Some superconductors admit magnetic fields in thin flux lines, or vortices. Any electrical current exerts a force on the vortices, and if that force is enough to make them move, they dissipate energy. In other words, the vortices cause electrical resistance, even though the material around them remains superconducting.

The high-temperature superconductors are all of this type, and the misbehaviour of magnetic flux quanta severely limits their usefulness by reducing the maximum current and magnetic field that they can bear. The obvious way to solve this problem is to pin the vortices down: if they can't move, they can't waste energy.

Vortices are naturally pinned in place by defects in the materials, but weakly. To investigate this ‘pinscape’, in the hope of understanding how to make the pinning more effective, a Japanese group led by Akira Tonomura made high-resolution videos of vortex motion (a ‘still’ is shown here). Looking at a piece of niobium at 4.5 K with an electron microscope, they watched flux lines stagger from one pinning site to another, propelled by a gradient in the magnetic field. The red thread marks such a path.

figure 1

Figure 1

Watching these moving pictures, David Grier of the University of Chicago was reminded of Brownian motion in colloids, and realized that the depth of the pinning-site potential wells has the same randomizing effect as the thermal energy of colloid particles — or of any liquid particle. He and his colleagues have now adapted their data-analysis techniques to the vortex movies, and use them to measure the strength of vortex-vortex interactions and the distribution of pinning potentials (C.-H. Sow et al. Phys. Rev. Lett. 80, 2693-2696; 1998). These numbers are already known for the simple low-temperature superconductor niobium, but the method could provide unique information about the more complicated high-temperature superconductors.