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Disorder goes deeper than the surface

Beauty may be only skin deep, but contrary to many textbooks, structural disorder is not.

Argonne theorists Dima Feldman and Valerii Vinokur reported in the Nov. 25 issue of Physical Review Letters that weak disorder on the surface of a material causes bulk disorder. Their discovery may force scientists and engineers to rethink the way they design diagnostic tools, electrical devices and research experiments that depend on structural order.

"The general belief was that if you have something [impure] only on the surface, then the bulk remains intact because the surface is just a teeny tiny part of everything," said Vinokur. "What we see now, is that this common wisdom is not the full truth."

Using mathematical techniques, some of which have been developed only within the last decade, Feldman and Vinokur demonstrated that weak impurities on a surface can destroy bulk order enough to alter the material's characteristic properties.

The molecules in structured materials are arranged much like bricks in a brick wall, Vinokur explained. The precision with which those "bricks" line up determines some of the material's characteristics, like conductivity or light-scattering ability.

As researchers and engineers control the structural purity of materials they use, they expect to be able to predict the characteristics those materials express, such as the conductivity of wires in an electrical device. But surface impurities are often disregarded, Vinokur said, which may lead to unusual results.

DISORDER - Argonne theorists demonstrated that small surface impurities may destroy long-range order. As molecules shift slightly to reduce physical strain, disorder propagates throughout the material.

"What you get may be different from what you expect, and then you may ascribe this difference to the wrong cause," he said. "You may never be able to design what you wish, because you don't know what is happening."

Understanding how bulk properties can be influenced by corruption on the surface will account for some of these differences, and make it possible to design more precise equipment. The knowledge may also be put to use as a non-destructive diagnostic tool to detect impurities on a hidden surface. For example, detecting a change in light-scattering ability of a liquid crystal display might suggest surface impurities that have caused disorder in the material. — Jennifer Ann Hutt

For more information, please contact Catherine Foster (312-252-5580 or cfoster@anl.gov) at Argonne.