Discovery of bismuth superconductivity at extremely low temperature jeopardizes theory

December 2, 2016 by Bob Yirka report
Artificially grown bismuth crystal illustrating the stairstep crystal structure, with a 1 cm3 cube of bismuth metal. Credit: Wikipedia

(—A team of researchers at the Tata Institute of Fundamental Research in India has found that cooling a sample of bismuth to 0.00053 Kelvin caused the material to become a superconductor, putting at risk a decades-old theory regarding how superconductivity works. In their paper published in the journal Science, the team describes their cooling and testing approach and why they believe what they found will require physicists to rethink theoretical work that describes the conditions under which a metal can become superconductive.

After it was discovered that some metals can be superconductive under certain conditions (back in 1911), scientists have been hard at work trying to understand how they work so that they can take advantage of them. Much progress has been made—superconductivity is now commonly used in some devices such as particle accelerators—but the ultimate goal has yet to be reached—finding a metal that is superconductive at room temperature. In this new effort, the researchers looked at —a brittle reddish-gray metal that had been dismissed as a possible superconductor because it has such a small carrier density (one mobile electron is shared by 100,000 atoms).

To test the metal, the team drilled holes in a silver rod and then pushed bismuth crystals into them. They then covered the rod with a magnetic shield that was used to pass a over the bismuth samples. Sensors in the shield were sensitive enough to pick up magnetic field changes down to 10-18 Tesla. The team then chilled the construct they had made, watching for the point at which the magnetic field around the bismuth samples increased (an indicator of the Meissner effect)—that point arrived at 0.00053 Kelvin, revealing that the metal could be caused to be a superconductor when made cold enough.

The finding by the team has shed doubt on the reliability of the Bardeen-Cooper-Schrieffer theory, because the does not have enough electrons to allow for partnering up—the means by which most semiconductors operate without resistance. It also now represents the lowest carrier density superconductor. The work by the team also demonstrates that despite a significant amount of effort put into studying , it is still not very well understood.

Explore further: Superconductor's strange behaviour results in new laboratory tool

More information: O. Prakash et al. Evidence for bulk superconductivity in pure bismuth single crystals at ambient pressure, Science (2016). DOI: 10.1126/science.aaf8227

At ambient pressure, bulk rhombohedral bismuth (Bi) is a semimetal that remains in the normal state down to at least 10 mK. The superconductivity (SC) in bulk Bi is thought to be unlikely because of the extremely low carrier density. We observe bulk SC in pure Bi single crystals below 0.53 mK under ambient pressure with an estimated critical magnetic field of 5.2 μT at 0 K. SC in Bi cannot be explained by the conventional Bardeen-Cooper-Schrieffer theory because its adiabatic approximation does not hold true for Bi. Future theoretical work will be needed to understand SC in the non-adiabatic limit in systems with low-carrier density and unusual band structure, such as Bi.

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3 / 5 (6) Dec 02, 2016
Even if this result is confirmed, it might not threaten BCS. Perhaps we'll learn of another pathway to superconductivity? Even if it shows BCS to be incorrect or incomplete, that's fine too. We will have learned something which may open other doors to superconductivity.
1 / 5 (2) Dec 02, 2016
The best choice for atoms to produce superconductivity lie near the bottom and to the right of the Periodic Table. It is important to choose one that has an odd number of electrons with relatively low conductivity. This allows for atom structure to be mechanically periodic while also being electrically periodic with many electrons available. Bismuth has these characteristics. This is a rational choice for the mechanical and electronic structures that can allow superconductivity.
Dec 03, 2016
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1 / 5 (2) Dec 04, 2016
Or it is because our paradigm needs a serious shift. Maybe we've hold on for too long to obsolete theories, and we should should begin to make theories that respond to natural observation and not be pathologically clingy and narrow minded to the established theories. Ken wheeler talked about this "superconductivity " issue long ago. youtube com/watch?v=bBy7NnbFyMY

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