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Number 358, February 11, 1998 by Phillip F. Schewe and Ben Stein
SUPERCONDUCTIVITY-DEPENDENT FRICTION. A common parlor trick consists of yanking a tablecloth out from beneath an assortment of tableware. If you yank fast enough, the wineglasses remain where they are. At Northeastern University, this feat is acted out on a nanometer scale, with a thin (only one or two molecules thick) sliver of frozen nitrogen acting as the tableware, and a lead substrate as the tablecloth. Jacqueline Krim (617-373-2902) and her colleagues perform this kind of experiment in order to study friction at the atomic level. Despite the billion-dollar industrial importance of friction, it is still relatively little understood. In Krim's work a delicate quartz microbalance (with the lead substrate) is moved back and forth about 10 nm at rates of a million times per second, with the overlying nitrogen going along for the ride. With this approach the Northeastern researchers have measured what are probably the smallest frictional shear stresses yet seen (excepting only superfluids, which experience no friction). But a phenomenon even more interesting has emerged: when the lead substrate is chilled below its superconducting transition, the friction between the lead and the frozen nitrogen dramatically drops. This seems to represent a new and unexpected behavior of superconductors, and this has fascinated and puzzled theorists. (A. Dayo et al., upcoming article in Physical Review Letters.)
INK-JET PRINTING OF LIGHT-EMITTING POLYMERS onto a thin film has been demonstrated by a Princeton group (James Sturm, 609-258-5610), bringing about a new way to fabricate a light-emitting diode (LED) made of polymers. An LED is typically built by surrounding a semiconducting material with two electrodes. When an electron from one electrode and a hole from the other meet in the semiconductor, they can annihilate each other and release the energy as light. LEDs in which the semiconductor materials are polymers instead of inorganic materials such as gallium phosphide would be cheaper and easier to manufacture. To make polymer LEDs, the Princeton researchers replaced the ink cartridges of a conventional ink-jet printer with a polymer solution containing the semiconducting polymer polyvinylcarbazol (PVK) and a light-emitting dye dissolved in a chloroform solvent. The researchers printed this solution onto a thin polyester film coated with indium tin oxide (ITO), which served as one of the electrodes. Over the polymer layer they deposited a metal film, which served as the other electrode. With this technique, they produced LEDs emitting green light. In separate experiments, they used the ink-jet printer to make dot patterns of PVK mixed with either red, green, or blue dyes on the ITO-coated polyester film, although they have not yet used these patterned films to make LEDs. (T.R. Hebner et al., Applied Physics Letters, 2 February 1998.)
LIQUID CARBON is difficult to produce because a sample of solid carbon, melted quickly by a laser, will want to repose back into the form of graphite. Physicists at the Russian Academy of Sciences (Moscow) have sought to melt carbon with picosecond laser pulses, and report the observation of a liquid phase, the evidence being the fleeting presence of periodic stripes in microscopic pictures of the tiny (200 micron) spots on a graphite surface under bombardment. The researchers argue that the stripes could not be present in a fully solid phase. The liquid is scarcely glimpsed, however, before it quickly solidifies, partly into an amorphous carbon structure. (M.B. Agranat et al., Journal of Experimental and Theoretical Physics (JETP) Letters, a Russian journal translated into English by AIP.)
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