We have investigated the coupling between ${\mathrm{CuO}}_2$ layers in high-${\mathit{T}}_{\mathit{c}}$ 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 ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$, (${\mathrm{Pb}}_{\mathit{y}}$${\mathrm{Bi}}_{1\mathrm{-}\mathit{y}}$${)}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$, ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$, and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ and on a-axis-oriented ${\mathrm{YBa}}_2$${\mathrm{Cu}}_2$${\mathrm{O}}_7$ 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 Å. In our microwave emission experiments we were able to prove explicitly that every pair of ${\mathrm{CuO}}_2$ double or triple layers forms a working Josephson contact. An exception is ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, where only flux-flow behavior has been observed.

• Received 19 July 1993

DOI:https://doi.org/10.1103/PhysRevB.49.1327