How a Dash of Tin Turns a Film into an Insulator express
A thin film of niobium and tin conducts electricity without resistance — until you make it really thin. Then it suddenly becomes an insulator. A team of physicists tested what happens when you add a bit more tin to the film than the formula calls for.
The films were grown by sputtering onto a silicon substrate — kind of like spray-painting, but with atoms. In the tin-rich films, the crystal grains came out finer, and the boundaries between them were more pronounced. Such a film loses superconductivity at a thickness of about 11 nanometers — twice as thick as films with the exact composition. Beyond that, it behaves like an ordinary insulator.
Measurements in a magnetic field revealed another detail: in the more granular films, the superconducting state breaks down more easily. The superfluid stiffness, a measure of how firmly a superconductor holds onto its current, is noticeably lower in these films. It seems the extra tin atoms rattle the whole order, and superconductivity gives in earlier.
Such films are a convenient playground for studying the transition from conductor to insulator. By varying the composition, you can tune the thickness at which this transition occurs. And that means better understanding how disorder works in the nanoworld.
A thickness of 11 nanometers is about 40 atomic layers. If a sheet of paper were that thin relative to its size, it would cover a football field.
In Peter Watts's novel Blindsight, superconductors work at room temperature, but even there, the slightest disorder in the crystal nullifies their properties.