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.
as written by the authors arXiv:2604.23657
The temperature and magnetic field dependent magnetic penetration depth, $$\lambda_m(T,H)$$, was measured in a single crystal of a heavy fermion superconductor CeCoIn$$_5$$ using a frequency-domain tunnel diode resonator. In addition to the London penetration depth, which yields the superfluid density, measurements in a finite DC magnetic field provide Campbell penetration depth, $$\lambda_C(T,H)$$, which is directly linked to the true (unrelaxed) critical current density, $$J_c$$. The measured $$\lambda_C(H)$$ in CeCoIn$$_5$$ deviates significantly from the conventional $$\sim \sqrt{H}$$ behavior, and its slope changes abruptly at the characteristic magnetic field values. Considering that our sample is in the clean limit, we interpret this deviation as a fingerprint of the vortex lattice symmetry change. The temperature dependence $$J_c(T)$$ of CeCoIn$$_5$$ calculated from $$\lambda_C(T)$$ is nearly $$T$$-linear over the entire temperature range, also in stark contrast to expectations in a conventional type-II superconductor. Our results provide new evidence for unconventional superconductivity in CeCoIn$$_5$$ from the never-before-measured Campbell penetration depth.
Vortex filaments in a superconductor can form regular lattices, similar to crystal lattices, and they can be observed using magnetic microscopy.