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Super hot, superconducting thin film.

By making copper-oxide materials one layer of atoms at a time, Japanese materials scientists have created thin films that hint at record-high transition temperatures for superconductors.

In recent years, researchers have failed to push transition temperatures -- above which a material ceases to conduct electricity with no resistance -- much higher than 125 kelvins. So like other scientists, Tomoji Kawai of Osaka University and his co-workers have sought a better understanding of super-conductivity by controlling ever more precisely the structure and composition of copper-oxide thin films.

For the research, this Japanese group uses a technique called laser molecular-beam epitaxy. First, the researchers put the ingredients for the superconductor into a reaction chamber. Atoms knocked off these ingredients by the laser then settle onto a substrate and build up into a thin film. The researchers control the chemical makeup of the film by varying the timing and number of pulses aimed at each ingredient.

The reasearchers can stack up any number of layers of copper-oxide molecules. They have discovered that they can adjust the spacing between the layers of this so-called infinite-layer structure by sandwiching calcium and strontium atoms in different proportions between the layers. These sandwiched atoms prompt oxygen to squeeze in as well. The addition of oxygen improves superconducting properties, says Kawai. He and his coleagues also found that lowering the temperature of the film's substrate improves transition temperatures.

Thus, by inserting calcium as well as strontium between the layers and lowering the substrate temperature, they observed reliable indications of superconductivity up to 120 kelvins, he reports. That transition temperature represents a record for these layered copper-oxide materials, he adds. Kawai and his group have also observed unusual magnetic and resistance behavior in the film at 180 kelvins. But before he will believe that those observations suggest higher superconducting transition temperatures, "I need to see 150 kelvins." Kawai says.
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Author:Pennisi, Elizabeth
Publication:Science News
Article Type:Brief Article
Date:May 16, 1992
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