Nature Mater. 9, 550–554 (2010)

Shell effects are familiar in the electronic structure of atoms, and have also been seen in nuclei and clusters of atoms. Now, for the first time, physicists have observed effects owing to degenerate energy levels or shells in superconducting nanoparticles, confirming a number of recent theoretical predictions. The effects were seen in experiments with tin nanoparticles by Sangita Bose, of the Max Planck Institute for Solid State Research in Stuttgart, and co-workers. However, there was no evidence for these effects in experiments with lead nanoparticles.

Bose and co-workers grew tin and lead nanoparticles on a boron nitride/rhodium substrate and used a scanning tunnelling microscope to probe their properties. The nanoparticles had heights in the range of 1–35 nm and were cooled to a temperature of about 1 K to make them superconducting.

The researchers measured how the superconducting gap — the energy needed to split the electron pairs responsible for the superconductivity — varied with the size of the nanoparticle. They found that the gap in tin nanoparticles oscillated as a function of size, reaching values of 60% higher than the gap for bulk tin. Similar oscillations were not observed for lead because it has a much shorter superconducting coherence length.