Tunneling Spectroscopy of Bi(Pb)--Sr--Ca--Cu--O Superconductors

The tunneling characteristics for break junctions in polycrystalline Bi sub 1.7 Pb sub 0.4 Sr sub 2--x Ca sub 2+Y Cu sub 3+Z O sub 10+ delta samples fabricated by direct mixing of oxides--carbonates and via Bi(Pb)--Ca--O + Sr--Ca--Cu--O precursors have been investigated. Long time annealing of the samples causes decomposition in the form of the intergrowth faults (2212- in 2223-phase), which leads to the formation of the natural superlattice of the type S sub 1 -S sub 2 -S sub 1 -S sub 2 -.. and to an emergence of a reproducible vertical rise of current in the I-V characteristics for break junctions at voltages V approx = plus/minus 2 Delta /e. A record value of the characteristic voltage V sub c = I sub c R sub n = (31 plus/minus 4) m V at T = 4.2K has been detected. Evidence for an important role of the proximity effect in the samples has been obtained. For the samples the tunneling characteristics were strongly dependent on X, Y and Z. The samples with X = 0.2, Y = Z = 0 consist of an almost pure 2223-phase with a large gap parameter Delta = (31 plus/minus 1)meV (T = 4.2K). For these samples the subharmonic structure in the I-V characteristics, caused by the self-detection of Josephson radiation was observed. From I-V and dI/dV-V characteristics the tunneling density of states has been calculated. The advantage of the method of precursor synthesis for the effective modification of superconducting properties of Bi(Pb)--Sr--Ca--Cu--O has been demonstrated.