MIT researchers find superconductor law

Massachusetts Institute of Technology (MIT) scientists have uncovered the law governing thin-film superconductors. This breakthrough eliminates much of the trial and error for companies that manufacture superconducting photodetectors that can sense single photons and squids for super-accurate measurements of minute magnetic fields.

Other applications that may benefit include the voltage standard chip used by the National Institute of Technology (NIST), the world's first quantum computer from D-Wave Systems Inc., and numerous meteorology applications from Hypres Inc.

"The applications for thin-film superconductors today are squids, photodetectors, voltage standards, metrology and D-Wave's quantum computer," EE professor Karl Berggren told EE Times. He was assisted by Yachin Ivry, a postdoc in MIT's Research Laboratory of Electronics.

Today making thin-film superconductors involves a lot of trial and error, because there are no formulas that relate the different parameters. But with MIT's new mathematical law, new superconducting chips can be designed with the correct parameters determined ahead of time with his and Ivry's formula.

Ultra-thin superconducting film of niobium and nitrogen shows individual atoms, a view that helped MIT discover a universal law of superconductivity. (Source: MIT, Yachin Ivry)

"Understanding superconducting thin films makes designing them easier, when you know the relationship between critical temperature, resistivity and film thickness," Berggren said.

Perhaps the most important parameter is "critical temperature"—the temperature at which the material turns into a superconductor. Though that temperature can be optimised with MIT's new formula, unfortunately, it cannot be reduced to room temperature with it. "We can optimise the critical temperature, but unfortunately, the thinner the films, the lower the critical temperature."

However, super-cooled superconductor chips can be better engineered for applications ranging from quantum computing to integrated ultra-low power devices.

Berggren's lab—the Quantum Nanostructures and Nanofabrication Group, where Ivry works—has built circuits that use only one-hundredth the energy of non-superconducting chips performing the same function.

The scientists' latest test material is niobium nitride, a so-called high-temperature superconductor. By holding constant two of the three parameters—critical temperature, thickness and resistivity—they could see a clear relationship between the three parameters and a constant, which you can read about in their free paper "Universal scaling of the critical temperature for thin films near the superconducting-to-insulating transition."

Next the researchers tried out their new law on other superconductors and found it held for three dozen different superconductors (each of which had a different constant in the same formula depending on the regularity of their lattice).

- R. Colin Johnson
??EE Times