Investigating the Relationship Between Atomic Structure and Carrier Depletion at [001] tilt Grain Boundaries in YBCO
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In particular, grain boundaries facet on the scale of a few hundred nm and this leads to a predominantly asymmetric grain boundary plane. Efforts to link this grain boundary structure to the local electronic properties have focused on the use of electron energy loss spectroscopy (EELS) [10,11]. Profiles obtained from grain boundaries indicate that there is hole depletion occurring. There have been several theoretical models proposed for the observed Jc behavior of grain boundaries. For low-angle boundaries, it has been proposed that the reduction in J, is a result of the strain fields associated with dislocation cores in the grain boundary plane forming a reduced area of superconducting material [12]. These ideas have been extended [13], to show that these strain fields can result in a non-superconducting dislocation core and a suppression of the superconducting order parameter. Unfortunately, it is not straightforward to apply these models to high-angle grain boundaries, since the overlapping strain fields associated with the closely spaced dislocations are impossible to calculate. Other models for the Jc behavior have been based on the symmetry of the superconducting order parameter [ 14]. However, such models appear not to be able to account for the large decrease in J, at high-angle grain boundaries. The more recent theoretical models for grain boundaries in high-Ta superconductors have evolved to address the normal state properties of these materials [1]. In the normal state, the properties of high-T, superconductors are like any other semiconducting oxide material. In this case, transport across grain boundaries is dominated by the presence of localized charge states in the grain boundary plane. These charge states are compensated by carrier depletion zones either side of the boundary, with the resulting electrostatic potential being described as a back-to-back Schottky barrier. Hence, the properties of grain boundaries in high-Tc superconductors below the 89 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society
transition temperature are dominated by this depletion zone, which is devoid of mobile charge carriers. The fact that charge depletion zones are implicit in these models, leads to a plausible explanation for all of the observed transport measurements and microscopical characterizations. However, as with potential barriers in conventional oxides [15], the origin of these localized states is not known; only their presence is implied by the agreement with the transport measurements. Here we present a structural unit model for [001] tilt grain boundaries in YBCO. The structural unit model focuses on the particular atomic arrangements that exist in the dislocation core. This has the particular advantage over strain field models in that it can be applied equally well to low- and high-angle grain boundaries. For [001] tilt boundaries in YBCO, the structure of all boundaries (0-450) can be constructed from only two distinct structural units [16]. In the case of the most prevalent asymmetric
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