Vortex Pinning and Critical Current Enhancements in High-T _c Superconductors with Fission-Generated Random Columnar Defects

Abstract

Randomly oriented columnar defects can efficiently increase and stabilize the current density in many high-Tc superconductors. Irradiation with 0.8 GeV protons induces a prompt fission of heavy constituent nuclei, such as Hg, Tl, Pb, and Bi, in the cuprate compounds; then recoiling fragments generate randomly oriented columnar tracks. This markedly enhances the persistent current density J, elevates the irreversibility line to higher fields and temperatures, and reduces the temporal rate of current decay. The associated crystalline disorder depresses Tc by ~ 0.1–1 K per 1016 proton/cm². At optimal proton fluency, J is enhanced by one or more orders of magnitude (compared with unirradiated virgin materials) and the logarithmic decay rate d ln(J)/d ln(i) is diminished. However, we show that while these vortex pins greatly reduce thermally activated current decay in the highly anisotropic material Bi-2212, significant temperature-independent current decay remains, due to quantum tunneling of vortices. An analysis of the thermally-induced current decay in a “Maley” framework provides the effective pinning energy U(J, T) of irradiated materials, for comparison with the virgin superconductors. The influence of quantum decay on this analysis is shown.