Low-temperature magnetic relaxation inHgBa2Ca2Cu3O8+δsingle crystals with columnar defects

Abstract

Micrometer-sized Hall probes are used to measure the low-temperature magnetic properties of ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ single crystals with and without columnar defects. Introduction of columnar defects by heavy-ion irradiation suppresses the quasiexponential temperature dependence of the critical current, but only when the density of flux lines is less than the density of defects. The current-dependent creep activation energy $U(J)$ is found to be best described by the power law: ${U(J)=U}_{0}[{(J}_{c}{/J)}^{\ensuremath{\mu}}\ensuremath{-}1].$ At high current densities J, and low fields, there is a crossover to a quantum vortex creep process with a temperature-independent magnetic relaxation rate. On the low-J side of this crossover in the unirradiated sample, $\ensuremath{\mu}g~1,$ indicating that the thermally activated vortex creep process is consistent with collective creep. When the flux-line density exceeds the columnar defect density in the irradiated sample, the vortex creep process is characterized by $\ensuremath{\mu}\ensuremath{\sim}1.$ For flux-line densities less than the defect density, an anomalously large exponent is found $(\ensuremath{\mu}g~2),$ which is inconsistent with vortex transport occurring either by the nucleation and expansion of half-loop excitations or by variable-range hopping of vortex lines as described by the Bose-glass theory.