Physics of HTS natural, artificial and interface engineered junctions

In high-T_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_cJ (A/cm²), enhanced normal R_bn (Ωcm²) and leakage resistances R_bl(Tc)≥R_bn, where the degradation of j_cJR_bn≪Δ/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_cJ∝exp(-2κd), in j_cJR_bn≈(Δ/10e)exp(-κd), and in _jcJR²_bn≈j_cJR²_bl≈c≥c_o=10^-12 VΩcm² 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 φ≈2 eV, and with n_L≈10²¹/cm³ localized states causing R_bl∝1/n_L. For the first time their R_bl, j_cJRbn and j²_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/Nb₂O_5y and Nb/Al/AlO_x(OH)_y junctions shows ways out of the interface chemistry deadlock.