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Number 390, September 10, 1998 by Phillip F. Schewe and Ben Stein
NANOTUBE NANOLITHOGRAPHY. Carbon nanotubes have previously been used as tips in atomic force microscopes (AFM) for producing images. But now for the first time nanotube tips have been used as pencils for writing 10-nm-width structures on silicon substrates. Ordinary graphite pencils write by wearing themselves down, but this is not the case with nanotube pencils developed at Stanford by Hongjie Dai (hdai@chem.stanford.edu, 650-725-4518) and his colleagues. The robustness of the nanotube tips permits a writing rate- --0.5 mm/sec---five times faster than was possible with older AFM tips. The way the nanotube writes is for an electric field, issuing from the nanotube, to remove hydrogen atoms from a layer of hydrogen atop a silicon base. The exposed silicon surface oxidizes; thus the "writing" consists of narrow SiO2 tracks. The Stanford results should help the development of nanofabrication, since tip wear problems have been an obstacle to the use of probe microscopes in lithography and data storage at the nm size scale. (Dai, Franklin, and Han, Applied Physics Letters, 14 September 1998; figure at Physics News Graphics)
KAONS INSIDE SUPERNOVAS. K mesons (kaons) are exotic, short-lived particles of interest not just to high-energy physicists but also to astrophysicists since the behavior of K's inside dense nuclear matter can place severe constraints on the dynamics of supernova explosions and the stability of neutron stars. Recent experiments at the GSI lab in Darmstadt, Germany (Peter Senger, 011-49-6159-712-652, p.senger@gsi.de) have looked for K's in violent collisions between gold nuclei (at a beam energy of 1 GeV/nucleon). In those collisions, the reaction zone is compressed to about 3 times normal nuclear density for a very short time, about 5 x 10-23 sec. Then, this nuclear fireball explodes and the gold nuclei disintegrate. During the hot and dense phase, strange mesons---mostly positively charged kaons---are created. These emerge preferentially out of the plane of the collision; apparently the high density of the reaction zone offers the kaons nowhere to escape but up or down. The pattern of kaon trajectories indicates that the effective mass of the kaon is altered in the extreme nuclear environment, in line with other experiments. These data have been explained by the suggestion that anti-kaons "condense" at nuclear densities above 3 times normal nuclear matter density. As a consequence, one can predict that a star with a 1.5-2 solar-mass iron core will not subsequently be able to sustain itself as a neutron star following a supernova explosion but would instead collapse into a black hole. (Y. Shin et al., Physical Review Letters, 24 Aug.)
A PERIODIC TABLE FOR ARTIFICIAL ATOMS. At the atomic level, confinement leads to quantization: electrons trapped in atoms possess only certain discrete energies. All of this applies as well to quantum dots, which are sculpted from tiny layers of semiconductor and metal. By modulating an external electric field, electrons can be added to or subtracted from the dot one by one. In effect the dots are artificial atoms and, like real atoms, can be sorted into a Periodic Table according to their complement of electrons. The electrons reside in two- dimensional orbits. This and the fact that the dots (small as they are) are so much bigger than regular atoms permits the study of quantum effects not seen before. For one thing, more magnetic flux (from an external magnet) can fit inside a micron-sized dot than in an angstrom- sized atom; this can make magnetic interactions much more prominent. Physicists suspect that quantum dot molecules and even crystals will be a novel arena for building tailored materials and for exploring new quantum physics. (New Scientist, 29 August 1998.)
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