Intrinsic Josephson effects and intrinsic tunneling in high-T_c 
superconductors

Paul Mueller
Physikalisches Institut III, Universitaet Erlangen-Nuernberg, Germany

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 A, a single crystal of 3  m 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 high-T_c superconductors as well as of
the organic superconductor k-(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. In order to access these atomic
scale devices, vertical structures on single crystal surfaces and
epitaxial thin films were fabricated. Recent progress includes
sub-micron devices and access to single layers. Using this technology
we were able to investigate the intrinsic quasiparticle tunneling
characteristics of Bi_2Sr_2CaCu_2O_8 near optimal doping. We 
charcterize the gap-to-pseudogap evolution and conclude that the 
superconducting gap and the pseudogap arise from independent 
quasiparticle excitations on the same energy scale.
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