The short coherence length in high-Tc superconductors (5-15Å) makes an atomic scale understanding of the electronic properties at defects and interfaces essential for device applications. This understanding is particularly relevant for grain boundaries in YBa2CU3O7-δ (YBCO), where although extensive studies have shown a clear exponential decrease in critical current with misorientation angle, the absolute value can vary by several orders of magnitude at any given misorientation angle.

Figure 1 shows Z-contrast images of an [001] low-angle tilt boundary and a 30° [001] asymmetric tilt grain boundary. An interesting feature of both of these boundaries is that there appear to be sites where two atom columns are too close together. However, the problem of like-ion repulsion can be avoided if the columns are taken to be partially occupied. Insight into the effect of this partial occupancy can be obtained through the use of bond-valence sum analysis. Here, the formal valence of an atom is made up of contributions from all of its nearest neighbors, the magnitude of which are determined by the bond length.