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At low temps, bismuth superconducts: despite few free electrons, element loses electrical resistance.

An oddball superconductor is the first of its kind--and if scientists are lucky, its discovery may lead to others.

At a frigid five ten-thousandths of a degree above absolute zero, bismuth becomes a superconductor--a material that conducts electricity without resistance. Physicists at the Tata Institute of Fundamental Research in Mumbai, India, report the feat online December 1 in Science.

Bismuth, a semimetallic element, conducts electricity less efficiently than an ordinary metal. It is unlike most other known superconductors in that it has very few mobile electrons. Consequently, the prevailing theory of superconductivity doesn't apply.

New ideas--either a different theory or a tweak to the standard one--are needed to explain bismuth's superconductivity, says theoretical physicist Marvin Cohen of the University of California, Berkeley. "It might lead us to a better theory of superconductivity with more details."

An improved theoretical understanding might then point to other new superconductors, potentially ones that work at more practical temperatures, says study coauthor Srinivasan Ramakrishnan.

Physicists' ultimate goal is to find a superconductor that operates at room temperature. Such a material could replace standard metals in wires and electronics, providing massive energy savings and technological leaps, from advanced supercomputers to magnetically levitated trains. Ramakrishnan and collaborators chilled ultrapure crystals of bismuth while shielding the crystals from magnetic fields. Below 0.00053 kelvins (about -273[degrees] Celsius), the researchers observed a hallmark of superconductivity known as the Meissner effect, in which the superconductor expunges magnetic fields.

In the standard theory of superconductivity, electrons partner up in a fashion that removes resistance to their flow, thanks to the electrons' interactions with ions in the material. But the theory works only for materials with many free-floating electrons. A typical superconductor has about one mobile electron for each atom in the material, while in bismuth each electron is shared by 100,000 atoms.

Bismuth has previously been made to superconduct when subjected to high pressure, when formed into nanoparticles or when its atoms are disordered, rather than neatly arranged in a crystal. But under those conditions, bismuth behaves differently, so the prevailing superconductivity theory still applies. The new result is the first sign of super-conducting bismuth in its normal form.

Caption: Pure bismuth, like the cube (left), superconducts below five ten-thousandths of a kelvin. Impure crystals of bismuth (right) have iridescent patterns due to oxidation.


Please note: Illustration(s) are not available due to copyright restrictions.

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Title Annotation:MATTERS & ENERGY
Author:Conover, Emily
Publication:Science News
Date:Dec 24, 2016
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