QUASI-PARTICLE TUNNELING IN BI-SR-CA-CU-O THIN-FILMS

Tunnel junctions were formed by depositing a thin layer of Al2O3(20 A) and Al counter electrodes onto sputtered and annealed Bi-Sr-Ca-Cu-O films. Differential conductance spectra displayed low conductivity at low bias, and two pairs of peaks, symmetric with respect to zero bias, which correlate in energy and relative magnitude with the two superconducting Bi-Sr-Ca-Cu-O phases present in the film (110 and 85 K). The measured values of the peak positions and the reduced gap parameter are 1=1821 meV(85-K phase), 2=2528 meV (110-K phase), 2 /kBTc=5.5 0.6 for both phases. The shape of the conductance curves at higher bias is parabolic. This effect can be explained by the presence of a composite barrier. Such a barrier is likely to be the result of insulating or semiconducting grain surfaces in addition to the artificial barrier. © 1990 The American Physical Society.