A review of weak/strong links and junctions in high-Tc superconductors as a transition to a Mott insulator

In high-T-c superconductors (HTS), which are layered, doped Mott insulators, weak links occur easily in preparation and growth, being weakened further by irradiation, by impurities and by disorder. CuO-plane weak links are the major obstacle for HTS currents, both dc and radio frequency. They are tunnel junctions that show reduced critical Josephson currents j(cJ) (A cm(-2)) and enhanced normal R-bn (Omega cm(2)) and leakage resistances R-bl (T < T-c) greater than or equal to R-bn where the degradation of j(cJ)R(bn) much less than Delta/e with the superconducting energy gap A is specific to HTS. The easy occurrence of weak links and their degradations already in the normal superconducting state are consequences of the space-wise transition to a Mott insulator seam with n(L)(parallel to) localized states by reduced wavefunction overlap at surfaces and by chemical or spatial disorder. The tunnel degradations show up in j(cJ) alpha exp(-2kappad), in j(cJ)(parallel to)R(bn)(parallel to) approximate to Delta/10e exp(-kappad) and in j(cJ)(parallel to)R(bn)(parallel to2) approximate to j(cJ)(parallel to)R(bl)(parallel to2) approximate to const greater than or equal to c(0) = 10(-12) V Omega cm(2) for all NCCO, YBCO, BSCCO and TBCCO junctions grown naturally or artificially to date. In-plane tunnel barriers of width d(parallel to) > 0.2 nm and of height phi(parallel to) approximate to 2 eV, housing n(L)(parallel to approximate to) 10(21) cm(-3) localized states causing R-bl(parallel to) alpha 1/n(L)(parallel to), are quantified for the first time by the resonant tunnel model in agreement with all experimental data, especially R-bl-(parallel to), j(cJ)(parallel to)R(bn)(parallel to)- and j(cJ)(parallel to)Rparallel to(bn)(2) degradations. In interface engineered junctions, a modified, seemingly crystalline (YBCO)* layer of low barrier height phi(c) less than or similar to 20 meV and width dc greater than or equal to 1-5 nm, covered by the standard Mott insulating YBCO, acts as a controllable and integrable tunnel barrier. Perpendicular weak links with the quasi-insulating blocking layer as the tunnel barrier are dominated by d(perpendicular to) greater than or equal to 0.8 nm and by n(L)(B) < 10(21) cm(-3), as the density of intermediate states depending strongly on doping yields j(cJ)(perpendicular to)R(bn)(perpendicular to) and j(cJ)(perpendicular to)R(perpendicular to2)bn values which are larger than for in-plane junctions. Comparing HTS junctions with Nb/Nb2O5-y and Nb/Al/AlOx(OH)(y) junctions shows the way out of this interface chemistry deadlock.