Low-angle grain boundaries in ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$ with high critical current densities

The grain-boundary network in high-$T_c$ coated conductors consists of a large number of low-angle grain boundaries with many types of misorientation. Using the bicrystal technology we have measured the critical current densities of the relevant ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$ grain boundaries as a function of the grain-boundary angle. We find that in the low-angle regime [010]-tilt boundaries and [100]-twist boundaries reduce the critical current density much less than [001]-tilt boundaries. Transmission electron microscopy reveals a low defect density in the [010]-tilt boundaries and we find that ${\text{CuO}}_2$ planes cross these boundaries without interruption.

Figure 1

(Color online) Illustrations of (a) [001]-tilt, (b) [010]-tilt valley-type, (c) [010]-tilt roof-type, and (d) [100]-twist grain boundaries in a cubic material.

Figure 2

(Color online) $J_c$ of various types of bicrystalline grain boundaries in ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$ films measured at (a) 77 K and (b) 4.2 K as a function of misorientation angle. Please note that there are two data points for the $6°$ [100]-twist grain boundaries.

Figure 3

(Color online) AFM micrographs of (a) $\theta =4°$ and (b) $\theta =8°$ [010]-tilt valley-type grain boundaries, (c) $\theta =4°$ and (d) $\theta =8°$ [010]-tilt roof-type grain boundaries, and (e) $\theta =4°$ and (f) $\theta =8°$ [100]-twist grain boundaries. The micrographs were taken in air using the tapping mode.

Figure 4

(Color online) AFM micrographs of a (a) $\theta =6°$ [001]-tilt grain boundary and (b) a $\theta =6°$ [010]-tilt valley-type grain boundary. Cross-sectional HA-ADF STEM images of (c) a $\theta =6°$ [001]-tilt grain boundary and (d) a $\theta =6°$ [010]-tilt valley-type grain boundary.

Figure 5

(Color online) $J_c$ of various types of $\theta =6°$ grain boundaries (gb) and of the corresponding grains in magnetic fields $B$ applied in the boundary plane parallel to the substrate normal measured at (a) and (b) 77 K and (c) and (d) 4.2 K. The measured [100]-twist boundary is the one of the two $6°$ [100]-twist grain boundaries that has the higher $J_c$ as shown in Fig. 2.