Text: http://www.nature.com/nsu/020916/020916-19.html Crystal remembers sounds Information-technology material re-emits sounds it hears. 23 September 2002 PHILIP BALL Squeezing lithium niobate produces an electric field. © Oklahoma State University Crystal Growth Laboratory Scientists have discovered a crystal that answers back. They sent a sound wave into the material, there was a quiet pause, then it suddenly emitted the same sound. The material, lithium niobate, is used in information technology. Its acoustic memory might help manufacturers to assess crystal quality, suggest its discoverers, Mack Breazeale and co-workers at the University of Mississippi. This acoustic quirk of lithium niobate might be connected to its other unusual and useful electrical properties. It is piezoelectric, producing an electric field when squeezed, and electric fields change the way light passes through it. This makes it suitable for use in fibre-optic telecommunications and holographic memories, where laser beams read information in and out. Each lithium niobate crystal is a patchwork of so-called ferroelectric domains. Breazeale's team suspects that the frequency of the delayed echoes a crystal produces is related to the size of these domains, which determine the material's suitability for various applications. Sound idea Lithium niobate seems to store acoustic energy temporarily, rather like a compressed spring stores mechanical energy. How is not clear, but the researchers reckon the acoustic wave squeezes the material as it passes through. This produces electric fields within the crystal, which in turn move the electrically charged atoms that the solid contains - just as a breeze passing through a cornfield stores energy by bending all the stems. When the acoustic input stops, the ions move back, but not all in the same direction - the movement is divided into domains, separated by boundaries where the direction changes. As the ions spring back, they release the stored energy as a delayed acoustic wave. This causes each domain to ring like a bell. The strongest echo corresponds to the resonant frequency of the domains, which, like that of a bell, depends on size.

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