TRAPPING A SINGLE NANOPARTICLE BETWEEN TWO ELECTRODES has been controllably achieved for the first time, enabling researchers to deposit individual nanoparticles onto surfaces and offering possibilities such as single-nanoparticle switches. Researchers (Cees Dekker, Delft University of Technology, dekker@qt.tn.tudelft.nl) construct a circuit containing two platinum electrodes separated by as little as 4 nm--a gap that the researchers believe to be a world record. To trap nm-scale molecules or clusters, they immerse the electrodes in a solution containing the nanoparticles. Applying a voltage to the electrodes polarizes each particle and attracts a particle to the gap between the electrodes. Once a particle bridges the gap, current flows through the circuit, and a resistor then sharply reduces the electric field, discouraging any additional nanoparticles from entering the gap. In principle, this electrostatic-trapping technique can work for any polarizable nanoparticle; it has been demonstrated for nanometer-scale clusters of palladium (Pd) atoms, carbon nanotubes, and a 5 nm-long chain of thiophene (a conducting polymer). The researchers have also studied the properties of single electrons as they cross a Pd nanocluster between the electrodes. (A. Bezryadin et al., Applied Physics Letters, 1 September; images and more info available at Physics News Graphics)

QUARK STARS represent one segment on the sliding scale of collapsed stars stretching from white dwarfs to black holes. In between lie neutron stars, in which self-gravitation has forced electrons to merge with protons to form neutrons. At higher density, some of the nuclear matter may exist in the form of hyperons, heavy versions of neutrons which can be made artificially at accelerators on Earth. Hyperons are normally unstable and quickly decay, but would survive indefinitely inside neutron stars. Up to this point, the nucleons inside a neutron star are still baryons; that is, they each consist primarily of three quarks. But at higher density still, the baryons can melt, creating the quark-gluon plasma state being sought at the CERN collider in Geneva and (in the next few years) at the RHIC collider on Long Island. However, if physicists don't hurry, astrophysicists might spot evidence for the quark matter first. Rapidly spinning neutron stars (pulsars) gradually shed energy and angular momentum in the form of radio emissions and an electron- positron stellar wind. This causes the star to contract, jacking up the pressure a bit, making conditions more favorable for the creation of hyperons and quark matter. According to Norman Glendenning of LBL (nkg@csa.lbl.gov) and his colleagues S. Pei (Beijing Normal University) and F. Weber (Ludwig-Maximilians University of Munich) one in a hundred pulsars is undergoing the baryon-melting phase transition. They suggest ways in which this transition could be detected, and they look forward to the advent of a new "quark astronomy."(Norman Glendenning et al., 1 September Physical Review Letters; image available at Physics News Graphics.)

AT THIS YEAR'S PHYSICS OLYMPIAD , held in Sudbury, Ontario July 13-21, 1997, students from 56 nations took challenging experimental and theoretical exams. The Russian team received 4 gold medals, China earned 3, and Australia garnered 2. Everyone on the US team brought home a medal; Boris Zbarsky of Rockville, Maryland won a gold medal. (AAPT Announcer, Sept.1997; For more information contact Patrick Knox, American Association of Physics Teachers, 301-209-6430.)