Number 71, March 13, 1992 by Phillip F. Schewe and Ben Stein|
THE CONDUCTANCE OF A CROSSED-WIRE TUNNEL JUNCTION can be reversibly changed by a factor of 15 through the magnetic interaction between a tunneling current and what may be a single electron trapped in the junction. Stephen Gregory of Bellcore (908-758-2847) has made measurements of conductance under low temperature conditions in a junction in which electrons tunnel between a pair of crossed tungsten 10-micron wires separated by a monolayer of helium gas (forming in effect an atomic-sized near contact). By applying a mechanical strain to the junction, Gregory found that he could switch between two distinct conductance states. The experimental data suggested to him that he was switching on and off a strong magnetic interaction, known as the Kondo effect, an interaction, studied in other areas of condensed matter physics, in which conduction electrons scatter strongly from the magnetic moments of impurity atoms. Because neither the tungsten nor the helium is magnetic, Gregory believes that the magnetic moment doing the scattering arises from an electron in an impurity atom in the junction. A tunnel junction with a single isolated magnetic dipole would not only provide optimal conditions for studying the Kondo effect but would also lead to novel quantum devices. (Physical Review Letters, 30 March 1992.)
BLUE DWARF GALAXIES seem to have been the predominant type of galaxy several billion years ago. This notion, based on infrared observations recorded by Lennox Cowie of the University of Hawaii late last year, have caused cosmologists to rethink the sequence of galaxy evolution. The chief question is what happened to the blue dwarfs. Did they merge into the much larger galaxies we now see, or are they still out there but so dim as to be undetectable? Cowie plans to search for other galaxies that may have co-existed with the blue dwarfs. (Science, 28 February 1992.)
THE "TEXTURE" THEORY OF COSMOLOGY holds that in the early universe Higgs fields, the hypothetical fields responsible for disturbing the equivalence of the physical forces, may have taken on different values in different regions of the universe and that at the borders between these domains huge concentrations of energy would have occurred. Such "topological defects" in space-time would exist over a range of sizes and would give rise to gravitational clumping of matter also over a range of scales. The texture theory departs from the inflationary model of cosmology in predicting that galaxies would form earlier and that distributions of clusters and superclusters would be denser. The texture theory also predicts that in general galaxies cluster more densely than dark matter and that unwinding textures would leave behind distinctive hot or cold spots on the cosmic microwave background at a level of one part in 10,000. This latter prediction should be testable as new COBE data become available. (Scientific American, March 1992.)
B FACTORIES WILL NOT BE BUILT , according to new DOE and NSF budget reports. Particle physicists at Stanford and Cornell had sought to build electron-positron colliders dedicated to producing B mesons, whose decays, it is thought, would provide valuable information about the little-understood violation of CP invariance.