Implications of d(x**2-y**2) Symmetry for Grain-boundary-based High-Tc Devices

Grain boundaries in high-Tc superconductors are extensively used as high-quality Josephson junctions in device applications and in fundamental studies of high-Tc superconductivity. Characteristic features of large-angle grain boundaries, such as the dependence of the critical current on applied magnetic fields, are highly anomalous and cannot be explained by conventional Josephson-junction models and a superconducting order parameter with s-wave symmetry. Direct imaging with scanning SQUID microscopy provides evidence that magnetic flux is generated spontaneously in these boundaries. This is in accordance with a model based on a d(x**2-y**2) symmetry component of the superconducting order parameter and with the faceted character of the grain boundary. These observations have several significant implications both for the understanding of the properties of grain boundaries in high-Tc superconductors and for their applications.