Geometry dependence of intergranular currents in sintered YBa 2Cu 3O 7−δ

Abstract

Using standard four-point terminal measurements, the intergranular critical current densities j J c( T, B) of sintered YBa 2Cu 3O 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 ≤ μ 0 H 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.