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

The basic building unit of transition metal oxides are O-octaedra being edge connected to blocks (Nb₂O_5-y, . . . ) or planes (STO, . . . HTS) being separated by crystallographic shear planes. Shear planes allow easy creation of oxygen vacancies which then permit fast electron or hole hopping along those planes. The connected O-octaedra blocks or planes are structurally and electronically rigid, yielding, e.g., overdoped Mott insulators in HTS. In high T_c superconductors CuO-plane transport dominate and CuO-in-plane weak links occur easily in preparation and growth, being the major obstacles for HTS transport currents. CuO-plane weak links are tunnel junctions with reduced critical Josephson currents j_cJ(A/cm²), enhanced normal R_bn(Wcm²), and leakage resistances R_bl(T < T_c) ³ R_bn, where the degradation of j_cJR_bn << D/e with D the superconducting energy gap is specific to HTS. Those degradations are consequences of the space-wise transition to a Mott-insulator seam with n_L^|| localized states by reduced wave function overlap by surfaces and by spatial or bonding disorder, with d^|| > 0.2 nm as tunnel barrier width of height f @ 2eV housing n_L^|| @ 10²¹/cm³ localized states. To avoid the intrinsic Mott insulating seam, crystalline (YBCO)* is used in interface engineered junction (IEJ), which is a dielectric with a low barrier height f < 0.1 eV and n^L << 10²¹/cm³, simulating Nb/Al/AlO_x(OH)_y- junctions showing ways out of this interface chemistry dead lock.