What is a superconductor?

A superconductor is defined as a substance that offers no resistance to the electric current when it becomes colder than a critical temperature.
Some of the popular examples of superconductors are aluminium, magnesium diboride, niobium, copper oxide, yttrium barium and iron pnictides.

How mercury becomes superconductor?

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.

Ans. Bardeen-Cooper-Schrieffer (BCS) theory

Scientists 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.
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 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 side-lined, 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).

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.