PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institut

PHYSICS NEWS UPDATE
A digest of physics news items by Phillip F. Schewe, American
Institute of Physics
Number 173  April 12, 1994

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 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, that the electron pairing in their high-temperature
superconductor is d-wave in nature.  (Science News, 2 April 1994.)

INTERNAL COMBUSTION ENGINES, on the molecular level,
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.)