Critical currents near 10⁶ A cm^-2 at 77 K in neutron-irradiated single-crystal YBa₂Cu₃O₇

THE defects in as-grown bulk YBa₂Cu₃O₇ have been shown to act as weak flux pinning sites, leading to a reversible magnetization¹, magnetization decay² and non-zero resistivity³ in moderate magnetic fields at temperatures near the transition temperature, T_c. These effects limit the technological utility of the as-grown material³. This is not entirely surprising, as both single crystals and polycrystalline materials are grown under conditions expected to minimize the defect density, but the weak pinning in these materials has nevertheless raised the issue of a possible intrinsic limitation to the critical current density. Here we report the attainment of a critical current density, J_c, of ∼6 × 10⁵ A cm^-2 at 77 K and 9 kOe, in a single crystal of Yba₂Cu₃O₇ irradiated with fast neutrons to introduce defects. (In both irradiated and unirradiated crystals, the critical current density was inferred by measuring the magnetic moment resulting from an induced persistent current.) This hundredfold enhancement of J_c relative to unirradiated crystals demonstrates that artificially induced defects can act as strong flux pinning sites, and analysis of the defects should increase our understanding of flux pinning in high-T_c superconductors.