Pinning action of correlated disorder against equilibrium properties of HgBa₂Ca₂Cu₃O_x

We report significant alteration of the equilibrium properties of the superconductor HgBa2Ca2Cu3Ox 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 Meq 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 Meq. 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. © 2004 The American Physical Society.