Pinning action of correlated disorder against equilibrium properties of HgBa₂Ca₂Cu₃O_x

We report significant alteration of the equilibrium properties of the superconductor HgBa₂Ca₂Cu₃O_x when correlated disorder in the form of randomly oriented columnar tracks is introduced via induced fission of Hg nuclei. From studies of the equilibrium magnetization M_eq and the persistent current density over a wide range of temperatures, applied magnetic fields, and track densities up to a “matching field” of 3.4 T, we observe that the addition of more columnar tracks acting as pinning centers is progressively offset by reductions in the magnitude of M_eq. Invoking anisotropy-induced “refocusing” of the random track array and incorporating vortex-defect interactions, we find that this corresponds to increase in the London penetration depth γ; this reduces the vortex line energy and consequently reduces the pinning effectiveness of the tracks.