Superconductivity is the phenomenon where â when the temperature of a material is reduced below a critical value â its resistance to the flow of electric current abruptly vanishes. This state is associated with the emergence of a quantum character that makes superconductors useful for quantum computing. Superconductors thus have the potential to usher in a new era of computing.
However, something has been holding us back. Discovered in 1911 and theoretically understood only in the 1950s, superconductivity is a low-temperature phenomenon. In the 1960s and 1970s, physicists made fervent efforts to synthesise materials with higher critical temperatures. Their ultimate hope is to find a material that becomes a superconductor at room temperature and ambient pressure.
In May this year, a group of scientists from Germany reported in a peer-reviewed paper that lanthanum hydride has a critical temperature of 250K (approximately -20Âº C) but under extremely high pressures, exceeding a million-times atmospheric pressure. The confirmed the material enters a superconducting state by measuring its electrical resistance as well as identifying a characteristic way in which the critical temperature changes when a magnetic field is applied. Besides these methods, it is too difficult to ascertain superconductivity by directly probing any quantum effects.
The question arises: Why would anyone be interested in a material that performs only under such extreme conditions? As the researchers state clearly in their paper, their work was motivated by theories that suggested that clathrate compounds â compounds including lanthanum hydride, whose molecules are arranged in a certain interesting way â could be superconductors at high pressures.
What is the significance of this finding? Will it be a game changer? With a superconductor that functions at near room temperature but only under very high pressure, the answer is ‘no’ vis-Ã -vis quantum technologies.
However, the significance here is really for the science of superconductivity, which has evolved in a laborious way, with many unexpected discoveries. One of the more notable examples is the cuprate superconductor, which was the first superconducting material known to have a critical temperature above 30 K.
Cuprate superconductors are not fully understood yet in theory. They fall in the class of non-BCS superconductors. ‘BCS’ stands for John Bardeen, Leon Cooper and Robert Schrieffer, who first explained why metals like mercury become superconducting.
The atomic structure of metals that conduct electricity involves the atoms’ nuclei being fixed in place while the electrons move freely through the metal. Occasionally, these electrons might bump into the nuclei and slow down â perceived as resistance. But in some metals, the jiggling motion of the nuclei encourages free-flowing electrons to pair up with each other below a critical temperature. These electron pairs flow unimpeded through the material, with zero resistance.
In the early 1980s, physicists believed that this mechanism of superconductivity severely limited the critical temperature. If they wanted materials with higher critical temperatures, they figured they had to look among the cuprate compounds, where the electrons have motivations other than the jiggling motion to pair up.
The new paper is significant because it brings the ‘jiggling mechanism’ back into the limelight. Physicists laid the conceptual foundations of the theory used to predict the critical temperatures of materials in the 1960s and 1970s. Modern computers and materials-modelling techniques developed over the last several decades have helped turning these into quantitative information and guided us on what kind of experiments we’ll need to perform. The paper re-motivates the central question: can we design materials with structures that are conducive to superconductivity at ambient conditions?
For example, the atoms inside clathrate compounds are arranged in a cage-like pattern that assists with the jiggling. This work will certainly motivate scientists to examine this issue with state-of-the-art modelling techniques, perhaps even augmented by ideas from machine learning and artificial intelligence.
Bernd Matthias, a well-known scientist who contributed greatly to developing superconducting materials in the 1970s, laid down a set of rules to look for better superconducting materials. They are, in brief:
- Higher symmetry structures are better
- Materials with a higher density of electrons are better
- Oxygen is bad
- Magnetism is an enemy of superconductivity
- Insulators are useless
- GO NOWHERE NEAR A THEORIST
The discovery of cuprate superconductors in 1987 can be said to have shattered the first five rules in one shot, and the new paper has brought down the last Matthias rule standing.
Vijay B. Shenoy is an associate professor at the Centre for Condensed Matter Theory in the Department of Physics, Indian Institute of Science, Bengaluru.
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