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

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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.)


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