductivity,
some theorists believe, the Cooper pairs are in "d-wave" states
having an angular momentum of two units. At the March APS meeting in Pittsburgh,
John Kirtley of IBM reported on an experiment measuring the spontaneous
magnetic flux through tiny high-temperature superconducting rings. The
flux equaled integral multiples of the basic unit of magnetic flux (defined
as Planck's constant divided by twice the charge of the electron) when
the ring consisted of one or two crystal grains. However, the measured
flux proved to be only half the basic flux unit when the ring was made
of three grains, a configuration which has a different quantum effect on
the supercurrent as it flows around the ring than does a ring with only
two grain boundaries. This half-integral flux had never been directly observed
before, and the IBM scientists believe that their finding supports, but
does not yet prove, the notion that the electron pairing in their high-temperature
superconductor is d-wave in nature. (Science News, 2 April 1994.)
MOLECULAR LEVEL INTERNAL COMBUSTION ENGINES , convert chemical energy
into mechanical motion by burning single molecules. No such motor has yet
been made artificially, but examples abound in biology, where the "fuel"
is adenosine triphosphate (ATP), the basic energy-carrying molecule in
cells. Two examples of "protein motors" are myosin, which slides
past actin filaments to produce muscle contraction, and kinesin, which
transports material in cells. The size of the kinesin motor is 12 nm, 50
times smaller than the smallest transistor now made. Marcelo Magnasco (212-327-8542)
of Rockefeller University and NEC Research Institute has developed a general
framework for such motors which describes the relationship between their
state of motion and the rate at which they consume chemical fuel. Magnasco's
description paves the way for a fundamental, physics-based understanding
of motor proteins and provides insights into designing artificial ones.
(Upcoming article: 18 April, Physical Review Letters.)
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