TIFR discovery challenges theory of superconductivity

A team of researchers has discovered semi-conductivity in bismuth at around -273 C°

December 07, 2016 12:21 am | Updated 03:46 am IST - CHENNAI

Dr. S. Ramakrishnan’s team showed that bismuth  semi-metal shows superconductivity.

Dr. S. Ramakrishnan’s team showed that bismuth semi-metal shows superconductivity.

Researchers at the Tata Institute of Fundamental Research (TIFR), Mumbai, have made a landmark discovery that challenges the conventional understanding of superconductivity.

A team, lead by Dr. S. Ramakrishnan of the Department of Condensed Matter Physics and Material Sciences at TIFR, has discovered bismuth semi-metal in bulk form becoming a superconductor when the temperature is lowered to 530 microKelvin (about -273 degree C), which is three orders of magnitude higher than the theoretical prediction. The results were published in the journal Science .

“The Bardeen-Cooper-Schrieffer (BCS) theory [which explains superconductivity in most low Tc superconductors] cannot explain the superconductivity seen in bismuth,” said Dr. Ramakrishnan, the corresponding author of the paper. “The discovery demands a new theory and a new mechanism to understand superconductivity in bismuth. This discovery provides an alternative path for discovering new superconducting materials which are very different from the conventional superconductors.”

Superconductors are materials that conduct electricity with no resistance whatsoever. To become superconductors, the element should have mobile electrons, and these electrons should come together to form pairs, known as Cooper pairs. Unlike other elements in the periodic table, bismuth has unusual phenomenon — while metallic superconductors have one mobile electron per atom, bismuth has only one mobile electron per 100,000 atoms. Since carrier density is so small, people did not believe that bismuth will superconduct.

Also, bismuth’s electronic energy (Fermi energy) is comparable to the lattice (phonon) energy. “So the conventional BCS theory and its extensions which assume that Fermi energy is two to three orders of magnitude higher than phonon energy is not valid in bismuth. We know that if we prove superconductivity in bismuth, it will be from a different mechanism,” Dr. Ramakrishnan said.

“Superconductivity in bismuth is puzzling. Even at 10 milliKelvin, people did not find superconductivity in bismuth. So they gave up nearly 20 years back,” said Om Prakash Shukla from TIFR and the first author of the paper.

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