Pinning action of correlated disorder against equilibrium properties of HgBa 2 Ca 2 Cu 3 O x

We report significant alteration of the equilibrium properties of the superconductor ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ 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 ${M}_{\mathrm{eq}}$ 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 ${M}_{\mathrm{eq}}.$ 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 $\ensuremath{\lambda};$ this reduces the vortex line energy and consequently reduces the pinning effectiveness of the tracks.