Intrinsic Josephson effects in layered superconductors
The large anisotropy and the extremely short coherence lengths of the high-Tc superconductors suggest that the layered crystal structure is mapped onto a periodic modulation of the superconducting order parameter. Even an ideal single crystal should consist of a stacked series of superconducting and non-superconducting layers. Three-dimensional phase coherence is provided by Josephson currents between the layers. As the typical interlayer distance is approximately 15 Å, a single crystal of 3 μm thickness should behave like a stack of 2000 Josephson junctions. This hypothesis is proved in every detail by measurements of the DC as well as the AC Josephson effects on single crystals of Bi2Sr2CaCu2O8, (Bi1−y Pby)2Sr2CaCu2O8, Tl2Ba2Ca2Cu3O10 and Pr2−x CexCuO4. Microwave emission experiments at frequencies between 3.5 and 95 GHz reveal explicitly the number of junctions in the samples. This number is given by the crystal thickness divided by 15 Å, i.e. every pair of CuO2 bilayers forms a Josephson junction.
Similar results, including microwave emission, 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.
The modulation of superconductivity on an atomic scale opens up a new application: the crystals by themselves are superconducting devices without the need of artificial treatments. The basic unit of those devices is a cell with the dimensions of the Ginzburg-Landau coherence length parallel to the layers and the interlayer distance i.e. a cube with 15·15·153 which contains only 150 atoms.
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