The atomic-scale origins of grain boundary superconducting properties

Due to the extremely short coherence lengths of the high-Tc superconductors, defects such as grain boundaries are obvious barriers to the flow of supercurrent. Within a few months of the discovery of these materials, it was shown how the critical current dropped four orders of magnitude as the grain boundary misorientation increased from zero to 45 degree. Even today, there is no quantitative understanding of this behavior. A qualitative understanding is however possible through atomic resolution Z-contrast imaging on YBa2Cu3O7-δ and SrTiO3 bicrystal grain boundaries, combined with bond-valence-sum analysis. The Z-contrast image of a YBa2Cu3O7-δ low angle grain boundary shows the same kind of reconstructed dislocation cores as seen in SrTiO3, containing reconstructions on both the Cu and Y/Ba sublattices. An image of an asymmetric 30 degree boundary in YBa2Cu3O7-δ shows the same units and unit sequence as expected for SrTiO3. YBa2Cu3O7-δ boundaries are wavy because of their non-equilibrium growth process, and therefore mostly asymmetric in nature, although small segments have the symmetric structure. It seems reasonable to assume that boundaries of other angles will also have similar structures in these two materials