Lifetime analysis of positronium annihilating in nanometer voids is used to study the thermal expansion behavior of thin, Si-supported polystyrene films near the glass transition temperature T g . A reduction in void volume expansion is correlated with a reduction in the apparent T g as film thickness decreases. Our results can be fitted using a three-layer model incorporating a 50 Å constrained layer at the Si interface and a 20 Å surface region with reduced T g .[S0031-9007(97)02458-7]

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Transition to a superconducting zero-resistance state at 155 K is observed for the first time in bulk material. A new five-element compound has been synthesized with nominal composition YiBa2Cu3F20^. Fluorine plays a critical role in achieving this effect. X-ray diffraction and electron microprobe analysis indicate that the samples are multiphasic. Evidence is presented that the samples contain superconducting phases with onset temperatures considerably above 155 K. Magnetic measurements suggest a fluxtrapping effect below 260 K, and diamagnetic deviations from Curie-Weiss behavior in the range 250 K < r< 100 K indicate a Meissner effect in a small superconducting volume fraction.

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