The immutable laws that govern our universe – such as those that reign over the observable world in classical mechanics and those that rule the atomic physics world – are at the core of all of our scientific principles. They not only provide consistent, repeatable, and accurate rules that allow calculations and experiments to be tested or verified, they also help us make sense of the workings of the cosmos. MIT researchers claim to have discovered a new universal law for superconductors that, if proved accurate, would bring the physics of superconductors in line with other universal laws and advance the likes of superconducting circuits for quantum and super low-power computing.
Superconducting materials have no electrical resistance at temperatures close to absolute zero, which means that very small amounts of energy are required to induce electrical currents in them. Devices, such as computer processors, built from these materials would theoretically be expected to use many hundreds of times less energy than conventional circuits.
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However, until recently, the correlation between the physical and electrical parameters of superconductors has been largely based on assumptions from standard theoretical physics and no one single law on these functions had been previously proven.
The new mathematical relationship discovered by the researchers involving material thickness, temperature, and electrical resistance, on the other hand, so far appears to hold true in all superconductors.
Prior to the MIT research, other theoretical work had previously indicated that the critical operating temperature in a superconductor was a function of the thickness of the film from which it was made or its measured electrical resistance at room temperature. However, when the team grew superconductors from niobium nitride atoms, these theories did not seem to hold true.
"We saw large scatter and no clear trend," said Yachin Ivry, a postdoctoral researcher in MIT’s Research Laboratory of Electronics. "It made no sense, because we grew them in the lab under the same conditions."
To attempt to understand this anomaly between theory and practice, the researchers decided to conduct a number of experiments with the growing of the superthin film to see if they could produce more consistent results. To do this, they kept one of two parameters constant: the thickness of the material or its "sheet resistance" (the material’s resistance per unit area). They then measured any changes in critical temperature whilst varying either of these parameters.
As a result of this work, a pattern of repeatable behavior resulted. The team was able to show that sheet thickness (d) multiplied by the critical temperature (Tc) equaled a constant divided by sheet resistance (Rs) raised to a particular power, thereby providing the universally-applicable thin-film superconductivity relationship formula: dTc(Rs).
"We were able to use this knowledge to make larger-area devices, which were not really possible to do previously, and the yield of the devices increased significantly. Thin films are interesting scientifically because they allow you to get closer to what we call the superconducting-to-insulating transition," said Ivry. "Superconductivity is a phenomenon that relies on the collective behavior of the electrons. So if you go to smaller and smaller dimensions, you get to the onset of the collective behavior."
The researchers believe that application of their research will provide greater insights into thin-film superconductivity, which could see improvements in the likes of super-sensitive photodetectors and quantum computing semiconductors.
"This is very convenient for technical applications, because there is a lot of spreading of the results, and nobody knows whether they will get good films for superconducting devices," says Claude Chapelier, a superconductivity researcher at France’s Alternative Energies and Atomic Energy Commission. "By putting a material into this law, you know already whether it’s a good superconducting film or not.”
The findings of the team's research were published in the journal Physical Review B