Researchers at the Tata Institute of Fundamental Research (TIFR), Mumbai have made a landmark discovery that challenges the conventional understanding of superconductivity. A team led 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,” says 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 (mobile electron 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 says.
“Superconductivity in bismuth is puzzling. Even at 10 milliKelvin people did not find superconductivity in bismuth. So they gave up nearly 20 years back,” says Om Prakash Shukla from TIFR and the first author of the paper.
Scientists had earlier succeeded in making bismuth to superconduct, but bismuth was in the form of nanowires, amorphous, nanofilm etc. “When bismuth is made into nanowires or nanofilms the characteristics change dramatically as it becomes more metallic due to enhancement of electron density. So the conventional BCS theory can explain superconductivity in these systems,” Shukla says. But in this case, the TIFR researchers were able to make ultrapure bismuth crystals to superconduct.
Though superconducting materials are commonly used, as in the case of CERN, the Holy Grail is to find a material that superconducts at room temperature. “Bismuth is a very low temperature superconductor material. But once the theory and mechanism of superconductivity in bismuth is known one can find materials similar to bismuth but can show superconductivity at a higher temperature,” says Dr. Ramakrishnan.
The BCS theory came about in 1957, 46 years after the first experimental discovery of superconductivity in mercury in 1911. But Dr. Ramakrishnan is optimistic that it will not take long for theorists to explain how his team experimentally showed superconductivity in bismuth, which is a semi-metal.