A superconductor does not let a magnetic field penetrate its interior. But if the field is strong enough, it does get in — not as a continuous flux, but as individual vortex filaments, like tiny tornadoes. Inside each filament, superconductivity is destroyed, and around it flows a persistent current. These filaments are not stationary: under the influence of a current, they can move, and then the superconductor loses its remarkable properties.
To hold the vortices in place, the material contains defects — imperfections in the crystal lattice. The vortices latch onto them like grains of sand catching on the roughness of a streambed. If the current is small, the vortices only shift slightly and then return. This displacement can be measured: it is called the Campbell depth, after the Canadian physicist who first described the effect. The stiffer the vortex pinning, the smaller this depth.
In the new experiment, scientists measured the Campbell depth in a CeCoIn₅ crystal — a superconductor with unusual properties. It turned out that as the magnetic field increases, the vortex lattice does not just compress; it abruptly changes its symmetry: the filaments rearrange into a different pattern. It is as if a crowd walking across a square suddenly reorganized from a square grid into a triangular one. Such jumps indicate that superconductivity in this material is not like the conventional kind.
The critical current, the maximum current a superconductor can carry without loss, also turned out to be unusual. Usually it drops rapidly with increasing temperature, but here it decreases almost linearly, as if the vortices cling more weakly to defects when heated. This is yet more evidence that CeCoIn₅ is a superconductor with an unconventional electron pairing mechanism.
Vortex filaments in a superconductor can form regular lattices, similar to crystal lattices, and they can be observed using magnetic microscopy.