Number 45, August 30, 1991 by Phillip F. Schewe and Ben Stein
AN ATOMIC SWITCH , whose "on" and "off" state depends on the position of a single atom, has been devised by researchers at IBM Almaden Research Center (contact: Don Eigler, 408-927-2172). Using STM technology, the IBM scientists have been able to manipulate a single xenon atom reversibly to and from nickel substrates. Voltage pulses change the position of the atom and designate the state of the switch, which can be determined by measuring the conductance between the tip and surface: if the xenon atom is on the substrate surface, the conductance is low and the switch if "off"; if the atom is instead drawn to the tip, the conductance is high and the switch is "on." This achievement may facilitate the design of electronic devices of atomic dimensions. Although the reading and writing of data to such devices would be painfully slow with present methods, the entire contents of the Library of Congress---estimated to take up the equivalent of 250,000 compact disks---would fit on a single twelve-inch disk composed of atom switches. (Nature, 15 August, 1991.)
LIMITS ON THE ELECTRON ANTINEUTRINO MASS have been further tightened by observations of the beta decay of molecular tritium by scientists at Los Alamos National Lab (John Wilkerson, 505-667-9429). In this latest update on the long-running effort to determine whether neutrinos have a nonzero mass---an issue with implications for cosmology---the Los Alamos scientists have established an upper limit of 9.3 eV for the mass of the electron antineutrino, markedly different from the mass range of 17-40 eV reported by a team at the Institute of Theoretical and Experimental Physics (ITEP) in Moscow. (Upcoming article in Physical Review Letters.)
A NEW MECHANISM FOR THE DIFFUSION OF ATOMS deposited on metal surfaces has been observed at Sandia National Lab. Previously it was thought that the only transportation option for atoms deposited on metal surfaces was for them to move on the surface, between the bumpy rows of metal atoms. But new results show that the deposited atoms can actually displace the substrate atoms and effectively trade places with their original hosts, a process which can be more feasible energetically than for the newcomer to move between the surface rows of atoms. This process has been observed for aluminum atoms on an aluminum surface and for platinum on platinum, as well as for other systems including Ir-Ir, Pt-Ni, and Re-Ir. (Science News, 24 August 1991.)
ELECTRON HOLOGRAPHY WITH ATOMIC RESOLUTION has been achieved by an IBM Zurich and Dalhousie University (Nova Scotia) collaboration. According to Hans-Werner Fink of IBM, the advent of electron holography has lagged behind optical holography using lasers because, for electrons, a coherent source of reference waves is difficult to come by; another problem is the aberrations caused by the lens system magnifying the hologram. The IBM-Dalhousie scientists used an ultra-sharp tip (tapering to a single atom), providing a coherent point source of electrons, to image thin gold films with atomic resolution, without the use of lenses. Moreover, unlike conventional high-energy transmission electron microscopy, the low-energy electron holographic technique may be well suited to work with biological samples. (Upcoming article in Physical Review Letters.)
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