In 1911, Dutch physicist Heike Kamerlingh Onnes discovered superconductivity in mercury. He found that at a very low temperature, called the threshold temperature, solid mercury offers no resistance to the flow of electric current.

The BCS theory

  • Scientists later classified mercury as a conventional superconductor because its superconductivity could be explained by the concepts of Bardeen-Cooper-Schrieffer (BCS) theory.
  • While scientists have used the BCS theory to explain superconductivity in various materials, they have never fully understood how it operates in mercury — the oldest superconductor. A group of researchers from Italy recently set out to “fill this gap”, as they wrote in their November 3 paper in the journal,Physical Review B.
  • The researchers used “state-of-the-art theoretical and computational approaches” and found that “all physical properties relevant for conventional superconductivity… are anomalous in some respect” in mercury.
  • In a testament to their strategy, they were able to work out a theoretical description for superconductivity in mercury that predicted its threshold temperature to within 2.5% of the observed value.
  • In BCS superconductors, vibrational energy released by the grid of atoms encourages electrons to pair up, forming so-called Cooper pairs. These Copper pairs can move like water in a stream, facing no resistance to their flow, below a threshold temperature.
  • By including certain factors that physicists had previously sidelined, the group’s calculations led to a clearer picture of how superconductivity emerges in mercury. For example, when the researchers accounted for the relationship between an electron’s spin and momentum, they could explain why mercury has such a low threshold temperature (around –270°C).

Coulomb repulsion

  • Similarly, the group found that one electron in each pair in mercury occupied a higher energy level than the other. This detail reportedly lowered the Coulomb repulsion (like charges repel) between them and nurtured superconductivity.
  • Thus, the group has explained how mercury becomes a superconductor below its threshold temperature.
  • Their methods and findings suggest that we could have missed similar anomalous effects in other materials, leading to previously undiscovered ones that can be exploited for new and better real-world applications.



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