INTRINSIC JOSEPHSON EFFECTS IN HIGH-T-C SUPERCONDUCTORS

We have investigated the coupling between CuO2 layers in high-T-c superconductors by direct measurements of all dc and ac Josephson effects with current flow in the c-axis direction. The measurements have been performed on small single crystals of Bi2Sr2CaCu2O8, (PbyBi1-y)(2)Sr2CaCu2O8, Tl2Ba2Ca2Cu3O10, and YBa2Cu3O7-x and on a-axis-oriented YBa2Cu2O7 thin films. The results clearly show that all materials behave like stacks of superconductor-insulator-superconductor Josephson junctions. The current-voltage characteristics exhibit large hystereses and multiple branches, which can be explained by a series connection of highly capacitive junctions. From the modulation of the critical current in a magnetic field parallel to the layers, we infer a junction thickness of approximately 15 Angstrom. In our microwave emission experiments we were able to prove explicitly that every pair of CuO2 double or triple layers forms a working Josephson contact. An exception is YBa2Cu3O7, where only flux-flow behavior has been observed.