Intrinsic Josephson effects in high-Tc superconductors.

Abstract

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{\ensuremath{-}}\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{\ensuremath{-}}\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 \AA{}. 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.