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Number 404, November 30, 1998 by Phillip F. Schewe and Ben Stein
POSITRONIC WATER, a molecule consisting of an oxygen atom bonded to two positronium (Ps) atoms (each consisting of a positron and an electron bound together) instead of hydrogen atoms (proton-electron systems), has been predicted to exist by researchers (David Schrader, Marquette University, 414-288-3332). Positronic water could prove to be a rich playground for exploring electromagnetic interactions between trios of charged particles; such interactions are much rarer in an ordinary atom or molecule since all of the charged particles outside the nucleus are electrons which repel one another. Using a "Monte Carlo" technique that analyzed the various configurations of the 10 electrons and 2 positrons that would exist in a positronic water molecule, the researchers found that the energy associated with a positronic water molecule (abbreviated Ps2O) is 1.27 eV less than that for a dissociated system of Ps and PsO, suggesting that stable positronic water can be formed, although with chemical bonds about 4 times weaker than those which exist in water. The model predicts an average lifetime of 220 ps for a Ps2O molecule. To realize this molecule experimentally, however, researchers need to create higher- concentration positron sources than presently available. To form Ps2O one might shoot nanosecond-length pulses of cold positron beams onto a metal surface covered with a sprinkling of oxygen atoms. (Jiang and Schrader, Physical Review Letters, 7 December 1998.)
ANTI-MEISSNER EFFECT. Chill a superconductor in the presence of a magnetic field. At the critical temperature superconductivity happens---resistance drops to zero---but at first only in an archipelago of tiny islands at the material's surface. If the sample is smooth enough, which is the case at Andre Geim's lab (geim@sci.kun.nl) at the University of Nijmegen in the Netherlands, the superconductivity will extend uninterrupted all the way around the sample like a sheath 100 to 1000 atoms deep, trapping any magnetic flux that is in the sample. As the sample is cooled further the superconductivity regime takes over more and more of the interior, tightening the noose around the flux already inside. This in turn makes room for more flux to enter the outer precincts of the sample. In effect the sample attracts and promotes magnetism. This is an example of a paramagnetic (magnetic enhancement) Meissner effect, in contrast to the diamagnetic (magnetically neutralizing) Meissner effect customarily observed in superconductors. Geim's experiment, using micron-sized disks, is the first to measure this effect with a precision of better than one magnetic flux quantum. From his results, Geim argues two points: (1) that if samples have smooth enough surfaces the anti-Meissner effect would be more typical of superconductor behavior than the conventional Meissner effect; and (2) that since his sample is made of aluminum (a low-temperature superconductor) the anti-Meissner effect cannot be exclusive to high-temperature materials, suggesting that the effect cannot serve as direct evidence in favor of the idea that superconductivity in high-temperature materials is characterized by charge carriers that are so called d-wave Cooper pairs. (Nature, 12 November 1998.)
THE NUMBER OF NEW PHYSICS BACHELOR'S RECIPIENTS in the US has again gone down a new report shows. The most recent data applies to the class of 1997, for which 3826 physics degrees were awarded, 8% less than the previous year and 24% lower than the recent high in 1989; female students accounted for 19% of the degrees. About 41% of the graduates directly sought jobs. The median starting salary for civilian work was $34,800. ( AIP "1997 Bachelor's Degree Recipients Report"; contact Patrick Mulvey, pmulvey@aip.org.)
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