Characteristic microstructure in small Bi-2212 grains showing the Wohlleben effect as revealed by High-Resolution Electron Microscopy

By means of high-resolution transmission electron microscopy and electron d iffraction we show that samples of Bi-2212 high-temperature superconductors which exhibit the Wohlleben effect have an extremely polydomain microstruc ture on a mu m length scale. In directions perpendicular to the c-axis smal l crystallites are separated by well-defined atomically sharp (less than or similar to 1 nm) interfaces which provide good contacts between the domain s in the ab-planes. The observed microstructure strongly favors the explana tion of the Wohlleben effect due to spontaneous currents flowing in the clo sed paths between differently oriented unconventional (d-wave) superconduct ors.