Physics of HTS Natural, Artificial and Interface Engineered Junctions

In high-Tc superconductors, being layered, doped Mott insulators, in-plane weak links occur easily in preparation and in growth, and are prone to deteriorate further. CuO-plane weak links are the major obstacles for HTS transport currents, both dc and rf. Weak links are tunnel Junctions showing reduced critical Josephson currents jcJ (A/cm^2), enhanced normal Rbn (flcm") and leakage resistances Rbi(T < Tc) > Rbn, where the degradation of JcJ R(bn) << (delta)/e is specific to HTS. The ease of occurrence of weak links and their degradations are consequences of the transition to a Mott-insulator seam by reduced wave function overlap at surfaces, by underdoping, and by spatial or bonding disorder. The degradations show up in jcJ (proportionl to) exp(- 2Kd), in jcJ R(bn) ~ ( (delta)/10e)exp(- Kd), and in jcJRbn ~ jcJRbl ~ c > co = 10-12 Vf (omega)cm2 for all NCCO, YBCO, BSCCO, and TBCCO junctions grown naturally or artificially to date, with d > 0.2 nm as tunnel barrier width of height (phi) ~ 2 eV, and with nL ~ 10^21 /cm^3 localized states causing Rbl (proportionl to) l/nl. For the first time their Rbl, jcJ Rbn and j2JRbn degradations are quantified by the resonant tunnel model, even for interface engineered junctions (IEJ), in agreement with experimental data. Comparison of HTS junctions with Nb/Nb2O5y and Nb/AI/AIOx (OH)y junctions shows ways out of the interface chemistry deadlock.