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

摘要

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) >= R_(bn) where the degradation of j_(cJ)R_(bn) " DELTA/e with the superconducting energy gap DELTA 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) propor. to exp(-2kd), in j~(||)_(cJ)R~(||)_(bn) approx =DELTA/10e exp(-Kd) and j~(||)_(cJ)R~(||2)_(bn) approx = j~(||)_(cJ)R~(||2)_(bl) approx =const >= 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~(||) > 0.2 nm and of height phi~(||) approx =2 eV, housing n_L~(||) approx = 10~(21) cm~(-3) localized states causing R~(||)_(bl) propor. to 1_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 phi_c< approx = 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~(perpendicular) >= 0.8 nm and by n~B_L < 10~(21) cm~(-3), as the density of intermediate states depending strongly on doping yields j~(perpendicular)_(cJ)R~(perpendicular)_(bn) and j~(perpendicular)_(cJ)R~(perpendicular 2)_(bn) values which are larger than for in-plane junctions. Comparing HTS junctions with Nb/Nb_2O_(5-y) and Nb/Al/AlO_x(OH)_y junctions shows the way out of this interface chemistry deadlock.