Quasiparticle tunneling in Bi-Sr-Ca-Cu-O thin films

Tunnel junctions were formed by depositing a thin layer of ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$(20 Å) 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 ${\Delta }_1$=18–21 meV(85-K phase), ${\Delta }_2$=25–28 meV (110-K phase), 2Δ/$k_B$ $T_c$=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.