Physics of HTS natural, artificial and interface engineered junctions

Abstract

In high-T/sub c/ 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 j/sub cJ/ (A/cm/sup 2/), enhanced normal R/sub bn/ (/spl Omega/cm/sup 2/) and leakage resistances R/sub bl/(T<T/sub c/)/spl ges/R/sub bn/, where the degradation of j/sub cJ/R/sub bn//spl Lt//spl 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 j/sub cJ//spl prop/exp(-2/spl kappa/d), in j/sub cJ/R/sub bn//spl ap/(/spl Delta//10e)exp(-/spl kappa/d), and in /sub jcJR//sup 2//sub bn//spl ap/j/sub cJ/R/sup 2//sub bl//spl ap/c/spl ges/c/sub o/=10/sup -12/ V/spl Omega/cm/sup 2/ 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 /spl phi//spl ap/2 eV, and with n/sub L//spl ap/10/sup 21//cm/sup 3/ localized states causing R/sub bl//spl prop/1/n/sub L/. For the first time their R/sub bl/, j/sub cJRbn/ and j/sup 2//sub cJRbn/ 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/Nb/sub 2/O/sub 5y/ and Nb/Al/AlO/sub x/(OH)/sub y/ junctions shows ways out of the interface chemistry deadlock.