When Two Superconductors Become One
Most superconductors have more than one energy band where electrons can pair up to flow without resistance. But the complex behavior of such multiband superconductivity has been difficult to study experimentally. Electrons scatter off crystal defects and jump between bands, making the material behave as a single-band superconductor through the so-called interband proximity effect. Now Qili Li at the Karlsruhe Institute of Technology in Germany and his colleagues have demonstrated a method for overcoming this hurdle [1]. Their experiments rigorously test current theories of multiband superconductors and lay the groundwork for accessing many predicted quantum effects in these systems.
The researchers used the fact that superconductors made of pure lead have two conduction bands that electrons rarely jump between. This weak coupling enables the formation of two distinct superconducting states. To observe such states, Li and his colleagues built a high-resolution scanning tunnelling microscope operating at millikelvin temperatures. They then studied the material’s electronic properties in and around nanometer-sized crystal defects to determine how scattering off the defects makes it easier for electrons to jump between the two bands.
The team’s observations show exactly how defects in pure-lead superconductors couple the material’s two bands. The results also reveal the impact of different band-coupling strengths on the measured energy spectrum, confirming a model of multiband superconductivity first proposed almost 60 years ago. Li and his colleagues suggest that this control over the band coupling could enable investigations of various quantum phenomena in these systems, ranging from self-sustaining solitary waves called solitons to swirling magnetic vortices that have fractional flux.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- T. Gozlinski et al., “Visualization of defect-induced interband proximity effect at the nanoscale,” Phys. Rev. Lett. 136, 056401 (2026).