Pinning Action of Correlated Disorder against Equilibrium Properties of HgBa${}_2$Ca${}_2$Cu${}_3$O${}_x$: a Delicate Balance

Abstract

We report significant alteration of the equilibrium properties of the superconductor HgBa${}_2$Ca${}_2$Cu${}_3$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 $M_{eq}$ 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 Tesla, 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 increases in the London penetration depth $\lambda$; this reduces the vortex line energy and consequently reduces the pinning effectiveness of the tracks.

8 pages, 9 figures