In-plane weak links and tunnel junctions in high Tc-superconductors (HTS)

•Juergen Halbritter
Forschungszentrum Karlsruhe, IMF 1, Postfach 3640, D-76021 Karlsruhe, Germany

The culprit of superconducting transport in HTS is asymmetry, and related to this, in-plane weak links degraded strongly shown by jCJ << jCP @ 4×108 A/cm2, by jCJRbn << D/e (sc energy gap) and by enhanced boundary resistance Rbn >> Rsh @ 10-11 Wcm2 (Sharvin resistance). The degradations are due to quasi two-dimensional metallic conduction in CuO-planes being separated by quasi-insulated blocking layers of thickness d^ ³ 0.8 nm driving a metal-insulator transition. Hence, any perturbation yields insulating plaques in the CuO-planes with plenty of localized states nL (x, e) therein given by intrinsic and extrinsic interface relaxation. The resonant tunneling vie nL describes quantitatively not only transport perpendicular to the planes and across external surfaces but also jbn of in-plane weak links which converts in the superconducting state to the Josephson tunnel current jcJ and to the normal leakage tunnel current jbl. Consequences of the localized states nL(x,e) as ïntrinsic defects" causing pair weakening, noise, jbl-enhancements, jcJ-reductions, electric ZE and magnetic ZHres rf residual surface impedance are elaborated. By resonant tunneling via nL @ 1021/cm states

jCJRbn @ D/eexp(- kd), jCJRbl2 @ jCJRbn2 ³ 10-12 VWcm2 and differences between artifical and natural junctions are quantified for the first time. Ways out of the ßurface chemistry dead lock" with its tunnel anomalies and residual losses are analyzed.