PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institut

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PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institute of Physics Number 179 May 17, 1994 THE RELAXATION OF A SINGLE DNA MOLECULE has been studied by attaching to it a 1-micron latex bead which then can be manipulated in a special configuration of lasers known as an "optical tweezers." Lit up by an infrared laser and viewed in a microscope, the DNA molecule can be sent through a series of maneuvers (including spelling out the letters "DNA") designed to measure the molecule's mechanical properties. Stanford physicist Steven Chu and his colleagues were also able to corroborate a theory of 1991 Nobel Laureate Pierre-Gilles de Gennes, who asserted that the movement of single polymer strand among other strands was analogous to one snake crawling past other snakes. (Thomas T. Perkins et al., Science, 6 May.) AN ION TEMPERATURE OF 429 MILLION K (37 keV) was achieved in the fusion experiments last December at Princeton's TFTR Tokamak, which for the first time used a fuel of 50% deuterium and 50% tritium in order to produce a record 6.2 megawatts of power (the 1991 experiments at the Joint European Torus, which produced the previous record of 1.7 MW, used a fuel of approximately 90% deuterium and 10% tritium). The central region of the TFTR plasma achieved a power density (from fusion reactions) of just over 1 megawatt/m**3, comparable to that expected in the first commercial reactors. There are encouraging signs that the alpha (helium-4) particles produced in the D-T reactions are directly heating electrons in the plasma. Alpha particle confinement and heating will be the focus of ongoing experiments at TFTR this year. (Upcoming articles in Physical Review Letters, May 23: J.D. Strachan et al., and R.J. Hawryluk et al.) A THOUSANDFOLD INCREASE IN THE MAGNETORESISTANCE (MR) in superlattice films has been observed by scientists at AT&T Bell Labs. The MR phenomenon, in which a material's electrical resistance is altered by a changing magnetic field, has already been used in magnetic recording heads. One figure of merit, the MR ratio, is the percentage change in resistance as an external magnetic field is switched between high and low values. The highest previous MR ratio was about 150% in a Fe- Cr multilayer film. The new MR ratio, obtained for 100 to 200-nm thick La-Ca-Mn-O films (grown epitaxially on a LaAlO3 substrate), is 127,000% at a temperature of 77 K and 1300% at room temperature. The field used was 6 Tesla. (S. Jin et al., Science, 15 April.) THE 1995 RESEARCH BUDGET REQUEST, submitted to Congress by President Clinton, calls for a 4% increase in R&D spending. Among physics programs at the Department of Energy, the 1995 request for high energy physics is $621.9 (all amounts are in millions of dollars), about the same as in 1994. The '95 request for nuclear physics is $300.8, down 14% from '94. The Basic Energy Sciences program, at $741.3, is down 6.2% from the previous year. The new request for magnetic fusion is $372.6, up 8.4% from '94, while the figure for inertial fusion is $176.5, down from $185.5. At the National Science Foundation, the 1995 figure for physics is $141.7 ($133.7 in '94), for materials research $185.5 ($175.6 in '94), and for geoscience $443.1 ($403.9 in '94). At NASA the physics and astronomy program calls for $1058.7 in 1995 ($1076.6 in '94), including $226.7 for the Hubble Space Telescope and $234.5 for the AXAF x-ray telescope. The planetary exploration request was $707.3, compared to $654.3 in '94. (Physics Today, April 1994.)

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