Geometry dependence of intergranular currents in sintered YBa₂Cu₃O_7-δ

Using standard four-point terminal measurements, the intergranular critical current densities j^J_c(T, B) of sintered YBa₂Cu₃O_7-δ samples are determined. The dependence of the critical current density on the sample geometry (height h and width b of parallel- epiped samples) is measured systematically for various temperatures (5K ≤ T ≤ 91 K) and low applied external magnetic fields (0 mT ≤ μ₀H_ext ≤ 10 mT). It is shown that the intergranular critical current density increases on reducing the sample height or width, if no external magnetic field is applied to the sample. The results are discussed using a model assuming the currents flowing in a surface layer of the sample. A reduction of the height or width of the sample changes the circumferences and not the current itself, thus implying an enhancement of the critical current density, j^J_c(T, B). It is found that the increase of j^J_c(T, B) on reducing the sample dimensions is limited by the Josephson penetration depth, λ_J(T). This penetration depth, λ_J(T), turns out to be a very important parameter to reach high intergranular critical current densities in sintered high-T_c superconductors.