Number 21, February 13, 1991 by Phillip F. Schewe and Ben Stein
THE QUANTUM HALL RESISTANCE IS THE SAME in gallium arsenide and silicon to within an uncertainty of 3.5 x 10-10. In 1980 Klaus von Klitzing discovered that under conditions of low temperature and high magnetic field, the Hall resistance for an electric current flowing in a semiconductor surface layer did not rise linearly with increasing magnetic field (as in the classic Hall effect observed since the 19th century) but instead exhibited plateaus in which the Hall resistance was equal to a fractional number times the ratio h/e2, where h is Planck's constant and e is the charge of the electron. This quantization of the Hall resistance has proved to be so precise that it became, as of January 1, 1990, the primary standard of resistance. Now scientists at the National Physical Laboratory and the University of Nottingham in Great Britain have compared directly (using a sensitive cryogenic comparator bridge) the Hall resistance in a GaAs/AlGaAs heterojunction with that in a silicon field effect transistor and found them to be the same to a new level of precision. Equivalently, the scientists claim that they have measured the von Klitzing constant, presumably equal to h/e2, with "a substantially lower uncertainty than that claimed for any previous measurement." This strengthens further the notion that the quantum Hall effect is independent of the host material. (A. Hartland et al., 25 Feb. 1991 issue of Physical Review Letters.)
EVIDENCE FOR A NEUTRINO WITH A MASS OF 17 keV has accumulated in labs at Oxford, Ontario (Guelph), and Berkeley (Eric Norman, LBL, 415-486-5088). Beta decay experiments with nuclei such as tritium, sulphur-35, and carbon-14 have uncovered anomalies in the energy spectrum of the electrons coming from the decays. The shape of the spectra has been interpreted to mean that the beta-decaying nuclei usually emit a neutrino which has little or no mass, but that occasionally (a few percent of the time) a neutrino with a mass of 17 keV is being emitted. This may be a tau neutrino---whose mass, along with that of the electron neutrino and the mu neutrino, has not been definitively measured---or it may be some new kind of neutrino. The discovery of so massive a neutrino would have implications for particle physics and for cosmology. (New Scientist, 2 Feb. 1991.)
DOES DARK MATTER CONSIST OF BARYONS? Joseph Silk of the University of California at Berkeley (415-642-2113) believes that compact stellar remnants such as neutron stars and white dwarfs (objects made from baryonic matter like protons and neutrons) rather than the hypothesized weakly interacting massive particles may account for much of the missing mass in galaxy halos and clusters. He suggests ways of testing this theory. (Science, 1 Feb. 1991.)
A RECORD HIGH MATTER DENSITY in an inertial-confinement fusion experiment has been achieved. S. Nahai of Osaka University in Japan has reported that 8-10 kilojoule laser pulses were used to compress a carbon-deuterium-tritium target to a density of 600 tons per cubic meter or, equivalently, a peak density of 1032 electrons per cubic meter, a density comparable to that at the Sun's core. (Nature, 7 Feb. 1991.)
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