Grain boundaries in YBCO

Here we just point out that the (010)(001) tilt boundary in Fig. 10.8(b) is atomically abrupt, while the slight lateral lattice mismatch across the boundary 

Clearly, since the Cu02 planes, which play an essential role in the superconducting transport, are disrupted across the (010)(001) boundaries, they are expected to show weak-link behavior. However, since the Cu02 planes are continuous across (013)(013) boundaries, the latter may provide good supercurrent conducting paths across the 90° [100] tilt grain boundaries, resulting possibly in weak-link free behavior [10.43]. Measurements across individual 

The HREM image in Fig. 10.8(d) shows a curved boundary between the grain on the top and the grain on the bottom. The boundary has a 90°/[100] misorientation and, like the 90° [100] tilt boundaries, this boundary is structurally coherent and free of any second phase. A grain boundary along a horizontal plane in this image would be a pure twist grain boundary. 

However, in contrast to the previously described tilt grain boundaries the boundary is not planar and as well defined as in the (010)(001) boundary. Thus, although the grain boundary has mainly twist character, the grain boundary is of mixed type and it is evident that the crystalline structure does not abruptly change across the boundary. It appears possible that the irregular shapes of the boundary plane could increase the contact area of the Cu02 planes across the boundary. 

Consequently, as the dislocation cores get more closely spaced with increasing tilt angle, the well-coupled regions get smaller and smaller. Indeed, using the Read-Shockley formula together with the observed size of the cores, one 

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Further work on the misorientation dependence by Ivanov and coworkers indicated as a function of 6 an exponential decrease in the critical current across the interface in [001] tilt grain boundaries in YBCO films [10.7]. The critical current density [according to refs. 10.4, 10.7, 10.51, 10.59, and 10.60] for grain boundaries with misorientation about [001] is shown in Fig. 10.1. 

Fig. 10.8. HREM images of various 90° grain boundaries in YBCO (a) [100] tilt, a and b interchanged across boundary (b) [100] tilt grain boundary showing two distinct facets, the (013) symmetric boundary with the CuOi planes joining at the interface and the (010)(001) asymmetric grain boundary (c) same as the asymmetric boundary in (b), but viewed at 90°, with one grain in the [001] projection (d) tilt and twist facets combined (continues overleaf).

Fig. 10.9. Low-angle [001] tilt grain boundary in YBCO with extended and reconstructed dislocation cores (arrows) [10.66].

Fig. 10.10. (a) HREM image of one of the dislocation cores in Fig. 10.9. (b) Dislocation core model for [001] tilt grain boundary in YBCO. HREM simulations were performed for two models, (c) Model 1 HREM simulation assumes that the grain boundary core chemical composition is identical to bulk, (d) For the model 2 simulations the Y-Ba atomic columns were replaced by Cu. This model image closely resembles the observed HREM image (after [10.65]). 

In order to provide the opportunity for a direct analysis of structure-property correlations, measurements of structure and properties need to be performed on the same sample. Recent work in our laboratory has focused on the study of 45°/[001] tilt grain boundaries in YBCO [10.17, 10.19, 10.58-10.60]. Thin films were deposited epitaxially, using several techniques and deposition conditions. In each case grain boundaries were introduced into predetermined patterns suitable for electrical characterization. The individual thin-film bi-epitaxial grain boundary junctions then were electrically characterized by using... 

Fig. 10.12. Resistance-temperature relationship for a 45°/[001] tilt grain boundary in YBCO. In contrast to the grain characteristics, the grain boundary shows a foot structure, which is due to thermally activated phase slippage [10.17, 10.59].

These bicrystals are the substrates that have been typically used for the preparation of individual grain boundaries in YBCO thin films [11.1-11.3]. The reason that SrTiOs is chosen as the substrate for YBCO is primarily due to the closeness of the lattice parameters (3.905 A for SrTiOs compared with a = 3.81 A and b = 3.88 A in YBCO). In view of the fact that YBCO, and for that matter all of the high superconductors, have structures that are essentially perovskite, it is reasonable to assume that the structure of the YBCO grain boundary would follow that of the SrTiOs bicrystal. Indeed, in terms of the dislocation core models, the only difference between SrTiOs and YBCO are that the Sr columns are replaced by Y/Ba columns and the Ti—O columns are replaced by Cu—O columns. The structure in the [001] projection is the... 

Fig. 11.9. Maximum entropy image of a 30° asymmetrie [001] tilt grain boundary in YBCO, obtained from Fig. 11.3 by eonvolution with a narrow Gaussian. The struetural units of the boundary are the same as observed for SrXi03.

Fig. 14.11. Schematic illustration of four techniques to introduce grain boundaries in YBCO thin films, (a) Bi-crystal substrate technique, (b) Bi-epitaxial technique where a template layer is used to change the epitaxial orientation of the YBCO film with respect to the substrate, (c) Step-edge on the substrate, (d) Surface modification.

Fig. 10.7. HREM image of grain boundary in polycrystalline YBCO. The grain boundary appears free of impurity phases and is formed on the basal plane of one crystal, which borders a high-index plane in the second grain.

The definition of low-angle grain boundaries generally covers the range of misorientations from 0° to 10°. In this regime, the grain boundary plane can be considered to be a linear array of separated dislocation cores. For [001] tilt boundaries in YBCO, the boundary plane will be composed of [100] or [010] dislocations, as shown in Fig. 11.6 (for YBCO the small distortion between the a- and fi-axes needs to be incorporated for quantitative models, but structurally results in no observable differences in the dislocation cores). 

The structural unit model has been used successfully to predict the structures of grain boundaries in perovskite structured SrTiOs bicrystals [11.31-11.34]. The structural units observed for symmetric SrTiOs [001] tilt boundaries are shown in Fig. 11.8. In a similar manner to the isolated dislocation cores in YBCO, the structural units also appear to contain atomic positions where the cations are too close together. Again, depending on the structural unit, the close separation of the atomic columns can occur for either of the sub-lattice sites,... 

Fig. 14.17. Plan-view TEM micrographs of YBCO grain boundaries in the YBCO film shown in Fig. 14.16(b). (a) The shadowed region indicated in Fig. 14.16 is reflected in the YBCO film morphology. The width of the arrowed region in the TEM micrograph corresponds to the width of the shadowed region, (b) The dark region in (a) consists of YBCO where the unit cell is rotated 45° along the [001] axis. The rest of the film has a cube-on-cube orientation relationship with respect to the MgO substrate, (c) The YBCO grain boundaries consist of (100), (010) and (110) segments continues overleaf).

K. After wet chemical etching to form the resonator patterns, = 169000 pQ and 22000 pQ were calculated from 17 KQ values for the YBCO resonators on MgO (100) and LaAlO.i (100) substrates, respectively. The observed substrate dependence of / s seems due to high-angle grain boundaries in the YBCO on MgO resonator [202]. 

Another possibility for obtaining CSLs exists by approximating the crystal structure by pseudocubic or tetragonal unit cells. This approach of applying the CSL to non-cubic systems has been discussed in the literature by means of the constrained coincident site lattice [10.12] which has, among others, also been applied to YBCO grain boundaries. 

The specimens used in this study are laser ablated thin films of YBa2Cu30v (YBCO) [11.7] grown on SrTi03 bicrystal substrates. This method of boundary preparation has been chosen as it is known to produce clean grain boundaries,... 

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