Intrinsic Josephson effects in layered superconductors

Abstract

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-xPb x) 2Sr 2CaCu 2O 8, Tl 2Ba 2Ca 2Cu 3O 10, and Pr 2−xCe xCuO 4. For current flow perpendicular to the CuO 2 planes, the current-voltage characteristics show a series connection of hysteretic Josephson junctions. For Bi 2Sr 2CaCu 2O 8, 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 CuO 2 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 CuO 2 bilayers forms a Josephson junction. Similar results have been obtained very recently on single crystals of the organic superconductor κ-(BEDT-TTF) 2Cu(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.