INTRINSIC JOSEPHSON EFFECTS IN LAYERED SUPERCONDUCTORS

The large anisotropy and the extremely short coherence lengths of the high-T-c superconductors suggest, that the layered crystal structure is mapped onto a periodic order parameter modulation. Even an ideal single crystal should be a stack of Josephson junctions. The measurements were performed on small single crystals of (Bi-1-x Pb-x)(2)Sr2CaCu2O8, Tl2Ba2Ca2Cu3O10, and Pr2-xCexCuO4. For current flow perpendicular to the CuO2 planes, the current-voltage characteristics show a series connection of hysteretic Josephson junctions. For Bi2Sr2CaCu2O8, the modulation of the critical current in a magnetic field is consistent with a junction spacing of 1.5 nm, i.e. the distance between two CuO2 bilayers. By the incoherent microwave emission of the AC Josephson currents between 4.5 and 95 GHz, the number of emitting junctions can be counted. Indeed, the single crystals consist of a stack of Josephson junctions. Every pair of CuO2 bilayers forms a Josephson junction. Similar results have been obtained very recently on single crystals of the organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2). This observation supports the conclusion that in any layered superconductor with sufficiently high. anisotropy the superconducting order parameter is spatially inhomogeneous a priori.