A review of weak/strong links and junctions in high-Tcsuperconductors 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 (Ω cm 2 ) and leakage resistances R bl ( T < T c ) ≥ R bn where the degradation of j cJ R bn ≪ Δ/ e with the superconducting energy gap Δ 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 ∥ localized states by reduced wavefunction overlap at surfaces and by chemical or spatial disorder. The tunnel degradations show up in j cJ ∝ exp(−2κ d ), in j ∥ cJ R ∥ bn ≈ Δ/10 e exp(−κ d ) and in j ∥ cJ R ∥ 2 bn ≈ j ∥ cJ R ∥ 2 bl ≈ const ≥ c 0 = 10 −12 V Ω cm 2 for all NCCO, YBCO, BSCCO and TBCCO junctions grown naturally or artificially to date. In-plane tunnel barriers of width d ∥ > 0.2 nm and of height ϕ ∥ ≈ 2 eV, housing n L ∥ ≈ 10 21 cm −3 localized states causing R ∥ bl ∝ 1/ n L ∥ , are quantified for the first time by the resonant tunnel model in agreement with all experimental data, especially R ∥ bl -, j ∥ cJ R ∥ bn - and j ∥ cJ R ∥ 2 bn degradations. In interface engineered junctions, a modified, seemingly crystalline (YBCO)* layer of low barrier height ϕ c ≲ 20 meV and width d c ≥ 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 ⊥ ≥ 0.8 nm and by n B L < 10 21 cm −3 , as the density of intermediate states depending strongly on doping yields j ⊥ cJ R ⊥ bn and j ⊥ cJ R ⊥ 2 bn values which are larger than for in-plane junctions. Comparing HTS junctions with Nb/Nb 2 O 5− y and Nb/Al/AlO x (OH) y junctions shows the way out of this interface chemistry deadlock.