Current does not flow through a CC uniformly, like in a single crystal, but instead percolates due to local variations in Jc. These Jc variations are (among other things) a result of the multiple GBs and intra-grain regions in the conductor. This is illustrated at right, where a granular flux pattern can be seen in a YBa2Cu3O7-x RABiTS-type CC. Our previous work has shown that this grain structure is replicated directly from the underlying substrate. Most GBs introduced by the substrate have a Jc less than that of the intragrain, and flux flows more easily along these boundaries.
Current percolation may also be observed under an applied transport current.
Single Grain/Single Boundary Scale
It is also very useful to have knowledge of Jc and current flow at the single grain/single boundary level. Most of our understanding of thin film YBa2Cu3O7-x comes from films grown on STO single crystal or bicrystal substrates. In RABiTS-type CCs, each substrate grain acts as its own single crystal template, all with different vicinal angles and potentially different surface qualities. GBs in CC substrates are generally neither pure tilt nor pure twist (unlike most bicrystals). It is useful to know what effect these differences have on the YBa2Cu3O7-x.
Using optical photolithography, transport links have been placed across individual GBs and within individual grains in RABiTS-type CCs. This allows us to directly measure the Jc(H) properties of the intra-grain and GBs regions. These transport links are very small (~30μm long) and voltage-current measurements with sub-nanovolt voltage resolution ensure that Jc is evaluated at the 1uV/cm criterion.