Hamburg 2001 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 19: Korngrenzen in HTSL

TT 19.6: Vortrag

Donnerstag, 29. März 2001, 10:45–11:00, H

In-plane weak links and tunnel junctions in high T_c-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 j_CJ ≪ j_CP ≃ 4 × 10⁸ A/cm², by j_CJR_bn ≪ Δ/e (sc energy gap) and by enhanced boundary resistance R_bn ≫ R_sh ≃ 10⁻¹¹ Ωcm² (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^⊥≥ 0.8 nm driving a metal-insulator transition. Hence, any perturbation yields insulating plaques in the CuO-planes with plenty of localized states n_L (x, ε) therein given by intrinsic and extrinsic interface relaxation. The resonant tunneling vie n_L describes quantitatively not only transport perpendicular to the planes and across external surfaces but also j_bn of in-plane weak links which converts in the superconducting state to the Josephson tunnel current j_cJ and to the normal leakage tunnel current j_bl. Consequences of the localized states n_L(x,ε) as "intrinsic defects" causing pair weakening, noise, j_bl-enhancements, j_cJ-reductions, electric Z^E and magnetic Z_res^H rf residual surface impedance are elaborated. By resonant tunneling via n_L ≃ 10²¹/cm states j_CJR_bn≃ Δ/e exp(− κ d), j_CJR_bl² ≃ j_CJR_bn² ≥ 10⁻¹² VΩcm² and differences between artifical and natural junctions are quantified for the first time. Ways out of the "surface chemistry dead lock" with its tunnel anomalies and residual losses are analyzed.