Superconductivity observed in much smaller nanoparticles than previously believed possible

A superconductor is a solid that shows low electrical resistance (like most metals) around room temperature, but the resistance suddenly drops to zero when it is cooled below a specific temperature.

This has very significant implications in lossless power transmission as well as in high field magnets (like in MRI). In recent times, superconductors have found more exciting and futuristic applications in electronic circuits for quantum computation and as nano-electronic components such as ultrafast switches and nanoscale interconnects. So, it is important to ask whether superconductivity gets affected by a reduction in the size.

In other words, can a nanoparticle continue to show superconductivity down to arbitrarily small sizes?

This problem was addressed over 50 years ago by a leading theoretical physicist from USA, Prof. Philip Anderson, who showed that there is a fundamental limit in the size of a nanoparticle below which superconductivity cannot occur.

The ‘Anderson Limit’ is slightly different for different superconductors, but usually less than 10 nm. Prof. Anderson’s prediction has been experimentally verified in many superconductors and is widely accepted.

Anderson Limit re-examined: Recent research jointly carried out at TIFR, Mumbai and JNCASR, Bangalore, just published in the journal Nano Letters, questions the universal validity of this concept.

The TIFR group has conclusively shown that Tantalum (Ta) nanoparticles remain superconducting much below the Anderson Limit of 4nm.

Has Anderson’s theory, therefore, been disproved? Not quite! A theoretical analysis carried out by the JNCASR group shows that while Anderson’s theoretical framework remains valid, the size limit predicted by it can be “tweaked” to a large degree! The key issue, it appears, is that the average interatomic distance in Ta nanoparticles increases steadily (up to 4%) with a reduction in the particle size.

This means that the Ta crystal expands slightly when the particle size decreases. Why is this exciting? The lattice expansion that occurs in Ta nanoparticles is rather uncommon. However, it is possible to engineer this type of expansion in other nanostructured superconductors by growing them on substrate materials with a slightly larger interatomic spacing. Thus, our result indicates that it may be possible to allow superconductivity to occur in structurally engineered nanoparticles at sizes much lower than previously imagined!

Why is the lattice expansion important? Superconductivity is caused by a pairing up of some of the electrons that help to transport current in the material. The electron-electron bonding is provided by correlated vibrations (“phonons”) of the metal atoms.

The change in the interatomic spacing (that occurs in Ta due to size reduction) lowers the frequency of particular phonon modes, which in turn strengthens the electron-electron bonding and favours superconductivity even in nanoparticles that are smaller than the Anderson Limit.

The author is a Professor, Department of Condensed Matter Physics at the Tata Institute of Fundamental Research (TIFR)