TUNNELING SPECTROSCOPY OF BI(PB)-SR-CA-CU-O SUPERCONDUCTORS

The tunneling characteristics for break junctions in polycrystalline Bi1.7Pb0.4Sr2-XCa2+YCu3+ZO10+delta samples fabricated by direct mixing of oxides-carbonates (1) and via Bi(Pb)-Ca - O + Sr-Ca-Cu-O precursors (2) have been investigated. Long time annealing of the samples (1) 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 S1-S2-S1-S2-.. and to an emergence of a reproducible vertical rise of current in the I-V characteristics for break junctions at voltages V is-approximately-equal-to +/- 2DELTA/e. A record value of the characteristic voltage V(c) = I(c)R(n) = (31 +/- 4) m V at T = 4.2K has been detected. Evidence for an important role of the proximity effect in the samples (1) has been obtained. For the samples (2) 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 +/- 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.