A century later, a clearer picture of how mercury becomes a superconductor

Source: The post is based on the article “A century later, a clearer picture of how mercury becomes a superconductor” published in The Hindu on 20th January 2023.

What is the News?

More than 100 years ago, the physicist Heike Kamerlingh Onnes discovered that solid mercury acts as a superconductor. Now, for the first time, physicists have a complete microscopic understanding of why this is so. 

What is Superconductivity?

Superconductivity is the ability of a material to conduct electricity without any resistance. 

It is observed in many materials when they are cooled below a critical temperature that marks the transition to the superconducting state.

About the superconductivity in Mercury

In 1911, Heike Kamerlingh Onnes discovered superconductivity in mercury. Onnes had invented a way to cool materials to absolute zero – the lowest temperature possible. 

Using his technique, he found that at a very low temperature, called the threshold temperature, solid mercury offers no resistance to the flow of electric current. It was a watershed moment in the history of physics. 

Methodologies to determine superconductivity in mercury: Superconductivity in Mercury can be explained by the following theories:

BCS (Bardeen-Cooper-Schrieffer) Theory: 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. This could explain why mercury has such a low threshold temperature (around –270°C).

Spin-Orbit Coupling(SOC) Theory: SOC is the way an electron’s energy is affected by the relationship between its spin and its momentum.

– SOC gave a better view of the phonons’ energies and explained why mercury has such a low threshold temperature (approx. –270º C).

Coulomb repulsion (a.k.a. ‘like charges repel’) between two electrons in each pair: The superconducting state is determined by a balance between an attractive interaction between electrons, mediated by phonons and the repulsive Coulomb interaction (electrostatic repulsion between negative charges).

– The electrons are able to overcome their repulsion and pair up because the phonons have a very low frequency and the electrons have a relatively higher frequency, allowing the interacting electrons to avoid each other in time.

– The researchers found that, in mercury, one electron in each pair occupied a higher energy level than the other, a detail that reduced the Coulomb repulsion and nurtured superconductivity.