Campbell penetration depth of a superconductor in the critical state

The magnetic penetration depth λ(T, H, j) was measured in the presence of a slowly relaxing supercurrent j. In single crystal Bi₂Sr₂CaCu₂O₈ below approximately 25 K, λ(T, H, j) is strongly hysteretic. We propose that the irreversibility arises from a shift of the vortex position within its pinning well as j changes. The Campbell length depends upon the ratio j/j_c where j_c is the critical current defined through the Labusch parameter. Similar effects were observed in other cuprates and in an organic superconductor.