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A single-crystal route to tunneling.

A Josephson junction generally consists of a thin layer of an electrical insulator sandwiched between two slabs of super-conducting material. Extraordinarily sensitive to changes in electromagnetic fields, such junctions lie at the heart of SQUIDs, superconducting quantum interference devices used for mapping tiny variations in magnetic fields, whether in geological formations or in the human brain.

Now researchers have found that a single crystal of a copperoxide superconductor can by itself act as a set of Josephson junctions, R. Kleiner and co-workers at the Walther Meissner Institute in Garching, Germany, observed this effect in a high-temperature superconductor made up of bismuth, strontium, calcium, copper, and oxygen.

"Although our results are strongly influenced by crystal imperfections, they show the possibility of using small ... single crystals as naturally grown series arrays of Josephson junctions;' the researchers say,

One can picture this particular superconductor as stacked layers of strontium and bismuth atoms (together with oxygen) separating sheets of copper. and oxygen atoms. The bismuth and strontium layers act as insulators between the superconducting copper oxide sheets, creating conditions under which pairs of electrons can "tunnel" through the barriers separating superconducting layers. A crystal only 3 microns tall would contain approximately 2,000 Josephson junctions arranged in a stack.

"This is a significant demonstration of true Josephson behavior, which few other types of high-temperature junctions have shown;' physicist Colin Pegrum of the University of Strathclyde in Glasgow, Scotland, comments in the July 16 NATURE. "Whilst their findings do not form the basis of a direct method to engineer true junctions for device applications, they give some insight into the mechanism of interlayer superconductivity in the material."

Such insights may also suggest ways of improving current methods of fabricating Josephson junctions, which normally involve the establishment of tunneling between adjacent superconducting grains or crystals (SN: I/11/92, p. 30). Kleiner's group described their experiments in the April 13 PHYSICAL REVIEW LETTERS.
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Publication:Science News
Article Type:Brief Article
Date:Aug 8, 1992
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