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Superconducting silicon.

Under extreme pressure (thousands of atmospheres) solids can change their crystal structure and with it their properties. For example, carbon becomes diamond rather than graphite. Consisdering silicon, earth's most abundant element and the one on which much of modern technology depends, three physicists from the University of California at Berkeley, Michel M. Dacorogna, Kee, J. Chang and Marvin L. Cohen, calculated what would happen to it under pressure.

At normal pressure silicon has the same crystal structure as diamond and is, electrically, a semiconductor. Under more and more pressure, Dacorogna, Chang and Cohen predicted that silicon should go through a series of metal-like crystalline states--first the configuration known as beta-tin, then simple hexagonal and finally hexagonal close-packed. In these configurations and at sufficiently low temperatures silicon should become a superconductor.

As they were about to present their calculations to the American Physical Society meeting, they received news from a French group that is performing actual experiments with pressured silicon. Early results show that under pressure silicon becomes a superconductor at a temperature of 5 kelvins. With more pressure applied, the transition to superconductivity occurs at a higher temperature, 8 kelvins. Cohen says these two figures correspond to his group's calcuations. He expects that the hexagonal close-packed state will show an even higher superconducting transition temperature. In the meantime, he and his theoretician colleagues plan to start a calculation for another common semiconductor, germanium.

The French experiments are done by putting a minute piece of silicon between two anvils made of diamond and clamping on the pressure. Then the sample is dropped into a Dewar flask for chilling. The hard part is putting electrodes on the sample to measure the conductivity.
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Title Annotation:silicon under extreme pressure
Author:Thomsen, Dietrick E.
Publication:Science News
Date:Apr 6, 1985
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